Extreme Thrombocytosis in Patients With Malignancy; Complications, Management and Outcomes

ABSTRACT Introduction Extreme thrombocytosis (EXT; platelet count >1000 × 10 9 /L) is a rare haematological abnormality that may be primary or reactive. Data on its incidence and management in patients with malignancy are limited. We retrospectively evaluated the incidence, management, and complications of EXT in patients with malignancy over a 12‐year period. Methods Two‐hundred and twenty patients with malignancy experienced at least one episode of EXT during the study period. Patients with a known or subsequent diagnosis of myeloproliferative neoplasm (MPN) were excluded from analysis. Clinical data were collected from electronic patient records. Results We identified 198 patients (57% female), with a median age of 55 years (range 1–86) and median follow‐up of 55 months. The median peak platelet count was 1112 × 10 9 /L (range 1001–2593). Most patients (77%) had solid tumours, lung cancer being most common (25%). MPN screening was performed in 11 patients (5.5%), with no positive results. Thrombocytosis resolved in 70% but persisted in 28%. Antiplatelets were prescribed in 16% and anticoagulation in 18%, almost exclusively for venous thrombosis. Thrombosis occurred in 42 patients (21%), predominantly venous (83%); 60% had concurrent thrombocytosis. This exceeds the reported 4%–6% thrombotic rate in patients with cancer. Among 44 paediatric patients, none received thromboprophylaxis, and four (9%) developed thrombosis. Overall mortality was 64.5%, predominantly from progressive malignancy. Conclusions EXT in patients with malignancy was associated with a higher‐than‐expected thrombotic risk. Persistent thrombocytosis was common, yet MPN screening was infrequent. These findings support further investigation of persistent EXT and evaluation of thromboprophylaxis in prospective studies. Trial Registration The authors have confirmed clinical trial registration is not needed for this submission.