PurposeThe WHO diagnostic criteria underscore the role of bone marrow (BM) morphology in distinguishing essential thrombocythemia (ET) from early/prefibrotic primary myelofibrosis (PMF). This study examined the clinical relevance of such a distinction.MethodsRepresentatives from seven international centers of excellence for myeloproliferative neoplasms convened to create a clinicopathologic database of patients previously diagnosed as having ET (N = 1,104). Study eligibility criteria included availability of treatment-naive BM specimens obtained within 1 year of diagnosis. All bone marrows subsequently underwent a central re-review.ResultsDiagnosis was confirmed as ET in 891 patients (81%) and was revised to early/prefibrotic PMF in 180 (16%); 33 patients were not evaluable. In early/prefibrotic PMF compared with ET, the 10-year survival rates (76% and 89%, respectively) and 15-year survival rates (59% and 80%, respectively), leukemic transformation rates at 10 years (5.8% and 0.7%, respectively) and 15 years (11.7% and 2.1%, respectively), and rates of progression to overt myelofibrosis at 10 years (12.3% and 0.8%, respectively) and 15 years (16.9% and 9.3%) were significantly worse. The respective death, leukemia, and overt myelofibrosis incidence rates per 100 patient-years for early/prefibrotic PMF compared with ET were 2.7% and 1.3% (relative risk [RR], 2.1; P < .001), 0.6% and 0.1% (RR, 5.2; P = .001), and 1% and 0.5% (RR, 2.0; P = .04). Multivariable analysis confirmed these findings and also identified age older than 60 years (hazard ratio [HR], 6.7), leukocyte count greater than 11 × 109/L (HR, 2.01), anemia (HR, 2.95), and thrombosis history (HR, 2.81) as additional risk factors for survival. Thrombosis and JAK2V617F incidence rates were similar between the two groups. Survival in ET was similar to the sex- and age-standardized European population.ConclusionThis study validates the clinical relevance of strict adherence to WHO criteria in the diagnosis of ET and provides important information on survival, disease complication rates, and prognostic factors in strictly WHO-defined ET and early/prefibrotic PMF.

Accurate prediction of thrombosis in essential thrombocythemia (ET) provides the platform for prospective studies exploring preventive measures. Current risk stratification for thrombosis in ET is 2-tiered and considers low-and high-risk categories based on the respective absence or presence of either age > 60 years or history of thrombosis. In an international study of 891 patients with World Health Organization (WHO)-defined ET, we identified additional independent risk factors including cardiovascular risk factors and JAK2 V617F. Accordingly, we assigned risk scores based on multivariable analysis-derived hazard ratios (HRs) to age > 60 years (HR ‫؍‬ 1.5; 1 point), thrombosis history (HR ‫؍‬ 1.9; 2 points), cardiovascular risk factors (HR ‫؍‬ 1.6; 1 point), and JAK2V617F (HR ‫؍‬ 2.0; 2 points) and subsequently devised a 3-tiered prognostic model (low-risk ‫؍‬ < 2 points; intermediaterisk ‫؍‬ 2 points; and high-risk ‫؍‬ > 2 points) using a training set of 535 patients and validated the results in the remaining cohort (n ‫؍‬ 356; internal validation set) and in an external validation set (n ‫؍‬ 329).