Essential Thrombocythemia: Analysis of Risk Factors for Thrombotic Events in a Series of 306 Patients.

Essential thrombocythemia (ET) is a chronic myeloproliferative disorder characterized by peripheral thrombocytosis and abnormal proliferation of megakariocytes in the bone marrow. Even thought thrombosis is frequently associated to ET, the risk factors of this clinical complication are still controversial. The aim of this retrospective, single institution study was to investigate clinical and laboratory characteristics associated with the occurrence of thrombotic events, with the purpose of identifying subgroups of patients who could benefit from antiaggregant and/or cytostatic treatment. 306 consecutive ET patients (109 men and 197 females, median age 58 yr) diagnosed between January 1979 and December 2002 were included in the study. At the time of analysis, 196 patients were still alive with a median follow up of 96 months. The following variables were investigated for the association with thrombotic complications: age, platelet count, previous history of thrombotic events, time from diagnosis, treatment with antiaggregant/cytostatic drugs, and cardiovascular risk factors such as arterial hypertension, obesity, hypercolesterolemia, diabetes, cigarette smoking. At the time of last follow up, 46 patients (15%) experienced at least one thrombotic event. The occurrence of thrombotic events was observed in 26/64 (40.6%) patients with previous history of thrombosis and in 20/242 (8.3%) patients with no previous history of thrombosis (p<0.0001 Fisher’s exact test, odd ratio 7.6). A significant difference between the two groups of patients was also confirmed when Kaplan Meier estimates of thrombosis-free survival were compared by log-rank test (p<0.0001). By logistic regression, platelet number at diagnosis did not associate with occurrence of thrombosis in the whole patient population. When patients without previous history of thrombosis were stratified according to the number of cardiovascular risk factors (none vs one vs more than one), a significant correlation with occurrence of thrombotic events was observed (Mantel-Haenszel Chi-square 5.47, p<0.05). This study confirms that history of thrombosis is strongly related with risk of further thrombotic events in patients with ET, whereas platelet number at diagnosis does not seem to represent a prognostic factor. In patients with no previous history of thrombosis, the presence of other cardiovascular risk factors has to be taken into account when establishing the therapeutic approach.