2025 BSH指南：无JAK2、CALR或MPL突变的血小板增多症的调查和管理

Correspondence

Anna L. Godfrey, Department of Haematology, Cambridge University Hospitals NHS Foundation Trust, Cambridge, UK

Email: anna.godfrey1@nhs.net

BSH Administrator, British Society for Haematology, 100 White Lion Street, London N1 9PF, UK.

Email: bshguidelines@b-s-h.org.uk

Funding information

British Society for Haematology

KEYWORDS

diagnosis, myeloproliferative, platelets, thrombocytosis, triple negative

METHODOLOGY

This guideline was compiled according to the BSH process at (https://b-s-h.org.uk/media/16732/bsh-guidance-development-process-dec-5-18.pdf). The Grading of Recommendations Assessment, Development and Evaluation (GRADE) nomenclature was used to evaluate the levels of evidence and to assess the strength of recommendations. The GRADE criteria can be found at http://www.gradeworkinggroup.org and are summarised in appendix 3 of the guidance

document linked above. Literature review details are in the Supplementary Appendix.

REVIEW OF THE MANUSCRIPT

Review of the manuscript was performed by the British Society for Haematology (BSH) General Haematology Task Force and BSH Guidelines Committee. It was also on the members' section of the BSH website for comment. It has

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been reviewed by members of the UK Blood Cancer Research Network MPN Group, the NHS England Haematological Malignancies Genomics Working Group and patient representatives from MPN Voice; these organisations do not necessarily approve or endorse the contents.

INTRODUCTION

Since previous BSH guidelines covering investigation and management of thrombocytosis,$^{1-3}$ there have been considerable advances in the biology, diagnosis and management of essential thrombocythaemia (ET), particularly relating to myeloproliferative neoplasm (MPN) driver mutations in JAK2, CALR and MPL. However, many patients with unexplained thrombocytosis are negative for these mutations. Optimal application and interpretation of bone marrow biopsy and broader genomic testing have not been defined in this context and real-world practice varies.$^{4}$ This guideline provides expert consensus on investigation and management of patients with thrombocytosis lacking classical MPN driver mutations, focusing on practical diagnostic and management strategies appropriate for UK haematology practice.

AETIOLOGY OF THROMBOCYTOSIS

Table 1 shows the main primary and secondary causes of thrombocytosis.

Reactive thrombocytosis

Over 80% of thrombocytosis is secondary to causes such as iron deficiency, infection, cancer or inflammation ('reactive' thrombocytosis, Table 1).$^{6}$ Thrombopoietin (TPO), the primary regulator of megakaryopoiesis, is predominantly produced by the liver and cleared from plasma via receptors (thrombopoietin receptor, TPOR, encoded by the MPL gene) on megakaryocytes and platelets.$^{7}$ Plasma TPO concentration is therefore regulated by platelet mass, although hepatic production is also increased by acute phase reactants (e.g. interleukin-6). Additionally, other hormones and cytokines can modulate megakaryopoiesis independently of TPO. There is an association between platelet count and increased body mass index, most significant in pre-menopausal women$^{8}$,$^{9}$ and with metabolic syndrome,$^{10}$,$^{11}$ although there are no defined 'normal ranges' for these contexts.

Clonal myeloid disorders

Thrombocytosis is a cardinal feature of ET and can occur in polycythemia vera (PV) and primary myelofibrosis (PMF). Constitutive activation of MPL/TPOR signalling is driven by recurrent somatic mutations in JAK2, CALR and MPL,$^{12}$ most often JAK2 c.1849G>T (p.Val617Phe, JAK2 V617F; 50%~60%

|Primary|Essential thrombocythaemia Other myeloid neoplasm (e.g. PV, CML, PMF, MDS with low blasts and 5q deletion, MDS/MPN with SF3B1 mutation and thrombocytosis) Hereditary Idiopathic| |---|---| |Reactive (secondary)|Iron deficiency Bleeding Infection Inflammatory/autoimmune conditions (e.g. rheumatoid arthritis, systemic lupus erythematosus) Non-myeloid malignancy Tissue damage (e.g. trauma, surgery, burns) Hyposplenism/splenectomy Autoimmune haemolytic anaemia Obesity| |Drug-related|Thrombopoietin receptor agonists Erythropoiesis-stimulating agents Bone marrow recovery following cytotoxic chemotherapy Other drugs$^{5}$ (e.g. baricitinib, all-trans retinoic acid, vinca alkaloids)| |Spurious|Fragmented or microspherocytic red blood cells (e.g. iron deficiency anaemia, burns) Cryoglobulinaemia Cytoplasmic fragments of leukaemic blasts Microorganisms (e.g. candidaemia)|

of ET), frameshift variants in CALR exon 9 (25%–35% of ET) or MPL exon 10 variants (particularly Trp515/Ser505; 5%–10% of ET). Non-hotspot JAK2 or MPL mutations occur in a small minority.$^{4,13,14}$ ET cases lacking JAK2, CALR and MPL mutations ('triple-negative') comprise 10%–15% of cases$^{13,15}$ and are diagnosed on histology.$^{16,17}$ Most of these patients, who are predominantly young females, lack definitive evidence of a clonal origin to their thrombocytosis.$^{18-20}$ Compared to other ET genotypes, triple-negative patients exhibit lower risks of myelofibrotic transformation and thrombosis, with highly favourable survival.$^{13,15,21,22}$

Less common clonal causes of thrombocytosis include BCR::ABL1-positive chronic myeloid leukaemia (CML), myelodysplastic neoplasms (MDS) with chromosome 5q deletion and MDS/MPN overlap syndromes including chronic myelomonocytic leukaemia and MDS/MPN with SF3B1 mutation and thrombocytosis. Very rarely thrombocytosis is a feature of myeloid/lymphoid neoplasms with gene rearrangements, such as ETV6::ABL1 and JAK2 rearrangements,$^{16}$ usually associated with additional blood count abnormalities.

Clonal haematopoiesis (CH) refers to the presence of clonal blood cell populations in otherwise healthy individuals, often driven by somatic mutations in a set of established genes, for example, DNMT3A, TET2 and ASXL1.$^{16,23-26}$

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CH increases with age in both individuals with and without thrombocytosis and is commonly detected in older patients undergoing investigation for thrombocytosis.$^{4}$,$^{14}$,$^{27}$,$^{28}$ Initial studies estimated the prevalence of CH at ~10% of healthy individuals aged over 70.$^{23}$ With more sensitive sequencing techniques detecting mutations at variant allele fractions (VAFs) <1%, prevalence increases to over two-thirds, if not all, elderly individuals.$^{29}$,$^{30}$ There are no clear platelet count differences between individuals with and without CH (excluding JAK2)$^{31}$ and whether common CH-associated mutations directly drive thrombocytosis in some patients is currently unknown. However, a finding of CH-associated mutations may indicate emergent haematological malignancy, particularly those present at higher VAF (e.g. >10%), in multiple genes, in less commonly mutated genes, in younger individuals and/or in the presence of additional raised counts and/or cytopenias. Of note, some genetic alterations, in particular SH2B3 mutations and chromosome 20 deletions, while not typical primary MPN drivers, have been reported in triple-negative ET and share mechanisms that could drive thrombocytosis.$^{13}$,$^{32}$,$^{33}$

Germline variants associated with thrombocytosis

High penetrance germline variants in several genes (e.g. SH2B3, EGLN1/EPAS1, 14q32.2 duplication) can predispose to the development of acquired MPN in rare cases,$^{34,35}$Individuals often have a strong family history of MPN, haematological malignancy or other cancers. Germline genetic factors also contribute to the natural platelet count variation between individuals, through largely polygenic and complex mechanisms.$^{36,37}$

Hereditary thrombocytosis (HT), with autosomal dominant or recessive inheritance, comprises a rare, heterogeneous group of disorders most commonly driven by pathogenic germline variants in JAK2, MPL or THPO$^{38}$⁻$^{42}$ (Table 2). Individual familial cohorts have been reported where germline SH2B$^{33}$ or GSN$^{44}$ variants clustered with the HT phenotype, although these are likely to be very $^{45}$. The clinical phenotype in HT appears variable; most patients are asymptomatic and identified incidentally.$^{46}$ Some mutations are associated with extra-haematological effects such as distal limb defects (THPO variants),$^{47}$ autoimmune manifestations and hepatosplenomegaly (biallelic SH2B3 variants).$^{43}$ A minority of reports have described increased thrombotic and/or haemorrhagic risk; however, studies are limited by small sample size, short follow-up and insufficient data on additional risk factors.

During investigation of thrombocytosis, uncommon germline variants in JAK2, MPL, THPO or SH2B3 may be identified by next-generation sequencing (NGS). Lack of high-quality variant-specific data on associated clinical phenotypes and long-term outcomes can make interpretation of their clinical significance challenging. There are also no defined criteria to discriminate between a thrombocytosis 'trait' (a genetic characteristic, e.g. platelet count slightly above normal range) and a

hereditary ‘disorder’. Positive family history or long history of stable thrombocytosis may increase suspicion for HT.

APPROACH TO THE INVESTIGATION OF THROMBOCYTOSIS

Patients with persistent isolated thrombocytosis (platelet count >450×10$^{9}$/L) should be reviewed clinically and secondary causes (Table 1) considered with a thorough history and examination. Assessment should include symptoms of headaches, erythromelgia or other microvascular disturbances, cardiovascular risk factors, thrombotic or haemorrhagic events, family history of thrombocytosis, body mass index, palpable splenomegaly and targeted assessment of any symptoms or signs suggesting non-haematological malignancy.$^{65,66}$

Stage 1 investigations

First-line investigations (Figure 1) include blood film examination for spurious thrombocytosis (e.g. cryoglobulinaemia) or features of other myeloid diseases (e.g. myeloid left shift, basophilia, monocytosis, dysplasia), iron studies and inflammatory markers (e.g. C-reactive protein). Imaging should be considered, including chest X-ray for patients aged over 40, abdominal ultrasound if splenomegaly cannot be excluded clinically, and other targeted investigations such as ultrasound or computed tomography (CT) imaging if non-haematological malignancy or other systemic disease is suspected clinically.$^{66}$ Serum thrombopoietin levels do not distinguish between reactive and clonal thrombocytosis$^{68,69}$ and are not routinely recommended.

If no secondary cause is found, peripheral blood assays for canonical mutations in JAK2, CALR and MPL should be undertaken. Recommended minimum limits of detection are 1%–3% VAF for JAK2 V617F and 5% VAF for CALR exon 9 and MPL exon 10 variants; methods are discussed more fully in the BSH Good Practice Paper.$^{71}$ Laboratories are increasingly moving to peripheral blood screening with NGS mini-panels, which often sequence larger gene regions than recognised hotspots and may detect non-canonical mutations outside JAK2 V617 and MPL W515/S505, in a minority of patients investigated.$^{4}$ Clinicians should be aware of their laboratory's peripheral blood screening techniques, including gene regions covered and sensitivity, since these may influence subsequent testing strategies. BCR::ABL1 rearrangement should be excluded if JAK2, CALR and MPL screening is negative, especially if there are other features suspicious for CML, using assays that detect typical and atypical transcripts.$^{2,72}$

Bone marrow histology

Histological assessment of bone marrow (BM), requiring an adequate trephine biopsy specimen with high-quality

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|Genes|Mutations|Clinical phenotype if documented|References| |---|---|---|---| |JAK2|JAK2 V617I|Three patients with significant thrombosis events at a young age; others in kindred asymptomatic|41| |JAK2|JAK2 R564Q/L|Can be symptomatic thrombocytosis. One provoked thrombotic episode described. May predispose to, or present with, concurrent haematological neoplasms|48, 49| |JAK2|JAK2 R867Q|Two family members had thrombosis within described kindred|50| |JAK2|JAK2 L611S|Thrombocytosis detected in a 3-month-old child|51| |JAK2|JAK2 S755R/R938Q|No documentation of thrombosis in described kindred|50| |JAK2|JAK2 H608N|No documentation of thrombosis or splenomegaly in described kindred|52| |JAK2|JAK2 T875N|History of cerebrovascular infarction in one of described kindreds|53| |MPL|MPL R102P|No clearly documented risk of thrombosis|54| |MPL|MPL S505N|Described kindreds have high risk of thrombotic complications, and development of splenomegaly and marrow fibrosis after the age of 20. This variant is more commonly somatically acquired but occasionally may be germline|55, 56| |MPL|MPL P106L|No clearly documented risk of thrombosis. Mainly significant when homozygous|57, 58| |MPL|MPL R170H|Low risk of thrombosis|46| |MPL|MPL V501A|No clearly documented risk of thrombosis|59| |MPL|MPL K39N|No clearly documented risk of thrombosis. Mainly significant when homozygous|60| |SH2B3|SH2B3 Arg148Profs*40|Homozygous variant: splenomegaly, neutrophilia, leucoerythroblastosis and autoimmune features in childhood; isolated thrombocytosis in adulthood. No thrombotic events|43| |SH2B3|SH2B3 Val402Met|Homozygous variant: splenomegaly and autoimmune features in childhood. Thrombotic tendency|43| |Nucleotide change|Nucleotide change|Nucleotide change|Nucleotide change| |THPO|G --> C transversion in the splice donor of intron 3 of THPO|Microcirculatory disturbances that respond well to aspirin|39| |THPO|G --> T transversion at position 516 of THPO mRNA|Autosomal dominant inheritance|61| |THPO|G --> A transition near the 5' donor site of THPO exon 3: Exon skipping|One older family member had a documented iliac artery thrombosis|62| |THPO|One-base deletion in the 5'-UTR of TPHO|Autosomal dominant inheritance|63| |THPO|G185T mutation in the 5'-UTR region of TPHO|Four affected family members with thrombocytosis and three of the four showed congenital transverse limb defects|64|

sections and staining, is a mandatory feature of the World Health Organization (WHO) and International Consensus Classification (ICC) diagnostic criteria for ET regardless of molecular results, specifically the finding of megakaryocyte proliferation and increased enlarged, mature megakaryocytes with hyperlobated nuclei.$^{16,17}$ Most patients with isolated thrombocytosis, an acquired JAK2, CALR or MPL mutation and no other blood film abnormalities will have ET, but where a trephine biopsy is performed, this allows exclusion of differential diagnoses such as prefibrotic myelofibrosis and baseline fibrosis assessment. BM histology is also recommended where a highly sensitive peripheral blood screening assay has detected JAK2 V617F at low VAF (e.g. 0.5%–1.0%); co-existent CALR or MPL mutation should be excluded and additional genomic studies considered, as

discussed below. Histology can be subtle in such cases and the reporting pathologist could be influenced by knowledge of molecular results. A diagnosis of CH rather than ET should be carefully considered if histology is inconclusive and/or there is an alternative reactive cause; interval trephine sampling may be helpful together with clinical monitoring for emergent MPN features.

In investigation of triple-negative thrombocytosis, BM aspirate and trephine biopsy may identify features suspicious for either MPN or another disorder (e.g. dysplasia, increased blasts, ring sideroblasts, eosinophilia, reduced storage iron on Perls stain). BM assessment should be performed routinely in individuals with unexplained cytopenias, monocytosis or other atypical blood film findings. Very occasionally a non-myeloid malignancy such as a

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![graph TD\n A1[Unexplained, persistent, isolated thrombocytosis >450 x 10⁹/L] --> A2[Clinical history & examination Blood film, iron studies, CRP Consider: CXR, abdominal ultrasound, targeted radiology]\n A2 --> A3[Peripheral blood screening: IAK2, CALR, MPL If negative: BCR:ABL1]\n A2 --> A4[Secondary or spurious cause]\n A3 --> A5[MPN Consider BM biopsy]\n A3 --> A6[Consider peripheral blood NGs-MGP panel and/or inherited thrombocytosis panel]\n A3 --> A7[Triple-negative thrombocytosis]\n A7 --> A8[Family history]\n A7 --> A9[Age <60 years without significant cardiovascular risk*, no previous vascular events Pits 450-600 x 10⁹/L and no pertinent symptoms³]\n A7 --> A10[Age ≥60 years without significant cardiovascular risk*, no previous vascular events Pits 600-1500 x 10⁹/L and/or unexplained pertinent symptoms‡]\n A7 --> A11[Age ≥60 years （consider at age 40-60 if significant cardiovascular risk factors¹ OR prior vascular events OR Pits >1500 x 10⁹/L）]\n A9 --> A12[No additional investigations]\n A10 --> A13[BM biopsy, NGs-MGP +/- cytogenetics may be considered]\n A11 --> A14[BM biopsy, NGs-MGP +/- cytogenetics should be considered]\n A12 -->|FBC monitoring| A15[If age 60 or significant change in blood counts or clinical features: Repeat PB screening⁶⁷; consider BM biopsy, NGs-MGP +/- cytogenetics]\n A13 -->|FBC monitoring| A16[If age 60 or significant change in blood counts or clinical features: Repeat PB screening⁶⁷; consider BM biopsy, NGs-MGP +/- cytogenetics]\n A14 -->|FBC monitoring| A17[If significant change in blood counts or clinical features: Repeat PB screening⁶⁷; consider BM biopsy, NGs-MGP +/- cytogenetics]\n](1/parseimages/figurec6b0572e89bd4398be3c443a459e3d28_0_2.png) plasma cell neoplasm may be identified, typically in older individuals.

In patients investigated for triple-negative thrombocytosis, BM histology reports are more likely to describe ET-like morphological features in those with higher platelet counts.$^{4}$ Many patients with modest thrombocytosis show normal megakaryocyte numbers, morphology and distribution or increased megakaryocytes without the characteristic morphological atypia (large, hyperlobated megakaryocytes) of ET. A proportion of patients, often young females with marked (e.g. >800 × 10$^{9}$/L) but unexplained thrombocytosis, show more striking morphology with increased megakaryocytes and ET-like atypical forms,

albeit with negative genomic investigations.$^{4,14}$ Definitive histological interpretation can, however, be challenging due to borderline features with only occasional atypical megakaryocytes. Furthermore, there is inter-individual variability in interpretation; one series found 18% of diagnoses were changed after expert histopathology review, most often rescinding a diagnosis of MPN.$^{14}$ Nonetheless, the final clinical diagnosis of patients investigated for triple-negative thrombocytosis in routine practice is strongly predicted by histology reports.$^{4}$ All BM histology should therefore undergo expert haematopathologist review, in conjunction with molecular and clinical features within a multidisciplinary team (MDT) setting.

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Additional genomic studies

Additional genomic studies

Assessment for a myeloid neoplasm on BM examination can be supplemented with extended NGS myeloid gene panel testing (NGS-MGP) and assessment of copy number abnormalities by conventional karyotype or single-nucleotide polymorphism (SNP) microarray$^{71}$ (Figure 1). NGS-MGP is negative in 70%–85% of patients investigated for triple-negative thrombocytosis, but a minority have (i) non-canonical, pathogenic acquired variants in JAK2 and MPL outside hotspot regions, (ii) putative pathogenic germline variants as discussed above and/or (iii) variants in genes found in other myeloid malignancies.$^{4,13,14,19,73}$ The Genomic Medicine Service within England specifies a minimum panel for MPN investigation including ASXL1, CALR, CBL, CSF3R, CUX1, DNMT3A, EZH2, HRAS, IDH1, IDH2, IKZF1, JAK2, KIT, KRAS, MPL, NFE2, NRAS, RUNX1, SETBTP1, SF3B1, SH2B3, SRSF2, TET2, TP53, U2AF1 and ZRSR2.$^{45}$ This service also allows patients who have exhausted ‘standard of care’ investigations to access whole genome sequencing (WGS),$^{45}$ and/or RNA fusion panel could be considered, although clear diagnostic utility for these modalities has not BEEN demonstrated.

Positive genomic findings must be interpreted in the context of clinical phenotype and BM morphology. Isolated variants in myeloid genes (e.g. DNMT3A, TET2, ASXL1), particularly at low VAF, are commonly found in older individuals with CH and normal blood counts and may not be causal for thrombocytosis.$^{23,24}$ Prior exposure to chemotherapy may be associated with CH variants in PPMID, CHEK2 or TP53.$^{74,75}$ In addition, while some non-hotspot JAK2 and MPL variants are recurrent in ET and are now established MPN drivers (e.g. MPL p.Ser204Pro), the significance of other rare acquired or germline variants in JAK2, MPL and SH2B3 must be assessed alongside published literature, local or public genomic databases, individual BM histology and clinical phenotype. National collaboration in collating and curating such variants with their associated phenotypes may assist interpretation in the future, ideally through a focused national MDT.

Patients with a putative germline variant in an MPN gene on BM NGS-MGP which is annotated ‘pathogenic’ or ‘likely pathogenic’ by appropriate criteria$^{76}$ should undergo confirmatory testing of a germline sample, preferably skin, nail or hair. Where there is a known family history of thrombocytosis, NGS-MGP may be performed on peripheral blood. However, standard NGS-MGP may not cover all relevant genes (e.g. THPO) and a dedicated inherited thrombocytosis panel may be required.

Choice of investigations

When selecting investigations for individual patients, the potential diagnostic yield of invasive and resource-intensive tests (e.g. BM biopsy, NGS-MGP) should be considered alongside the likely impact of positive findings on management. Figure 1 shows a suggested algorithm, including criteria for repeat testing that occasionally identifies a canonical

driver mutation or may be of value if panel contents are updated.$^{77}$ BM histology and complex genomic investigations are unlikely to influence the management of patients with negative peripheral blood screening tests aged under 60 years, without significant cardiovascular risk, prior vascular events or relevant symptoms, and with mild thrombocytosis (e.g. <600×10$^{9}$/L). It should be noted that this platelet threshold is based on consensus; patients may be counselled that a conservative investigation strategy does not completely exclude the possibility that WHO-defined features of ET might be identified if a BM biopsy was performed. For patients with low vascular risk, but higher platelet counts (e.g. 600–1500×10$^{9}$/L) or pertinent symptoms (e.g. headaches), BM biopsy and comprehensive genomic studies may be offered, particularly if local peripheral blood screening assays for JAK2, CALR and MPL target limited mutational hotspots, acknowledging that the yield from wider genomic studies is low and even patients with atypical histology will often not need therapy. Although BM histology and further genomic studies are also often negative in patients aged over 60, these should be considered where appropriate to assess for low-VAF or non-hotspot JAK2 or MPL mutations that would warrant therapy, alternative myeloid neoplasms or non-myeloid malignancy.

It is technically feasible to perform NGS-MGP on peripheral blood or bone marrow,$^{78,79}$ and some laboratories employ a broad panel even for initial peripheral blood screening for thrombocytosis. However, any positive findings usually require interpretation in the context of BM histology. Where comprehensive genomic investigation is warranted (Figure 1), this should usually be performed in conjunction with BM examination, preferably testing the BM sample especially if megakaryocyte morphology is abnormal. If a patient is unable or unwilling to undergo this assessment, and/or histological assessment is deemed very unlikely to affect their management significantly, then peripheral blood NGS-MGP could be considered as an additional screen for an underlying clonal disorder.

Paediatric considerations

Over 50% of paediatric patients diagnosed with ET are triple-negative, and negative clonality studies have been reported in a majority.$^{20,80,81}$ Similar histological features to adult ET were described in a small cohort.$^{82}$ Complication rates are low,$^{80,81,83,84}$ and in the absence of vascular events, symptoms or extreme thrombocytosis, most patients require no therapy.

Platelet count reference ranges increase in the first few months of life, then decrease towards adult ranges around school entry age.$^{85}$ A higher threshold for investigation of thrombocytosis can therefore be considered appropriate in young children (e.g. 650×10$^{9}$/L at 2 months, decreasing to 450×10$^{9}$/L by 6 years).$^{47}$ In children with thrombocytosis, following exclusion of secondary causes, testing of parental blood counts is suggested alongside JAK2, CALR and MPL

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mutations, BCR::ABL1 and an inherited thrombocytosis panel. The guidance for BM biopsy and further genomic investigations in Figure 1 can then be followed, although given the low likelihood of positive molecular findings or need for therapy, a higher platelet threshold (e.g. 800–1000 × 10$^{9}$/L) may be appropriate in children without prior thrombosis, haemorrhage or pertinent symptoms.

Recommendations

* Following exclusion of JAK2, CALR and MPL hotspot mutations and BCR::ABL1 with peripheral blood assays of appropriate sensitivity, bone marrow biopsy, NGS-MGP +/- cytogenetic analysis (karyotype or SNP microarray) should be considered in patients with un-explained persistent thrombocytosis (>450 × 10$^{9}$/L) who are aged over 60 years or with prior vascular event(s) or platelets >1500 × 10$^{9}$/L, and those aged 40–60 years with significant cardiovascular risk factors (Grade 1C).

$\bullet$ Patients aged under 60 years without significant vascular events or risk factors but more pronounced thrombocytosis (e.g. platelets 600–1500 × 10$^{9}$/L) and/or unexplained pertinent symptoms may be offered bone marrow biopsy, NGS-MGP +/- cytogenetic analysis (karyotype or SNP microarray) (Grade 2C).

* Bone marrow biopsy and additional genomic investigations are not required in most patients aged under 60 years without previous vascular events, cardiovascular risk factors or symptoms and with mild thrombocytosis (e.g. platelets <600 × 10$^{9}$/L); secondary causes should be fully excluded (Grade 2C).

$\bullet$ In children, higher platelet thresholds for investigation may be considered, particularly under 6 years (Grade 2C). Parental blood count testing and an inherited thrombocytosis panel should be included in those without reactive causes (Grade 1B).

$\bullet$ Assessment of triple-negative thrombocytosis should include review of histological and genomic findings within a specialist integrated haematological malignancy diagnostic service, summarised in an integrated report and discussion in the context of clinical findings at an appropriate MDT (Grade 1B).

INTEGRATED DIAGNOSTIC TERMINOLOGY

A proposed system for diagnostic terminology integrating histological and genomic findings is shown in Figure 2, with key features in Box 1.

Patients with acquired genomic abnormalities excluding JAK2, CALR and MPL

Patients with an acquired molecular or cytogenetic abnormality outside of JAK2, CALR and MPL (Figure 2)

have evidence of CH, although most variants are not MPN-specific. Although a clonal marker is a minor criterion supporting a diagnosis of ET by the WHO/ICC criteria, histological megakaryocyte atypia remains essential. Identification of a clonal genomic abnormality should prompt thorough clinical and BM evaluation for alternative myeloid pathology. This is particularly pertinent for patients with multiple pathogenic variants and/or variants at high VAF (e.g. >10%). Where an alternative myeloid neoplasm is absent and megakaryocyte atypia in keeping with ET is found, we consider subclassification as 'triple-negative ET with clonal marker(s)' to be helpful, to distinguish from the majority without clonal markers. A minimum VAF is suggested (Box 1), in accordance with other diagnostic entities such as clonal cytopenia of undetermined significance.$^{16}$

Cases of triple-negative thrombocytosis with an acquired genomic abnormality outside JAK2/CALR/MPL and without convincing classical 'proliferative' megakaryocyte morphology fall outside current diagnostic classifications. Considering the uncertain pathophysiology, natural history and vascular risk, a new diagnosis 'clonal thrombocytosis of undetermined significance' (CTUS) is proposed (Figures 2 and 3A). It should be noted that there is no current evidence that such patients have an increased risk of progression to classical ET, although CH in general is associated with an increased risk of haematological malignancy.$^{23,24}$ Further study of this group is warranted.

Patients with negative genomic studies

For patients without reactive causes in whom myeloid malignancy-associated genomic abnormalities have been excluded by NGS-MGP and cytogenetic analysis, a diagnosis of ET can still be made on histological features alone in the current WHO/ICC systems.$^{16,17}$ This approach has several limitations: (i) histological analysis is potentially subjective$^{4,14,86}$ and the robustness of these criteria has not been verified in triple-negative cases; (ii) many triple-negative cases have borderline histological features, particularly with modest thrombocytosis$^{4}$; (iii) ET driven by canonical driver mutations has a distinctive aetiology and natural history compared to patients with 'triple-negative ET',$^{13,21,22}$ but patient information and counselling are typically tailored towards the former; and (iv) there is no available clinical diagnosis for patients with significant thrombocytosis but neither reactive causes nor typical megakaryocyte atypia of ET. For these reasons, we propose alternative terminology derived from expert consensus that we consider helpful in clinical practice.

For patients lacking the characteristic morphological atypia (large, hyperlobated megakaryocytes) of ET, we propose the diagnosis 'idiopathic thrombocytosis of undetermined significance' (ITUS) (Figures 2 and 3B). Patients with classical ET-like megakaryocyte atypia would meet the current WHO diagnostic criteria for ET if reactive

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![graph TD\n A1[JAK2, CALR, MPL testing] -->|negative| A2[Extended genomic analysis(Myeloid gene panel+/- cytogenetics)]\n A1 -->|Acquired pathogenic variant| A3[JAK2, MPL or CALR- mutated ET]\n A1 -->|JAK2, MPL pathogenic germline variant| A4[Thrombocytosis with pathogenic germline variant （Hereditary thrombocytosis）]\n A1 -->|Other pathogenic germline variant| A4\n A2 -->|Acquired molecular or cytogenetic abnormality| A5[Bone marrow histology]\n A2 -->|Increased, atypical megakaryocytes （large, hyperlobated）| A6[Triple-negative ET with clonal marker(s)]\n A2 -->|Other histological abnormalities| A7[Other myeloid neoplasm e.g. PMF, MDS/MPN]\n A2 -->|Normal or increased megakaryocytes （no morphological atypia）| A8[Clonal thrombocytopsis of undetermined significance （CTUS）]\n A5 -->|Increased, atypical megakaryocytes （large, hyperlobated）2| A9[Idiopathic thrombocytosis with atypical megakaryocytes （ITAM） （Triple-negative ET without clonal markers2）]\n A5 -->|Normal or increased megakaryocytes （no morphological atypia）| A10[Idiopathic thrombocytosis of undetermined significance （ITUS）]\n](1/parseimages/figure2e527211e6f445c2b94c562171a1ba4d_0_2.png) thrombocytosis is excluded.$^{16}$ If making a diagnosis of ET, we suggest further subcategorising as 'triple-negative ET without clonal markers', facilitating discussion with the patient around the uncertain aetiology and likely indolent course. However, for such individuals, we propose that an alternative diagnosis 'idiopathic thrombocytosis with atypical megakaryocytes' (ITAM, Figures 2 and 3C) may be more appropriate than ET, for the reasons discussed above.

In proposing the distinction between ITUS and ITAM (wherein both groups lack a clonal marker but differ in whether megakaryocyte atypia is prominent histologically) and between CTUS and triple-negative ET with clonal marker(s) (wherein both groups have a clonal marker but differ in whether megakaryocyte atypia is prominent histologically), we acknowledge that histological discrimination is subjective and should be made following specialist haematopathology review and MDT discussion. The proposed diagnostic algorithm is intended to ensure that patients historically diagnosed with triple-negative ET are accommodated within current terminology and that cases can be subcategorised carefully in future studies of risk stratification and therapy; future audit of the clinical utility of these categories will be important. Conversely, young patients with mild thrombocytosis (e.g. <600×10$^{9}$/L) and low vascular risk may not warrant a BM biopsy that would discriminate histologically between ITUS and ITAM, as discussed above. A presumptive label of 'idiopathic thrombocytosis' may be pragmatically helpful in such cases.

MANAGEMENT OF THROMBOCYTOSIS WITHOUT JAK2, CALR AND MPL MUTATIONS

Reactive thrombocytosis

The management of RT is primarily of the underlying cause. Thromboembolic or bleeding complications appear rare but studies are heterogeneous$^{6,87}$ and cases require individual consideration; for example, splenectomy is associated with an increased thrombotic risk immediately post-operatively.$^{88}$ There is no evidence that therapies such as antiplatelet agents prevent complications in RT. Local guidelines for thromboprophylaxis should be used in hospitalised or recently discharged patients, considering the underlying condition and other risk factors.

A full blood count (FBC) should be monitored to ensure the thrombocytosis resolves as the reactive state is treated. Patients with iron deficiency, thrombocytosis and normal haematocrit should also have a repeat FBC to exclude an abnormal increase in haematocrit following iron replacement, which would warrant investigation for PV or hereditary erythrocytosis.$^{89}$

Recommendations

$\bullet$ Management of reactive thrombocytosis should focus on the underlying cause (Grade 1B). $\bullet$ Anticoagulant or antiplatelet medication is not routinely recommended, unless indicated due to other clinical factors (Grade 1C).

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BOX 1 Proposed diagnostic terminology for unexplained thrombocytosis without JAK2, CALR or MPL mutations

Idiopathic thrombocytosis of undetermined significance:

Persistent* thrombocytosis >450 × 10$^{9}$/L with no reactive cause;

$\bullet$ Negative broad genomic analysis including NGS-MGP;

Histology shows normal or increased megakaryocytes without significant morphological atypia.

Idiopathic thrombocytosis with atypical megakaryocytes (ITAM; triple-negative ET without clonal markers):

$\bullet$ Persistent* thrombocytosis >450 × 10$^{9}$/L with no reactive cause;

$\bullet$ Negative broad genomic analysis including NGS-MGP and cytogenetic analysis;

$\bullet$ Histology shows increased megakaryocytes with large forms showing hyperlobated nuclei.

Clonal thrombocytosis of undetermined significance (CTUS):

$\bullet$ Persistent* thrombocytosis >450 × 10$^{9}$/L with no reactive cause;

$\bullet$ One or more somatic mutations involving a specified set of clonal haematopoiesis driver genes$^{16}$ other than JAK2, CALR or MPL, with VAF ≥ 2% (≥4% for X-linked gene mutations in male patients), or a clonal cytogenetic abnormality;

$\bullet$ Histology shows: (i) normal or increased megakaryocytes without significant morphological atypia; (ii) no diagnostic features of another myeloid neoplasm).

Triple-negative ET with clonal marker(s):

$\bullet$ Persistent* thrombocytosis >450 × 10$^{9}$/L with no reactive cause;

$\bullet$ One or more somatic mutations involving a specified set of clonal haematopoiesis driver genes$^{16}$ other than JAK2, CALR or MPL, with VAF ≥ 2% (≥4% for X-linked gene mutations in male patients), or a clonal cytogenetic abnormality;

$\bullet$ Histology shows: (i) increased megakaryocytes with large forms showing hyperlobated nuclei; (ii) no diagnostic features of another myeloid neoplasm);

*Thrombocytosis must be persistent for at least 3 months before confirming a diagnosis, and a longer duration of surveillance may be preferable for mild or variable thrombocytosis. Clinical vigilance for any potential emerging reactive cause should continue throughout follow-up.

Hereditary thrombocytosis

There is no consensus on optimal approaches to the management of HT. Although some variants have been associated with an increased risk of vascular events,$^{55}$ others have favourable outcomes$^{57}$ and most cohorts lack the long-term follow-up data needed to estimate complication risks. Extrapolating data from ET to inform management may result in the overtreatment of patients at low risk of complications.$^{42}$

Low-dose aspirin could be considered as primary prophylaxis in a patient with a significant family history of unprovoked thrombosis associated with the same genotype and also in those deemed at high risk of cardiovascular complications due to other medical problems. Patients who develop unexplained venous thrombosis should be anticoagulated using standard protocols; these patients would be expected to have long-term anticoagulation, assuming acceptable bleeding risks. Cytoreduction should not be offered routinely but has been employed in patients with severe intractable symptoms associated with extreme thrombocytosis or recurrent thrombosis.$^{55}$ As the long-term complication risks may be unknown for individual patients, clinical monitoring including FBC and

blood film for disease progression and close monitoring of cardiovascular risks in a fashion akin to ET is advised. All patients with HT should be reviewed by an appropriate specialist MDT, which may include discussion in a genomics tumour advisory board with representation from clinical genetics, and enrolled on national registries if available.

Recommendations

$\bullet$ Patients with confirmed HT should be discussed in a multidisciplinary meeting, with specialist centre input if required (Grade 1C).

* Clinical monitoring for potential complications such as thrombosis, bleeding and myelofibrotic progression and optimisation of cardiovascular risk factors, similar to ET, is suggested (Grade 1C).

$\bullet$ Consider low-dose aspirin in the presence of other cardiovascular risks, if a family member with the same genetic abnormality has suffered an otherwise unexplained thrombotic episode or if the particular variant has been associated with high incidence of thrombosis (Grade 2C).

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 * Following unprovoked thrombosis, standard protocols should be followed including long-term secondary thromboprophylaxis (Grade 1C).

$\bullet$ Cytoreduction is not recommended routinely but could be considered if there is recurrent thrombosis despite anticoagulation, bleeding or other symptoms attributed to thrombocytosis (Grade 2C).

Idiopathic thrombocytosis of undetermined significance

Suggested management for the remaining diagnostic categories is summarised in Figure 4. For cases with thrombocytosis lacking secondary causes and with no clonal marker or atypical megakaryocyte morphology, considered ITUS, there is a lack of evidence to guide management. There is no evidence to support routine antiplatelet therapy in the absence of another definite indication such as background cardiovascular risk. In pregnancy, standard obstetric guidance should be followed, for example, for prevention of venous thromboembolism.$^{90}$ Regarding follow-up, patients with mild thrombocytosis (e.g. <600 × 10$^{9}$/L) with no evidence of a myeloid neoplasm or inherited disorder should be educated on any symptoms or signs that may prompt re-evaluation and community-based intermittent FBC monitoring could be considered (e.g. annually if platelet count stable). For younger patients, particularly those in whom initial investigations are limited to peripheral blood screening, comprehensive re-evaluation could be considered following significant change in blood counts, at the age of 60 years or in the event of a new, unprovoked vascular event as these represent time points where a change in management would occur if ET were diagnosed. For patients with equivocal evidence of a myeloid disorder, particularly borderline histological features, haematology follow-up may be appropriate to instigate routine re-evaluation at a future time point or in the event of change in phenotype.

Recommendations

* Routine use of antiplatelet agents is not recommended in the absence of other standard indications (Grade 1C). * Annual FBC surveillance is appropriate for most patients with a stable platelet count, with consideration for re-evaluation on significant change in blood counts, new clinical features (e.g. unexplained thrombosis) or (for younger patients) at age 60 (Grade 2C).

Idiopathic thrombocytosis with atypical megakaryocytes (triple-negative ET without clonal markers)

Patients falling into this proposed category have the typical megakaryocyte atypia of ET but lack an identifiable clonal marker. Such cases have traditionally been diagnosed with

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 FIGURE 4 Management recommendations for triple-negative essential thrombocythaemia (ET) with clonal marker(s), idiopathic thrombocytosis with atypical megakaryocytes (triple-negative ET without clonal markers), clonal thrombocytosis of undetermined significance and idiopathic thrombocytosis of undetermined significance. Note that when considering use of cytoreduction, platelet count targets should take the treatment indication and vascular risk into account (see text for details). Where the potential indication for therapy is a vascular event, consideration should also be given to the presence of other provoking factors. Aspirin in this figure refers to primary prophylaxis, assuming that individuals with established cardiovascular disease or other thrombotic events will be managed accordingly.

ET, and aspirin and/or cytoreductive therapy appropriate for ET have been used routinely in accordance with vascular risk,$^{21,22,91}$ although complication rates are low. Repeat BM and genomic assessment should be considered in patients contemplating commencement of cytoreduction where the original assessment was some years previously (e.g. >5 years).

There is no evidence to favour any particular cytoreductive agent, although in the absence of molecular confirmation of a clonal disorder, treatments without potential DNA-damaging mechanisms, such as interferon-alpha, may be preferred where possible. Where cytoreduction is used, platelet count targets should reflect the indication for cytoreduction; for example, aiming for resolution of symptoms where these were the primary indication; platelets <1500 × 10$^{9}$/L in those treated for extreme thrombocytosis; platelets <600 × 10$^{9}$/L in those treated because of age and/or vascular risk factors without prior events; platelets within the normal range in those with otherwise unexplained vascular events.

Recommendations

* Consider tertiary MDT review of bone marrow histology, in conjunction with molecular results (Grade 2B). * Optimise other cardiovascular risk factors (Grade 1B). * Consider primary prophylaxis with aspirin in those aged over 60 years or with additional cardiovascular risk factors (hypertension, diabetes mellitus,

hypercholesterolaemia or smoking) or for refractory symptoms (Gradness 2C).

$\bullet$ Consider cytoreduction in patients aged over 60, with a thrombotic or haemorrhagic event where the thrombocytosis may be implicated, persistent platelet count >1500 × 10$^{9}$/L, refractory symptoms or multiple cardiovascular risk factors. Interferon-alpha may be a favoured cytoreductive agent in the absence of clonal markers, if there are no contraindications (Grade 2B).

$\bullet$ In patients receiving cytoreduction, platelet count targets should take into account the treatment indication and vascular risk (Grade 1C).

$\bullet$ In patients undergoing invasive surgery, consider temporary cytoreduction in those with marked thrombocytosis (>1000 × 10$^{9}$/L) or reduced von Willebrand factor activity levels (Grade 2C).

Thrombocytosis with histology consistent with ET and a non-MPN driver mutation (triple-negative ET with clonal marker(s))

These patients would also have previously been managed as ET and the guidance for ITAM above should be followed. A low threshold for repeat BM and genomic assessment is advised in patients developing atypical features (e.g. unexplained or disproportionate cytopenias on cytoreduction or new monocytosis).

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Recommendations

* Follow guidance for ITAM, with a low threshold for repeat investigations if there are atypical features (Grade 2C).

Clonal thrombocytosis of undetermined significance

Patients with CTUS would not meet histological criteria for ET, and the natural history and complication risks are unknown from historical datasets. In the absence of specific evidence, suggestions given for ITAM above could be considered, but limiting cytoreduction to cases where there is concern that a pronounced thrombocytosis is contributing to refractory symptoms or an otherwise unexplained vascular event. Interventions for modifiable cardiovascular risk factors including smoking cessation are especially important since smoking increases the risk of CH and some, although not all, studies have identified CH to be associated with increased risk of cardiovascular disease.$^{92}$ When considering aspirin or cytoreductive therapy, it is particularly relevant in this group to counsel patients that the benefits of these agents on vascular risk are uncertain.

Although not meeting full criteria for CTUS (Box 1), these management principles are also suggested for patients with thrombocytosis in whom a very low-level JAK2 V617F mutation (e.g. VAF <1%) is identified in the absence of histology supporting MPN or other molecular findings. Repeat mutational testing (with VAF) is recommended if platelet count rises and could be considered every 2–3 years if counts are stable but a confirmed diagnosis of ET would influence management.

Recommendations

* Follow guidance for ITAM, considering cytoreduction only in cases where a pronounced thrombocytosis is thought to be contributing to refractory symptoms or an otherwise unexplained vascular event (Grade 2D). * Maintain a low threshold for repeat investigations if there are atypical features (Grade 2D).

AUTHOR CONTRIBUTIONS

All authors contributed to the conceptualisation, methodology, writing and review of the guideline.

ACKNOWLEDGEMENTS

All authors contributed to the guideline. The authors wish to thank Antria Siakalli for help in undertaking the initial literature review. The BSH General Haematology task force members at the time of writing this guideline were Sara Stuart-Smith, Savio Fernandes, Mohammed Altohami, John Brewin, Barbara De La Salle, Shereef Elmoamly, Jayne Parkes, Nita Prasannan, Joseph Sharif, Jennifer

Tam and Lugain Abdalla. The authors would like to thank them, the BSH sounding board and the BSH guidelines committee for their support in preparing this guideline, as well as Catherine Cargo, Mary Frances McMullin, Bethan Psaila and Tim Somervaille for their comments on the manuscript.

FUNDING INFORMATION

Funding was provided by the British Society for Haematology.

CONFLICT OF INTEREST STATEMENT

The BSH paid the expenses incurred during the writing of this guidance. All authors have made a declaration of interests to the BSH, which may be viewed on request. A.L.G.: advisory board, speaker honoraria and conference support from Novartis; consulting for Johnson & Johnson; speaker honoraria from MD-Education. A.A.K: educational support from AOP. A.M.: conference support and speaker honoraria from Novartis; speaker fees from GSK. A.J.I.: conference support, advisory board and speaker honoraria from Novartis; speaker fees from Incyte. N.C.P.C.: advisory board, speaker honoraria and research support from Novartis; honoraria from Ascentage and Blueprint. R.F.: advisory boards for GSK, Celgene, CTI Biopharma; conference support from BMS and Novartis. M.G.: advisory board for Sanofi, Pfizer, Janssen, SOBI; speaker honoraria from Amgen, SOBI, Pfizer, Novartis, Janssen; travel support from Janssen, Takeda, Novartis, Stemline Mernini. A.G.: honoraria from Novartis, BMS, AbbVie, Blueprint medicines and advisory board for Recordati. D.P.M.: advisory board, speaker honoraria and conference support from Novartis; speaker fees from GSK and AbbVie. A.J.W.: Speaker honoraria from Astellas, Gilead, Novartis, Servier, Pfizer, Jannsen and Jazz pharmaceuticals. C.N.H.: consulting/advisory board for AbbVie, AOP, Celgene/BMS, Constellation Pharmaceuticals, CTI BioPharma Corp, Galecto, Geron, Gilead, GSK, Imago, Incyte, Janssen, Keros, MSD, Morphosys, Promedior, Roche, Shire, Sierra Oncology, SOBI and Novartis; speaker honoraria from AbbVie, BMS, CTI BioPharma Corp, Geron, Sierra Oncology and Novartis; research funding from BMS, GSK, Constellation Pharmaceuticals and Novartis. A.J.M.: honoraria for consulting and speaker fees from Novartis, Celgene/BMS, AbbVie, CTI, MD-Education, Sierra Oncology, Medialis, Morphosys, Ionis, Mescape, Karyopharm, Sensyn, Incyte, Galecto, Pfizer, Relay Therapeutics, GSK, Alethiomics and Gilead; research funding from Celgene/BMS, Novartis, Roche, Alethomics and Galecto; co-founder and equity holder in Alethiomics Ltd., a spin-out company from the University of Oxford. J.N.: advisory board/conultancy and speaker honoraria from Bioskyrb and Incyte; consultancy/speaker fees from Astra Zeneca. The following members of the writing group have no conflicts of interest to disclose: M.A., J.G.

DATA AVAILABILITY STATEMENT

Data sharing not applicable to this article as no datasets were generated or analysed during the current study.

GODFREY ET AL.

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13

REVIEW PROCESS

Members of the writing group will inform the writing group chair if any new evidence becomes available that would alter the strength of the recommendations made in this document or render it obsolete. The document will be reviewed regularly by the relevant task force and the literature search will be re-run every 5 years to search systematically for any new evidence that may have been missed. The document will be archived and removed from the BSH current guidelines website,$^{93}$ for any addenda that may be produced after the initial publication.

DISCLAIMER

While the advice and information in this guidance are believed to be true and accurate at the time of going to press, neither the authors, the BSH nor the publishers accept any legal responsibility for the content of this guidance.

AUDIT TOOL

Blank Audit template can be found for the writing group to complete: https://b-s-h.org.uk/media/15658/audit-template-mar-2017.doc.

ORCID

Anna L. Godfrey https://orcid.org/0000-0003-1955-9499

Alesia A. Khan https://orcid.org/0000-0001-7641-2608

Mamta Garg https://orcid.org/0000-0002-3560-0132

Donal P. McLornan $\blacktriangleright$ https://orcid.

org/0000-0003-1224-091X

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SHB GUIDELINE FOR THROMBOCYTOSIS WITHOUT JAK2, CALR OR MPL MUTATIONS

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49. Stewart BL, Carraway HE, Alvarez AL, Lesmana H, Molina JC, Tu ZJ, et al. JAK2 p.R564 germ line variants associated with hered- itary thrombocythemia and hematologic neoplasms. Blood Adv. 2024;9:1534-43. 50. Marty C, Saint Martin C, Pecquet C, Grosjean S, Saliba J, Mouton C, et al. Germline JAK2 mutations in the kinase domain are responsible for hereditary thrombocytosis and are resistant to JAK2 and HSP90 inhibitors. Blood 2014;123(9):1372-83. 51. Aral B, Courtois M, Ragot S, Bourgeois V, Bottolier-Lemallaz E, Brandet C, et al. Germline JAK2 L611S mutation in a child with thrombocytosis. Haematologica. 2018;103(8):e372-873. 52. Rumi E, Harutyunyan AS, Casetti I, Pietra D, Nivarthi H, Moriggl R, et al. A novel germline JAK2 mutation in familial myeloproliferative neoplasms. Am J Hermatol. 2014;89(1):117-8. 53. Yoshimitsu M, Hachiman M, Uchida Y, Arima N, Arai A, Kamada Y, et al. Essential thrombocytosis attributed to JAK2-T875N germline mutation. Int J Hematol. 2019;110(5):584-90. 54. Bellanné-Chantelot C, Mosca M, Marty C, Favier R, Vainchenker W, Plo I. Identification of MPL R102P mutation in hereditary thrombocytosis. Front Endocrinol. 2017;8:235. 55. Teofili L, Giona F, Torti L, Cenci T, Ricerca BM, Rumi C, et al. Hereditary thrombocytosis caused by MPLSER505Asn is associated with a high thrombotic risk, splenomegaly and progression to bone marrow fibrosis. Haematologica. 2010;95(1):65-70. 56. Li K., Martini M, Rocca B, Amos CI, Teofili L, Giona F, et al. Evidence for a founder effect of the MPL-S505N mutation in eight Italian pedigrees with hereditary thrombocytemia. Haematologica. 2009;94(10):1368-74. 57. Alzahrani M, Al Turki S, Al Rajban W, Alshalati F, Almodaihsh F, Abuigsmin KA, et al. Pro106Leu MPL mutation is associated with thrombocytosis and low risk of thrombosis, splenomegaly and marrow fibrosis. Platelets. 2022;33(8):1220-7. 58. El-Harith EHA, Roel S, Ballmaier M, Germeshausen M, Frye-Boukhriss H, von Neuhoff N, et al. Familial thrombocytosis caused by the novel germ-line mutation p Pro106Leu in the MPL gene. Br J Haematol. 2009;144(2):185-94. 59. Borsani O, Pietra D, Casetti IC, Vanni D, Riccaboni G, Catricalà S, et al. Germline MPL mutations may be a rare cause of 'triple-negative' thrombocytosis. Ex Hematol. 2024;129:104127. 60. Moliterno AR, Williams DM, Gutierrez-Alamillo LI, Salvatori R, Ingersoll RG, Spivak JL. Mpl Baltimore: a thrombopoietin receptor polymorphism associated with thrombocytosis. Proc Natl Acad Sci USA. 2004;101(31):11444-47. 61. Ghilardi N, Wiestner A, Kikuchi M, Ohsaka A, Skoda RC. Hereditary thrombocythemia in a Japanese family is caused by a novel point mutation in the thrompoietin gene. Br J Haematol. 1999;107(2):310-6. 62. Prouzet-Mauléon V, Montibus B, Chauveau A, Hautin M, Migeon M, Ka C, et al. A novel thrombopoietin (THPO) mutation altering mRNA splicing in a case of familial thrombocytosis. Br J Haematol. 2020;190(2):e104-e107-07. 63. Kondo T, Okabe M, Sanada M, Kurosawa M, Suzuki S, Kobayashi M, et al. Familial essential thrombocythemia associated with one-base deletion in the 5'-untranslated region of the thrombopoietin gene. Blood. 1998;92(4):1091-6. 64. Graziano C, Carone S, Panza E, Marino F, Magini P, Romeo G, et al. Association of hereditary thrombocythemia and distal limb defects with a thrombopoietin gene mutation. Blood. 2009;114(8):1655-7. 65. Cheng J, Zeng Z, Ye Q, Zhang Y, Yan R, Liang C, et al. The association of pretreatment thrombocytosis with prognosis and clinicopathological significance in cervical cancer: a systematic review and meta-analysis. Oncotarget. 2017;8(15):24327-36. 66. Wang YH, Kang J, Zhi YF, Zhang Y, Wang ZQ, Zhou Q, et al. The pretreatment thrombocytosis as one of prognostic factors for gastric cancer: a systematic review and meta-analysis. Int J Surg. 2018;53:304-11. 67. National Institute for Health and Care Excellence. Suspected cancer: recognition and referenceal [Internet]. NICE guideline NG12. 2015 [cited 2025 Jun 23]. Available from: https://www.nice.org.uk/guidance/ng12

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68. Cerutti A, Custodi P, Duranti M, Noris P, Balduini CL. Thrombopoietin levels in patients with primary and reactive thrombocytosis. Br J Haematol. 1997;99(2):281-4. 69. Griesshammer M, Hornkohl A, Nichol JL, Hecht T, Raghavachar A, Heimpel H, et al. High levels of thrombopoietin in sera of patients with essential thrombocythemia: cause or consequence of abnormal platelet production? Ann Hematol. 1998;77(5):211-5. 70. Hippisley-Cox J, Coupland C, Brindle P. Development and validation of QRISK3 risk prediction algorithms to estimate future risk of cardiovascular disease: prospective cohort study. BMJ. 2017;357;2099. 71. Cross NCP, Godfrey AL, Cargo C, Garg M, Mead AJ, A British Society for Haematology Good Practice Paper. The use of genetic tests to diagnose and manage patients with myeloproliferative and myeloproliferative/myelodysplastic neoplasms, and related disorders. Br J Haematol. 2021;195(3):338-51. 72. Cross NCP, Ernst T, Branford S, Cayuela JM, Deininger M, Fabarius A, et al. European LeukemiaNet laboratory recommendations for the diagnosis and management of chronic myeloid leukemia. Leukemia. 2023;37(11):2150-67. 73. Cabagnols X, Favale F, Pasquier F, Messaoudi K, Defour JP, Ianotto JC, et al. Presence of atypical thrombopoietin receptor (MPL) mutations in triple-negative essential thrombocythemia patients. Blood. 2016;127(3):333-42. 74. Coombs CC, Zehir A, Devlin SM, Kishtagari A, Syed A, Jonsson P, et al. Therapy-related clonal hematopoiesis in patients with non-hematologic cancers is common and associated with adverse clinical outcomes. Cell Stem Cell. 2017;21(3):374-382.e4. 75. Reed SC, Croessmann S, Park BH. CHIP happens: clonal hematopoiesis of indeterminate potential and its relationship to solid tumors. Clin Cancer Res. 2023;29(8):1403-11. 76. Richards S, Aziz N, Bale S, Bick D, D. Das Gastier-Foster J, et al. Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology. Genet Med Off J Am Coll Med Genet. 2015;77(3):405-24. 77. Michail O, McCallion P, McGimpsey J, Hindley A, Greenfield G, McAllister R, et al. Mutational profiling in suspected triple-negative essential thrombocythaemia using targeted next-generation sequencing in a real-world cohort. J Clin Pathol. 2021;74(12):808-11. 78. Jansko-Gadermeir B, Leisch M, Gassner FJ, Zaborsky N, Dillinger T, Hutter S, et al. Myeloid NGS analyses of paired samples from bone marrow and peripheral blood yield concordant results: a prospective cohort analysis of the AGMT Study Group. Cancer. 2023;15(8):2305. 79. Huber S, Wossidlo N, Haferlach T, Hutter S, Walter S, Walkamp C, et al. Parallel genomic analysis from paired bone marrow and peripheral blood samples of 200 cytopenic patients. Leukemia. 2024;38(7):1626-9. 80. Ianotto JC, Curto-Garcia N, Lauermanova M, Radia D, Kiladjian JJ, Harrison CN. Characteristics and outcomes of patients with essential thrombocythemia or polycythemia vera diagnosed before 20 years of age: a systematic review. Haematologica. 2019;104(8):1580-8. 81. Zhang L, Chen X, Hu T, Xu Z, Yang W, Fu R, et al. Clinical and molecular characteristics of forty Chinese children with essential thrombocythemia: a single-center, retrospective analysis. Br J Haematol. 2023;201(3):520-9. 82. Putti MC, Pizzi M, Bertozzi I, Sabattini E, Micalizzi C, Farruggia P, et al. Bone marrow histology for the diagnosis of essential thrombocythemia in children: a multicenter Italian study. Blood. 2017;129(22):3040-2.

83. Teofili L, Giona F, Martini M, Cenci T, Guidi F, Torti L, et al. Markers of myeloproliferative diseases in childhood polycythemia vera and essential thrombocythemia. J Clin Oncol Off J Am Soc Clin Oncol 2007;25(9):1048-53. 84. Harriz, Z Kaizer H, Wei A, Karantanos T, Williams DM, Chaturvedi S, et al. Characterization of myeloproliferative neoplasms in the paediatric and young adult population. Br J Haematol. 2023;201(3):449-58. 85. Zierk J, Arzideh F, Rechenauer T, Haeckel R, Rascher W, Metzler M, et al. Age- and sex-specific dynamics in 22 hematologic and biochemical analytes from birth to adolescence. Clin Chem. 2015;61(7):964-73. 86. Wilkins BS, Erber WN, Bareford D, Buck G, Wheatley K, East CL, et al. Bone marrow pathology in essential thrombocythemia: interobserver reliability and utility for identifying disease subtypes. Blood. 2008;111(1):60-70. 87. Ho KM, Yip CB, Duff O. Reactive thrombocytosis and risk of subsequent venous thromboembolism: a cohort study. J Thromb Haemost JTH. 2012;10(9):1768-74. 88. Rottenstreich A, Kleinstern G, Spectre G, Da's N, Ziv E, Kalish Y. Thromboembolic events following splenectomy: risk factors, prevention, management and outcomes. World J Surg. 2018;42(3):675-81. 89. McMullin MF, Harrison CN, Ali S, Cargo C, Chen F, Ewing J, et al. A guideline for the diagnosis and management of polycythemia vera. A British Society for Haematology Guideline. Br J Haematol. 2019;184(2):176-91. 90. Royal College of Obstetricians and Gynaecologists. Reducing the Risk of Thrombosis and Embolism during Pregnancy and the Puerperium (Green-top Guideline No. 37a) [Internet]. 2015 [cited 2025 Mar 19]. Available from: https://www.rcog.org.uk/guidance/browse-all-guida nce/green-top-guidelines/reducing-the-risk-of-thrombosis-and-embolism-during-pregnancy-and-the-puerperium-green-top-guide line-no-37a/ 91. Barbui T, Tefferi A, Vannucchi AM, Passamonti F, Silver RT, Hoffman R, et al. Philadelphia chromosome-negative classical myeloproliferative neoplasms: revised management recommendations from European LeukemiaNet. Leukemia, 2018;32(5):1057-69. 92. Mangaonkar AA, Patnaik MM. Clonal hematopoiesis of indeterminate potential and clonal cytopenias of undetermined significance: 2023 update on clinical associations and management recommendations. Am J Hematol. 2023;98(6):951-64. 93. British Society for Haematology. Guidelines [Internet]. 2025 [cited 2025 Apr 13]. Available from: https://b-s-h.org.uk/guidelines/guide lines

SUPPORTING INFORMATION

Additional supporting information can be found online in the Supporting Information section at the end of this article.

How to cite this article: Godfrey AL, Khan AA, McGregor A, Innes AJ, Altohami M, Cross NCP, et al. Investigation and management of thrombocytosis without JAK2, CARL or MPL mutations: A British Society for Haematology Guideline. Br J Haematol. 2025;00:1–15. https://doi.org/10.1111/bjh.70260

GODFREY ET AL.

BJHaem UNIVERSITAS DEPUTER EYELITAL

68. Cerutti A, Custodi P, Duranti M, Noris P, Balduini CL. Thrombopoietin levels in patients with primary and reactive thrombocytosis. Br J Haematol. 1997;99(2):281-4. 69. Griesshammer M, Hornkohl A, Nichol JL, Hecht T, Raghavachar A, Heimpel H, et al. High levels of thrombopoietin in sera of patients with essential thrombocythemia: cause or consequence of abnormal platelet production? Ann Hematol. 1998;77(5):211-5. 70. Hippisley-Cox J, Coupland C, Brindle P. Development and validation of QRISK3 risk prediction algorithms to estimate future risk of cardiovascular disease: prospective cohort study. BMJ. 2017;357;2099. 71. Cross NCP, Godfrey AL, Cargo C, Garg M, Mead AJ, A British Society for Haematology Good Practice Paper. The use of genetic tests to diagnose and manage patients with myeloproliferative and myeloproliferative/myelodysplastic neoplasms, and related disorders. Br J Haematol. 2021;195(3):338-51. 72. Cross NCP, Ernst T, Branford S, Cayuela JM, Deininger M, Fabarius A, et al. European LeukemiaNet laboratory recommendations for the diagnosis and management of chronic myeloid leukemia. Leukemia. 2023;37(11):2150-67. 73. Cabagnols X, Favale F, Pasquier F, Messaoudi K, Defour JP, Ianotto JC, et al. Presence of atypical thrombopoietin receptor (MPL) mutations in triple-negative essential thrombocythemia patients. Blood. 2016;127(3):333-42. 74. Coombs CC, Zehir A, Devlin SM, Kishtagari A, Syed A, Jonsson P, et al. Therapy-related clonal hematopoiesis in patients with non-hematologic cancers is common and associated with adverse clinical outcomes. Cell Stem Cell. 2017;21(3):374-382.e4. 75. Reed SC, Croessmann S, Park BH. CHIP happens: clonal hematopoiesis of indeterminate potential and its relationship to solid tumors. Clin Cancer Res. 2023;29(8):1403-11. 76. Richards S, Aziz N, Bale S, Bick D, D. Das Gastier-Foster J, et al. Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology. Genet Med Off J Am Coll Med Genet. 2015;77(3):405-24. 77. Michail O, McCallion P, McGimpsey J, Hindley A, Greenfield G, McAllister R, et al. Mutational profiling in suspected triple-negative essential thrombocythaemia using targeted next-generation sequencing in a real-world cohort. J Clin Pathol. 2021;74(12):808-11. 78. Jansko-Gadermeir B, Leisch M, Gassner FJ, Zaborsky N, Dillinger T, Hutter S, et al. Myeloid NGS analyses of paired samples from bone marrow and peripheral blood yield concordant results: a prospective cohort analysis of the AGMT Study Group. Cancer. 2023;15(8):2305. 79. Huber S, Wossidlo N, Haferlach T, Hutter S, Walter S, Walkamp C, et al. Parallel genomic analysis from paired bone marrow and peripheral blood samples of 200 cytopenic patients. Leukemia. 2024;38(7):1626-9. 80. Ianotto JC, Curto-Garcia N, Lauermanova M, Radia D, Kiladjian JJ, Harrison CN. Characteristics and outcomes of patients with essential thrombocythemia or polycythemia vera diagnosed before 20 years of age: a systematic review. Haematologica. 2019;104(8):1580-8. 81. Zhang L, Chen X, Hu T, Xu Z, Yang W, Fu R, et al. Clinical and molecular characteristics of forty Chinese children with essential thrombocythemia: a single-center, retrospective analysis. Br J Haematol. 2023;201(3):520-9. 82. Putti MC, Pizzi M, Bertozzi I, Sabattini E, Micalizzi C, Farruggia P, et al. Bone marrow histology for the diagnosis of essential thrombocythemia in children: a multicenter Italian study. Blood. 2017;129(22):3040-2.

85. Zierk J, Arzideh F, Rechenauer T, Haeckel R, Rascher W, Metzler M, et al. Age- and sex-specific dynamics in 22 hematologic and biochemical analytes from birth to adolescence. Clin Chem. 2015;61(7):964-73. 86. Wilkins BS, Erber WN, Bareford D, Buck G, Wheatley K, East CL, et al. Bone marrow pathology in essential thrombocythemia: interobserver reliability and utility for identifying disease subtypes. Blood. 2008;111(1):60-70. 87. Ho KM, Yip CB, Duff O. Reactive thrombocytosis and risk of subsequent venous thromboembolism: a cohort study. J Thromb Haemost JTH. 2012;10(9):1768-74. 88. Rottenstreich A, Kleinstern G, Spectre G, Da's N, Ziv E, Kalish Y. Thromboembolic events following splenectomy: risk factors, prevention, management and outcomes. World J Surg. 2018;42(3):675-81. 89. McMullin MF, Harrison CN, Ali S, Cargo C, Chen F, Ewing J, et al. A guideline for the diagnosis and management of polycythemia vera. A British Society for Haematology Guideline. Br J Haematol. 2019;184(2):176-91. 90. Royal College of Obstetricians and Gynaecologists. Reducing the Risk of Thrombosis and Embolism during Pregnancy and the Puerperium (Green-top guideline No. 37a) [Internet]. 2015 [cited 2025 Mar 19]. Available from: https://www.rcog.org.uk/guidance/browse-all-guida en/green-top-guidelines/reducing-the-risk-of-thrombosis-and-embolism-during-pregnancy-and-the-puerperium-green-top-guide line-no-37a/ 91. Barbui T, Tefferi A, Vannucchi AM, Passamonti F, Silver RT, Hoffman R, et al. Philadelphia chromosome-negative classical myeloproliferative neoplasms: revised management recommendations from European LeukemiaNet. Leukemia, 2018;32(5):1057-69. 92. Mangaonkar AA, Patnaik MM. Clonal hematopoiesis of indeterminate potential and clonal cytopenias of undetermined significance: 2023 update on clinical associations and management recommendations. Am J Hematol. 2023;98(6):951-64. 93. British Society for Haematology. Guidelines [Internet]. 2025 [cited 2025 Apr 13]. Available from: https://b-s-h.org.uk/guidelines/guide lines

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