Hematocrit, White Blood Cells, and Thrombotic Events in the Veteran Population With Polycythemia Vera.

Background: Patients with polycythemia vera (PV), a chronic myeloproliferative neoplasm, have a greater morbidity and mortality risk than the general population, largely due to a high incidence of thrombotic events. Observations: Two recently published retrospective analyses from Parasuraman and colleagues used Veterans Health Administration (VHA) data to replicate, in a real-world population, findings from the prospective, randomized Cytoreductive Therapy in Polycythemia Vera (CYTO-PV) study. In the CYTO-PV study, hematocrit (Hct) level and white blood cell (WBC) count were shown to be independently associated with thrombotic event risk in patients with PV. In the VHA analysis, patients with Hct levels < 45% were found to have a significantly lower rate of thrombotic events compared to those with levels ≥ 45% (hazard ratio [HR], 1.61; P = .04). For WBC counts and thrombosis, patients with WBC ≥ 8.5 × 109/L were found to have a higher rate of thrombotic events compared to the reference cohort of WBC < 7 × 109/L (HR, 1.47; P < .01), and the rates were higher for those with WBC ≥ 11 × 109/L (HR 1.87; P < .001). Conclusions: The results from these analyses suggest the need for managing Hct appropriately to maintain levels < 45% and offer further support for the consideration of WBC counts in determining risk of thrombotic events. Studies are needed to clearly establish an optimal WBC count to inform updates to treatment guidelines.