High platelet count at diagnosis is a protective factor for thrombosis in patients with essential thrombocythemia.

To assess the role of platelet (PLT) count for thrombotic complications in Essential Thrombocythemia (ET), 1201 patients followed in 11 Hematological centers in the Latium region were retrospectively evaluated. At multivariate analysis, the following factors at diagnosis were predictive for a worse Thrombosis-free Survival (TFS): the occurrence of previous thrombotic events (p=0.0004), age>60years (p=0.0044), spleen enlargement (p=0.042) and a lower PLT count (p=0.03). Receiver Operating Characteristic (ROC) analyses based on thrombotic events during follow-up identified a baseline platelet count of 944×10/l as the best predictive threshold: thrombotic events were 40/384 (10.4%) in patients with PLT count >944×10/l and 109/817 (13.3%) in patients with PLT count <944×10/l, respectively (p=0.04). Patients with PLT count <944×10/l were older (median age 60.4years. vs 57.1years., p=0.016), had a lower median WBC count (8.8×10/l vs 10.6×10/l, p<0.0001), a higher median Hb level (14.1g/dl vs 13.6g/dl, p<0.0001) and a higher rate of JAK-2-V617F positivity (67.2% vs 41.6%, p<0.0001); no difference was observed as to thrombotic events before diagnosis, spleen enlargement and concomitant Cardiovascular Risk Factors. In conclusion, our results confirm the protective role for thrombosis of an high PLT count at diagnosis. The older age and the higher rate of JAK-2 V617F positivity in the group of patients with a baseline lower PLT count could in part be responsible of this counterintuitive finding.