Risk of bleeding in patients with essential thrombocythemia and extreme thrombocytosis.

Approximately 25% of patients with essential thrombocythemia (ET) present with extreme thrombocytosis (ExT), defined as having a platelet count ≥1000 × 109/L. ExT patients may have an increased bleeding risk associated with acquired von Willebrand syndrome. We retrospectively analyzed the risk of bleeding and thrombosis in ExT vs non-ExT patients with ET at Dana-Farber Cancer Institute and Massachusetts General Hospital from 2014 to 2022 to inform treatment decisions. We abstracted the first major bleed, clinically relevant nonmajor bleed (CRNMB), and thrombotic events from medical records. We identified 128 ExT patients (28%) and 323 non-ExT patients (72%). Cumulative incidence of bleeding was not different in ExT vs non-ExT patients (21% vs 13% [P = .28] for major bleed; 16% vs 15% [P = .50] for CRNMB). Very low and low thrombotic risk ExT patients were more likely to be cytoreduced than very low- and low-risk non-ExT patients (69% vs 50% [P = .060] for very low risk; 83% vs 53% [P = .0059] for low risk). However, we found no differences in bleeding between ExT and non-ExT patients when restricting the risk of bleed from diagnosis to cytoreduction start date (28% vs 19% [P = .29] for major bleed; 24% vs 22% [P = .75] for CRNMB). Cumulative incidence of thrombosis was also not different between ExT and non-ExT patients (28% vs 25%; P = .98). This suggests that cytoreduction may not be necessary to reduce bleeding risk based only on a platelet count of 1 million. We identified novel risk factors for bleeding in patients with ET including diabetes mellitus and the DNMT3A mutation.