Pre-fibrotic myelofibrosis (pre-PMF) and essential thrombocythemia (ET) are characterized by similarly increased rate of thrombotic events, but no study specifically analyzed risk factors for thrombosis in pre-PMF. In a multicenter cohort of 382 pre-PMF patients collected in this study, the rate of arterial and venous thrombosis after diagnosis was 1.0 and 0.95% patients/year. Factors significantly associated with arterial thrombosis were age, leukocytosis, generic cardiovascular risk factors, JAK2V617F and high molecular risk mutations, while only history of previous thrombosis, particularly prior venous thrombosis, was predictive of venous events. The risk of total thromboses was accurately predicted by the the international prognostic score for thrombosis in essential thrombocythemia (IPSET) score, originally developed for ET, and corresponded to 0.67, 2.05, and 2.95% patients/year in the low-, intermediate-, and high-risk categories. IPSET was superior to both the conventional 2-tiered score and the revised IPSET in this cohort of pre-PMF patients. We conclude that IPSET score can be conveniently used for thrombosis risk stratification in patients with pre-PMF and might represent the basis for individualized management aimed at reducing the increased risk of major cardiovascular events. Further refinement of the IPSET score in pre-PMF might be pursued by additional, prospective studies evaluating the inclusion of leukocytosis and/or adverse mutational profile as novel variables.

We applied a parametric Markov five-state model, on a well-characterized international cohort of 1,545 patients with polycythemia vera (PV; median age 61 years; females 51%), in order to examine the impact of incident thrombosis on the trajectory of death or disease progression. At a median follow-up of 6.9 years, 347 (23%) deaths, 50 (3%) blast phase (BP), and 138 (9%) fibrotic (post-PV MF) transformations were recorded. Incident thrombosis occurred at a rate of 2.62% pt/yr (arterial 1.59% and venous 1.05%). The probability of death, in the first 10 years, for 280 (18%) patients who developed thrombosis during follow-up was 40%, which was two-fold higher than that seen in the absence of thrombosis or any other transition state (20%; p < 0.01); the adverse impact from thrombosis was more apparent for arterial (HR 1.74; p < 0.01) vs venous thrombosis (p=NS) and was independent of other fixed (i.e., age, prior venous thrombosis, leukocytosis) or time-dependent (i.e., progression to BP or MF) risk variables. The transition probability to post-PV MF increased over time, in a linear fashion, with a rate of 5% capped at 5 and 10 years, in patients with or without incident thrombosis, respectively. The impact of thrombosis on transition probability to death or post-PV MF tapered off beyond 10 years and appeared to reverse direction of impact on MF evolution at the 12-year time point. These observations suggest thrombosis in PV to be a marker of aggressive disease biology or a disease-associated inflammatory state that is consequential to both thrombosis and disease progression.