Incidence and risk factors for bleeding in 1104 patients with essential thrombocythemia or prefibrotic myelofibrosis diagnosed according to the 2008 WHO criteria.

In an international study of 1104 patients with essential thrombocythemia (ET), a histological review according to the 2008 World Health Organization (WHO) criteria confirmed ET in 891 patients (WHO-ET, 81%), and revised the diagnosis to prefibrotic primary myelofibrosis (PMF) in 180 patients (PMF, 16%). Major bleeding during follow-up occurred in 55 (6%) WHO-ET and 21 (12%) PMF patients (P=0.009), at a rate of 0.79 and 1.39% patients per year, respectively, (P=0.039). In a multivariable analysis, predictors of bleeding included diagnosis of PMF (P=0.05; hazard ratio (HR) 1.74), leukocytosis (P=0.04; HR 1.74), previous hemorrhage (P=0.025; HR 2.35) and aspirin therapy (P=0.001; HR 3.16). The analysis restricted to patients with WHO-ET confirmed previous hemorrhage (P=0.043; HR 1.92) and aspirin (P=0.027; HR 2.24) as independent risk factors. The current study reveals that major bleeding associated with thrombocytosis might be relatively specific to PMF, as opposed to WHO-defined ET. Furthermore, it shows that low-dose aspirin exacerbates these hemorrhagic events of PMF. In contrast, thrombocytosis per se was not a risk factor for bleeding; however, low-dose aspirin had a synergistic hemorrhagic effect unmasking the bleeding tendency of patients with extreme thrombocytosis. These observations carry significant therapeutic implications in these two WHO entities.