Background: The 2022 WHO criteria do not list bone marrow cellularity as a criterion for ET diagnosis (dx) 1, but a review discussing the International Consensus Classification criteria states that the marrow should be normocellular for age, with rare cases of mild hypercellularity 2. The ELN also cited normocellularity as a key factor in differentiating ET from polycythemia vera (PV) or prefibrotic myelofibrosis (MF), further suggesting that normocellularity is the expected standard in ET 3. However, we encountered a number of cases in our hematology practice with increased cellularity in bona fide ET patients (pts). Therefore, we decided to more systematically review the marrow cellularity and its prognostic impact. Methods: Medical records were queried for pts with ET per the ICD, versions 9-10. The dx of ET per WHO 2022 was confirmed by manual review. Cases lacking marrow biopsy were excluded. Structured query language was used for automated extraction of clinical, lab, and molecular findings. We utilized our previously described natural language processing pipeline to extract cellularity information from marrow reports 4. All hematopathology reports of hypercellular marrows were manually reviewed by 3 observers for confirmation and a randomly selected subset of the core and clot sections were reviewed for histomorphology and cellularity by 2 expert hematopathologists and 1 histomorphologist, blinded to pt age and clinical information prior to their review. Quantitative cellularity was determined independently and then congruently for consensus. Qualitative cellularity was defined using (100-age) ± 20% as the normal range with increases above this range considered hypercellular for age. Disease progression was defined as either progression to post-ET MF (PETMF) or direct transformation to acute myeloid leukemia. Transformation to PV was defined as pts with a JAK2 mutation for whom PV was thoroughly excluded at the time of ET dx but who developed absolute erythrocytosis and met the PV WHO diagnostic criteria later in their disease course. We performed univariable and multivariable analysis of progression, PV transformation, thrombosis and mortality risk using Cox proportional-hazards models. Overall (OS), progression-free (PFS), PV-free (PVFS) and thrombosis-free (TFS) survival were estimated using Kaplan-Meier methods. Results: 382 pts met WHO defined ET with marrow dx, of which 145 had pre-treatment diagnostic marrows available for review at Weill Cornell. In the other 237, the diagnostic biopsies had either been performed after treatment was initiated or only the report was available. Thirty-three (23%) were described as hypercellular and 112 (67%) were not. Review of 20 randomly selected marrows from the hypercellular group confirmed age-adjusted hypercellularity ranging from 30 to 80%. The hypercellular pts were significantly older at dx compared to those without hypercellularity (59 vs 50 yrs, p <0.001, Table 1). We identified no other significant differences in blood counts at dx, sex, driver mutation or thrombosis history between the groups. Univariate and multivariate analyses demonstrated no significant difference between the groups in PFS (HR 1.29, p=0.871), TFS (HR 1.39, p=0.671) or OS (HR 0.50, p=0.487) using age, thrombosis history and sex as covariates. A Cox model could not be generated for PVFS due to the low number of events. The 15-year PFS for pts with hypercellularity vs those without was 97% and 94% respectively ( p=0.56, Figure 1), 15-year PVFS was 100% and 67% respectively ( p=0.12), 15-year TFS was 79% and 87% respectively ( p=0.53) and 15-year OS was 84% and 81% respectively ( p=0.74). Conclusion: Our data to date suggests that hypercellularity is of importance diagnostically but not prognostically. Nearly a quarter of our pts with WHO ET had marrow that was hypercellular for age at dx. Pts with increased marrow cellularity tended to be older but were clinically indistinguishable from the remainder in terms of driver mutation, blood counts and thrombosis history. Univariate and multivariate analyses demonstrate that hypercellularity does not confer increased risk of progression, thrombosis or mortality. Thus, hypercellularity alone should not be used as a factor for excluding ET dx. References: 1. Khoury et al. Leukemia, 2022. 2. Gianelli et al. Virchows Archiv, 2023. 3. Kvasnicka et al. Am J Hematol, 2017. 4. Sholle et al. IEEE Int Conf Healthc Inform, 2018.

Supplementary Figure 4 from Characterization of Murine JAK2V617F-Positive Myeloproliferative Disease

AbstractThe JAK2V617F mutation is present in almost all patients with polycythemia vera (PV), large proportions of patients with essential thrombocythemia and idiopathic myelofibrosis, and less frequently in atypical myeloproliferative disorders (MPD). We show that transplantation of JAK2V617F-transduced bone marrow into BALB/c mice induces MPD reminiscent of human PV, characterized by erythrocytosis, granulocytosis, extramedullary hematopoiesis, and bone marrow fibrosis, but not thrombocytosis. Fluorescence-activated cell sorting of bone marrow and spleen showed proportional expansion of common myeloid progenitors, granulocyte-monocyte and megakaryocyte-erythrocyte progenitors. Megakaryocyte and late erythroid progenitors were dramatically increased, with only modest expansion of early erythroid progenitors. Erythropoietin (Epo) receptor expression was reduced on early, but normal on late erythroblasts. Serum levels of Epo and granulocyte colony-stimulating factor, but not granulocyte macrophage colony-stimulating factor, were reduced, whereas tumor necrosis factor-α was increased, possibly exerting a negative effect on JAK2V617F-negative hematopoiesis. These data suggest that erythrocytosis and granulocytosis in JAK2V617F mice are the net result of a complex interplay between cell intrinsic and extrinsic factors. There were no thromboembolic events and no animals succumbed to their disease, implicating additional factors in the manifestation of human disease. The disease was not transplantable and prolonged observation showed normalization of blood counts in most JAK2V617F mice, suggesting that the mutation may not confer self-renewal capacity. (Cancer Res 2006; 66(23): 11156-65)

Supplementary Methods and Materials from Characterization of Murine JAK2^V617F-Positive Myeloproliferative Disease

Supplementary Table 1 from Characterization of Murine JAK2V617F-Positive Myeloproliferative Disease

Supplementary Methods and Materials from Characterization of Murine JAK2V617F-Positive Myeloproliferative Disease

Polycythaemia vera is associated with a reduced quality of life, a high rate of vascular events, and an intrinsic risk of disease evolution. The results of several randomised trials for the treatment of this disorder are now available, and both a new ropegylated formulation of interferon alfa-2b (ropeginterferon alfa-2b; 2018) and ruxolitinib (2015) have been approved in Europe. European LeukemiaNet (ELN) investigators have therefore deemed it appropriate to provide recommendations for the use of these drugs in clinical practice. An expert panel of 14 senior haematologists from ELN centres that had actively participated in previous ELN projects or relevant randomised trials, chaired by a member of the ELN Steering Committee, developed a list of clinical questions, and a methodologist established three patient, intervention, comparator, outcome (PICO) questions and systematically reviewed the evidence. Recommendations were approved by six Delphi consensus rounds and two virtual meetings (on Jan 26, 2021, and June 24, 2021). The expert panel recommended that patients with polycythaemia vera who are younger than 60 years and have not had previous thrombotic events should start cytoreductive drug therapy if at least one of the following criteria are fulfilled: strictly defined intolerance to phlebotomy, symptomatic progressive splenomegaly, persistent leukocytosis (>15 × 109 white blood cells per L), progressive leukocytosis (at least 100% increase if baseline count is <10 × 109 cells per L or at least 50% increase if baseline count is >10 × 109 cells per L), extreme thrombocytosis (>1500 × 109 platelets per L), inadequate haematocrit control requiring phlebotomies, persistently high cardiovascular risk, and persistently high symptom burden. Recombinant interferon alfa, either in the form of ropeginterferon alfa-2b or pegylated interferon alfa-2a, is the recommended cytoreductive treatment for these patients. The expert panel suggested that either interferon alfa or ruxolitinib should be considered for patients who are being treated with hydroxyurea but require a therapy change.

Introduction Essential thrombocythemia (ET) and polycythemia vera (PV) patients suffer from various symptoms that worsen quality of life (QOL), yet serial data on symptom changes resulting from therapy are sparse in the literature. Patient questionnaires from 2 large multicenter trials (MPN-RC 111, 112) were used to assess change in symptom burden and QOL over 12 months and impact of baseline symptom burden on subsequent change in ET / PV patients on hydroxyurea (HU) or pegylated interferon alfa-2a (PEG). Methods Trials MPN-RC 111 was a single-arm, open-label, phase II trial evaluating response to PEG in high-risk ET / PV patients with HU resistance/intolerance or splanchnic vein thrombosis (SVT; NCT01259817). MPN-RC 112 was a randomized, open-label, phase III trial comparing response to PEG versus HU in cytoreductive therapy naïve high-risk ET / PV patients diagnosed < 5 years ago (NCT01258856). Measures Patients reported disease-related symptoms via the validated Myeloproliferative Neoplasms Symptom Assessment Form (MPN-SAF), QOL via the European Organisation for the Research and Treatment of Cancer Core QOL Questionnaire (EORTC QLQ-C30), and (if applicable) PEG-related symptoms (flu-like symptoms, injection site irritation, blurry vision, vision change, flushing) at baseline, 3, 6, 9, and 12 months. Analysis Mixed models assessed mean changes from baseline in the MPN-SAF Total Symptom Score (TSS), MPN-SAF items, QOL, and PEG-related symptoms in MPN-RC 111, 112 PEG, and 112 HU patients. Mixed models also assessed the impact of baseline symptom burden (high [TSS ≥ 20] versus low) on subsequent change in PEG (MPN-RC 111 and 112) and HU patients. Results Patients Of the 135 enrolled MPN-RC 111 patients, 20 with SVT and 1 with no questionnaires were excluded. Of the remaining 114, 64 (56%) / 50 (44%) had ET / PV. Patients were 51% / 48% female. Median age was 65 / 64 years, and median time since diagnosis was 38 / 55 months. 31% / 22% had prior thrombosis, and 19% / 56% had splenomegaly. Of the 168 enrolled MPN-RC 112 patients (82 PEG, 86 HU), 2 with no questionnaires were excluded. Of the remaining 166, 79 (48%) / 87 (52%) had ET / PV. Patients were 50% / 33% female. Median age was 60 / 62 years, and median time since diagnosis was 3 / 3 months. 25% / 29% had prior thrombosis, and 11% / 37% had splenomegaly. Symptoms Questionnaire completion rates ranged from 90 - 99%, 87 - 100%, and 75 - 96% for on-treatment MPN-RC 111, 112 PEG, and 112 HU patients. At baseline, TSS (0 [absent] - 100 [worst imaginable]) and QOL (0 [very poor] - 100 [excellent]) means (SDs) were 19.5 (18.4) and 71.6 (20.1) for MPN-RC 111, 17.0 (13.6) and 67.9 (24.3) for MPN-RC 112 PEG, and 14.6 (11.4) and 73.8 (18.8) for MPN-RC 112 HU patients. On average, MPN-RC 111 patients had significant improvement of TSS, fatigue, abdominal pain, abdominal discomfort, dizziness, numbness, night sweats, and fever; MPN-RC 112 PEG patients had significant worsening of fever; and MPN-RC 112 HU patients had significant worsening of inactivity (no mean changes indicating improvement were observed). PEG patients had significant worsening of PEG-related symptoms. The greatest improvements occurred in the 46 (40%), 27 (33%), and 23 (28%) MPN-RC 111, 112 PEG, and 112 HU patients with high baseline symptom burden. On average, PEG patients with high baseline symptom burden had significant improvement of TSS, fatigue, early satiety, abdominal pain, abdominal discomfort, inactivity, headache, concentration, dizziness, numbness, insomnia, cough, night sweats, itching, bone pain, fever, weight loss, and QOL, while those with low baseline symptom burden had significant worsening of TSS, early satiety, headache, itching, and bone pain. On average, HU patients with high baseline symptom burden had significant improvement of TSS, early satiety, abdominal discomfort, headache, dizziness, numbness, insomnia, itching, and weight loss, while those with low baseline symptom burden had significant worsening of TSS, early satiety, abdominal discomfort, inactivity, concentration, and sexual desire/function (Figures 1 and 2). Conclusions Although no statistical comparisons were made across trials, overall improvements were seen in MPN-RC 111 but not 112. Patients with high baseline symptom burden experienced the greatest improvements in symptom burden and QOL during treatment with PEG or HU, which may explain the improvements seen in the more advanced patients in MPN-RC 111 compared to 112. Disclosures Mascarenhas: Celgene, Prelude, Galecto, Promedior, Geron, Constellation, and Incyte: Consultancy; Incyte, Kartos, Roche, Promedior, Merck, Merus, Arog, CTI Biopharma, Janssen, and PharmaEssentia: Other: Research funding (institution). Yacoub:Dynavax: Current equity holder in publicly-traded company; Ardelyx: Current equity holder in publicly-traded company; Cara Therapeutics: Current equity holder in publicly-traded company; Hylapharm: Current equity holder in private company; Incyte: Speakers Bureau; Agios: Honoraria, Speakers Bureau; Novartis: Speakers Bureau; Roche: Other: Support of parent study and funding of editorial support. Hoffman:Protagonist: Consultancy; Forbius: Consultancy; Novartis: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Dompe: Research Funding. Silver:PharmaEssentia: Speakers Bureau. Mesa:Bristol Myers Squibb: Research Funding; Incyte: Research Funding; AbbVie: Research Funding; Samus Therapeutics: Research Funding; Genentech: Research Funding; CTI BioPharma: Research Funding; Promedior: Research Funding; Sierra Oncology: Consultancy; LaJolla Pharmaceutical Company: Consultancy; Novartis: Consultancy.

Introduction: Recent advances in the prognostic scheme and treatment of primary and secondary myelofibrosis (MF) have resulted in an overwhelming amount of clinical information to assimilate. The authors believe a comprehensive review that summarizes the most recent published literature, could serve as guidelines for the practicing hematologist. Areas covered: The authors provide a summary of landmark articles regarding epidemiology, symptoms, and pathogenesis of disease. The authors conducted a systematic literature review to answer questions regarding differences between primary myelofibrosis (PMF) and secondary myelofibrosis (SMF), appropriate use and selection of the current risk-stratification models, early versus late treatment of MF and current practices in allogeneic hematopoietic stem cell transplantation (allo-HCT) for MF. The authors conclude the article with their clinical opinion based on their experience and literature review. The purpose of this article is to identify current practices, address any variation, identify and investigate conflicting results and produce statements to guide decision-making. Expert opinion: In this section, the authors advocate for and provide examples of a standardized way of incorporating future discoveries in the pathogenesis and risk-stratification models of MF. They also discuss the importance of using only one risk-stratification model for PMF and one for SMF and their reasoning for early instead of late treatment of MF.

Background: Myeloproliferative neoplasm (MPN) has been associated with pulmonary hypertension (PH) on the basis of small observational studies, but the mechanism and clinical significance of PH in MPN are not well established. The aims of this study were to expand understanding of PH in a well-characterized MPN cohort via study of PH-related symptoms, mortality risk, and cardiac remodeling sequalae of PH using quantitative echocardiographic methods. Methods: The population comprised a retrospective cohort of patients withMPNwho underwent transthoracic echocardiography: Doppler-derived pulmonary arterial systolic pressure applied established cutoffs for PH (>= 35 mm Hg) and advanced PH (>= 50 mm Hg); right ventricular (RV) performance was assessed via conventional indices (tricuspid annular plane systolic excursion [TAPSE], S') and global longitudinal strain. Symptoms and mortality were discerned via standardized review. Results: Three hundred one patients were studied; 56% had echocardiography-demonstrated PH (20% advanced) paralleling a high prevalence (67%) among patients with invasively quantified PASP. PH was associated with adverse left ventricular (LV) remodeling indices, including increased myocardial mass and diastolic dysfunction (P <=.001 for all): LV mass and filling pressure (P<.01) were associated with PH independent of LV ejection fraction. RV dysfunction by strain and TAPSE and S' increased in relation to PH (P <=.001) and was about threefold greater among patients with advanced PH compared with those without PH. Patients with RV dysfunction were more likely to report dyspnea, as were those with advanced PH (P<.05). During median follow-up of 2.2 years, all-cause mortality was 27%. PH grade (hazard ratio, 1.9; 95% CI, 1.1-3.0; P=.012) and TAPSE- and S'-demonstrated RV dysfunction (hazard ratio, 3.3; 95% CI, 1.3-8.2; P=.01) were independently associated with mortality; substitution of global longitudinal strain for TAPSE and S' yielded similar associations of RV dysfunction with death (hazard ratio, 3.2; 95% CI, 1.5-6.7; P=.003) independent of PH. Conclusions: PH is highly prevalent in patients with MPN and is linked to LV diastolic dysfunction; echocardiography-quantified RV dysfunction augments risk for mortality independent of PH.

Background: Information on the molecular and phenotypic correlates of acute myeloid leukemia secondary (S-AML) to post-polycythemia vera (post-PV) and post-essential thrombocythemia (post-ET) myelofibrosis (known as secondary myelofibrosis, SMF) is scant. Aims: The primary objective of this study is to assess the incidence and outcome of S-AML in a large cohort of SMF. To find out predictors of S-AML occurrence, we performed a whole investigation of risk factors at the time of SMF, also applying established prognostic models. Methods: The MYSEC (Myelofibrosis Secondary to PV and ET Collaboration) project was recently updated collecting 805 SMF cases. We performed time-to-event (S-AML) analysis with Fine & Gray competing risks model using either death or the time of stem cell transplantation as competing risks. To explore whether IPSS (International Prognostic Scoring System), DIPSS (Dynamic IPSS) and MYSEC-PM (MYSEC-Prognostic Model) predicted S-AML occurrence, we calculated the difference in Cumulative Incidence Function (CIF) among risk categories for each score and applied Gray's model for testing the homogeneity of CIFs. Results: Within 805 SMF patients, 71 (8.8%) developed S-AML. Median time from SMF to S-AML evolution was 1.6 years (range: 0.6 - 14.6). The S-AML incidence rate was 2.3 x100 person-year of follow up (CI 95%: 1.8 - 2.9 x100). In univariate analysis, we found a direct association between increased S-AML risk and platelets <150 x10^9/l, hemoglobin <10 g/dl, PV/ET duration > 14 years, blasts ≥ 3% and non-CALR genotype, and an inverse association with the use of JAK2 inhibitors. In a multivariate analysis, only blasts ≥ 3%, PV/ET duration > 14 years and non-CALR genotype maintained their association with an increased risk of S-AML (Table 1). No impact on S-AML risk was found as for leukocyte count, spleen size, symptoms, karyotype and age at SMF; neither imbalance was evident for gender, previous PV versus ET, and cytoreduction.Among IPSS, DIPSS and MYSEC-PM, only the latter score stratified SMF patients for S-AML risk (P = 0.004) (Figure 1a). The estimated hazard ratios (HRs) when changing score category were: 1.55 (CI 95%: 0.69 - 3.50) from low to intermediate-1, 1.09 (CI 95%: 0.51 - 2.34) from intermediate-1 to intermediate-2, and 2.42 (CI 95%: 1.01 - 5.77) from intermediate-2 to high risk. Comparing to lower risk groups, HR for high risk cases was 4.07 (CI 95%: 1.66 - 10.03). Among S-AML, 65 (91.5%) patients died and the median survival was 3.9 months (CI 95%: 2.4 - 6.8) (Figure 1b).Summary/Conclusion: In our dataset of 805 SMF patients, ∼ 9% evolved into S-AML. The S-AML incidence rate was 2.3 x100 person-year and the median survival dismal. In a multivariate analysis, blasts ≥ 3%, non-CALR genotype and longer duration of PV/ET correlated with an increased risk of S-AML. Besides, our data supports the relevance of MYSEC-PM in predicting S-AML evolution, especially for the opposite categories of low and high risks.

BACKGROUND Although recombinant interferon‐α (rIFNα) effectively treats patients with early myelofibrosis, the effect of driver and high molecular risk (HMR) mutations has not been considered. In this phase 2 study, for the first time, the authors correlate response to rIFNα treatment with driver and HMR mutations. METHODS Patients were diagnosed using World Health Organization or International Working Group for Myeloproliferative Neoplasms Research and Treatment criteria. Only patients who had low or intermediate‐1 Dynamic International Prognostic Scoring System scores with ≥15% hematopoietic bone marrow foci were included. History, symptom assessment, physical examination, and blood and bone marrow studies were performed. Genomic DNA was extracted from frozen cells, and next‐generation targeted sequencing of 45 genes was performed. Either rIFNα‐2b (0.5 million units subcutaneously 3 times weekly) or pegylated rIFNα‐2a (45 μg weekly) with escalation was initiated. All patients were followed at the authors' institution, and regular bone marrow biopsies were encouraged. International Working Group for Myeloproliferative Neoplasms Research and Treatment and European LeukemiaNet treatment response criteria were used. RESULTS Of 30 patients (16 women and 14 men; median age, 58 years), 22 were classified as low risk, and 8 were classified as intermediate‐1 risk. Two patients achieved complete remission, 9 achieved partial remission, 4 had clinical improvement, 7 had stable disease; 3 had progressive disease, 1 relapsed, and 4 died. There were 22 patients with JAK mutations, 6 with CALR mutations, and 2 with MPL mutations. Seventy‐three percent of patients improved or remained stable with acceptable toxicity, including 37% who achieved complete or partial remission. There was no correlation between treatment response and baseline driver mutations or Dynamic International Prognostic Scoring System scores. Of 8 poor responders, 3 had ASXL1 or SRSF2 mutations. CONCLUSIONS Early treatment with rIFNα in patients without HMR mutations may prevent the development of marked splenomegaly, anemia, and florid myelofibrosis. Molecular profiling at the time of diagnosis may predict prognosis and treatment response. Cancer 2017;123:2680‐87 . © 2017 American Cancer Society .

BACKGROUND:Thirty consecutive patients underwent hematopoietic stem cell transplantation for myelofibrosis (MF) at our institution. The median age at the time of transplant was 49 (range, 18-68) years, 74% of patients had advanced Dynamic International Prognostic Scoring System (DIPSS) scores, and 83% received reduced-intensity conditioning. PATIENTS AND METHODS:With a long follow-up of our patients, we analyzed disease and transplant variables that contributed to engraftment and outcomes. RESULTS:Neutrophil engraftment was achieved in 27 patients (90%) at a median time of 15 (range, 10-44) days, and 19 patients (63%) achieved platelet recovery at a median time of 18 (range, 8-100) days. Splenomegaly was associated with poor neutrophil engraftment (subdistributional hazard ratio [SHR], 0.42; 95% confidence interval [CI], 0.21-0.83; P = .01) and platelet engraftment (SHR, 0.18; 95% CI, 0.07-0.48; P < .001). Increased levels of lactate dehydrogenase (LDH) was associated with poor platelet engraftment (SHR, 0.39; 95% CI, 0.16-0.94; P = .04). The median follow-up for surviving patients was 49 (range, 3-155) months. The 1-year cumulative incidence of nonrelapse mortality (NRM) and relapse were respectively, 57% (95% CI, 29%-76%) and 25% (95% CI, 7%-48%). Increased levels of LDH was associated with high NRM (SHR, 2.82; 95% CI, 1.08-7.35; P = .03). The 4-year overall survival (OS) and relapse-free survival (RFS) were 44% (95% CI, 29%-67%) and 37% (95% CI, 23%-61%), respectively. In the multivariable model, splenomegaly and Eastern Cooperative Oncology Group (ECOG) performance status (PS) > 1 were associated with worse OS (hazard ratio [HR], 5.40; 95% CI, 1.19-24.56); P = .03) and RFS (HR, 3.78; 95% CI, 1.01-14.06; P < .05), respectively. ECOG PS > 1 was also associated with worse RFS (HR, 5.00; 95% CI, 1.31-19.14; P = .02). In this patient group with advanced disease, DIPPS score, Lille score, Janus-Associated Kinase V617F (JAK2 V617F) mutation status, and donor type did not predict transplant outcome. CONCLUSION:We confirm curative potential, but high NRM of allogeneic transplant for advanced MF.

Polycythemia vera (PV) is a clonal myeloproliferative neoplasm (MPN) almost always associated with mutations in JAK2 , most commonly the gain-of-function missense variant V617F. Results of recombinant interferon α-2b (rIFNα-2b) or pegylated rIFNα-2α (peg-rIFNα-2a) therapy in patients with PV

In this issue of Blood, Tefferi et al make the important observations that polycythemia vera (PV) is not a continuum from essential thrombocythemia (ET), that survival in ET is less than matched controls but of longer duration than in patients with PV and primary myelofibrosis (PMF), and that “triple negative” mutational status in PMF is an important adverse risk factor for blast transformation. Genetic profiling should be integrated into classical methods for profiling these diseases

Patients with IBD frequently have hematologic abnormalities suggestive of JAK2 mutated MPNs, but are traditionally classified as reactive processes. Haplotype 46/1 is a well-characterized genetic predisposition, common to both inflammatory bowel disease (IBD) and myeloproliferative neoplasms (MPN). In view of this shared genetic predisposition, we measured the frequency of the JAK2V617F mutation in IBD patients with thrombocytosis or erythrocytosis, in order to ascertain whether a higher than expected proportion of these patients may in fact have underlying MPNs. 1121 patients were identified with an active diagnosis of Crohn’s disease or ulcerative colitis, of which 474 had either thrombocytosis or erythrocytosis. Patients with abnormal counts were tested for the JAK2V617F mutation during routine follow-up visits. Interim analysis of first 23 patients tested was performed to assess whether the JAK2V617F positivity rate was statistically significant compared with known expected frequencies in a comparable control population. Of 23 patients, 13 patients had thrombocytosis and 10 had erythrocytosis. Three patients with thrombocytosis (23%), and 1 patient with erythrocytosis (10%), tested positive for JAK2V617F, exceeding the expected thresholds for statistical significance. In patients with IBD and thrombocytosis or erythrocytosis, a meaningful proportion may harbor an undiagnosed MPN, as indicated by clonal abnormalities such as JAK2V617F. These findings imply the need for increased testing of these patients for clonal hematologic abnormalities, and importantly, if found, suggest the need for therapeutic strategies with drugs, such as JAK2 inhibitors, in patients with both MPN and IBD.