Correlation between leukocytosis and thrombosis in Philadelphia-negative chronic myeloproliferative neoplasms

The evidence that leukocytes may contribute to the pathogenesis of thrombosis in Chronic Myeloproliferative Neoplasms is increasing but not definitive. To further enforces whether an increased leukocyte count is associated with thrombosis and whether this effect can be modulated by cytoreductive therapy, we analyzed the clinical course of 187 patients with Polycythemia Vera (PV) and Essential Thrombocythemia (ET) followed at two Italian Institutions over a period of 7 years. The association was measured at diagnosis or before thrombotic events: a multivariable analysis was carried out using data at baseline and time-dependent covariates. We found that white blood cells (WBC) count above 9.5 × 10⁹/L at diagnosis (baseline analysis) was associated with thrombosis during the follow-up (Hazard Ratio [HR] of 1.8, p 0.03). At the time-dependent analysis, therapy with hydroxyurea (HU), lowering by 35% the baseline WBC level, reduced such strength of association giving a HR of 1.3 (p value non significant). We found a trend between WBC level and thrombosis in untreated low-risk patients (RR of 1.9, 95% CI 0.9 to 3.1); in high-risk patients treated with HU this correlation was clearly lost (RR 1.1, 95% CI 0.2 to 2.7). Finally, we could not identify the presence of JAK2 ^V617F as a risk factor for thrombosis. Properly designed prospective studies should corroborate such results.