Leukocytosis at diagnosis and the risk of subsequent thrombosis in patients with low-risk essential thrombocythemia and polycythemia vera

BACKGROUND: Advanced age and a history of thrombosis were well-established risk factors for thrombosis in essential thrombocythemia (ET) and polycythemia vera (PV); cytoreductive therapy was indicated in their presence. Recent studies have suggested leukocytosis as an additional risk factor; however, such an association would be treatment-relevant in the context of low-risk disease. METHODS: In this retrospective study, a Cox proportional hazards model was used to determine the impact of various clinical and laboratory variables, including leukocytosis, on thrombosis-free survival (TFS). Arterial-specific or venous-specific TFS curves for different leukocyte count-defined risk groups were constructed by the Kaplan-Meier method and compared using the log-rank test. RESULTS: A total of 407 low-risk patients (254 with ET and 153 with PV) were considered. After a respective median follow-up of 104 months and 130 months, respectively, 47 (19%) patients with ET and 41 (27%) with PV experienced a total of 55 (41 arterial and 14 venous) and 46 (22 arterial and 24 venous) thrombotic events, respectively. Leukocytosis at the time of diagnosis, defined by a cutoff level of either 15 or 9.4 × 10⁹/L, did not appear to be predictive of either arterial or venous thrombosis during follow-up; similar results were obtained when analysis was restricted to patients with platelet counts of <1000 × 10⁹/L. Instead, advanced age was found to be significantly associated with arterial thrombosis in patients with PV and higher hemoglobin level with venous thrombosis in patients with ET. CONCLUSIONS: In the current retrospective study, leukocytosis at diagnosis did not appear to influence the risk of thrombosis in either ET or PV. However, a prospective study is required before leukocytosis is taken into account during treatment decisions in these disorders.