Background: Patients with polycythemia vera (PV) have a substantial risk of thrombotic events (TEs). The objective of the present analysis was to describe the association between white blood cell (WBC) levels and occurrence of TEs among patients with PV from a large real-world population.

Patients and methods: The present retrospective analysis using Veterans Health Administration claims data (October 1, 2005, to September 30, 2012) evaluated adult patients assigned to 4 WBC count categories (WBC count < 7.0, 7.0-8.4, 8.5 to < 11.0, and ≥ 11.0 × 10⁹/L) to compare the risk of TEs (reference, WBC count, < 7.0 × 10⁹/L group). Analysis was performed using a Cox proportional hazards model, considering WBC status as a time-dependent covariate.

Results: Of the 1565 patients with PV included in the present analysis, the WBC count was < 7.0 × 10⁹/L for 428 (27.3%), 7.0 to 8.4 × 10⁹/L for 375 (24.0%), 8.5 to < 11.0 × 10⁹/L for 284 (18.1%), and ≥ 11.0 × 10⁹/L for 478 (30.5%). Of the 1565 patients, 390 (24.9%) had experienced a TE during the study period. The mean follow-up ranged from 3.6 to 4.5 years. Compared with the reference group (WBC count < 7.0 ×10⁹/L), the hazard ratio for TEs was 1.10 (95% confidence interval [CI], 0.82-1.48; P = .5395), 1.47 (95% CI, 1.10-1.96; P = .0097), and 1.87 (95% CI, 1.44-2.43; P < .0001) for patients with a WBC count of 7.0 to 8.4, 8.5 to < 11.0, and ≥ 11.0 ×10⁹/L, respectively.

Conclusion: A positive, significant association between an increased WBC count of ≥ 8.5 ×10⁹/L and the occurrence of TEs was observed in patients with PV. The potential thrombogenic role of WBCs in patients with PV supports the continued inclusion of WBC count control in disease management and evaluation of the response to therapy.

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