The role of antiplatelet therapy as primary prophylaxis of thrombosis in low-risk essential thrombocythemia has not been studied in randomized clinical trials. We assessed the benefit/risk of low-dose aspirin in 433 patients with low-risk essential thrombocythemia (271 with a CALR mutation, 162 with a JAK2(V617F) mutation) who were on antiplatelet therapy or observation only. After a follow up of 2215 person-years free from cytoreduction, 25 thrombotic and 17 bleeding episodes were recorded. In CALR-mutated patients, antiplatelet therapy did not affect the risk of thrombosis but was associated with a higher incidence of bleeding (12.9 versus 1.8 episodes per 1000 patient-years, P=0.03). In JAK2(V617F)-mutated patients, low-dose aspirin was associated with a reduced incidence of venous thrombosis with no effect on the risk of bleeding. Coexistence of JAK2(V617F)-mutation and cardiovascular risk factors increased the risk of thrombosis, even after adjusting for treatment with low-dose aspirin (incidence rate ratio: 9.8; 95% confidence interval: 2.3-42.3; P=0.02). Time free from cytoreduction was significantly shorter in CALR-mutated patients with essential thrombocythemia than in JAK2(V617F)-mutated ones (median time 5 years and 9.8 years, respectively; P=0.0002) and cytoreduction was usually necessary to control extreme thrombocytosis. In conclusion, in patients with low-risk, CALR-mutated essential thrombocythemia, low-dose aspirin does not reduce the risk of thrombosis and may increase the risk of bleeding.

Center for Research and Innovation of MPN ; AOU Careggi and Department of Experimental and Clinical Medicine University of Florence Italy

Centre for Cancer Research and Cell Biology Queen's University Belfast UK

Department of Clinical Science and Education Karolinska Institute South Hospital Stockholm Sweden

Department of Hematology VU University Medical Center Amsterdam The Netherlands

Division of Hematology University Hospital Basel Switzerland

Haematology Department Guys' and St Thomas' NHS Foundation Trust London UK

Hematology and Medical Oncology Department Hospital Morales Messeguer IMIB Arrixaca UCAM Murcia Spain

Hematology Department Hospital Clínic IDIBAPS Barcelona Spain

Hematology Department Hospital Clínico Valencia Spain

Hematology Department Hospital del Mar IMIM UAB Barcelona Spain

Hematology Department Hospital Dr Negrín Las Palmas de Gran Canaria Spain

Hematology Department Hospital La Paz Madrid Spain

Hematology Department Hospital Príncipe de Asturias Alcalá de Henares Spain

Hematology Department Hospital Ramón y Cajal Madrid Spain

Hematology Department Hospital Universitario 12 de Octubre Madrid Spain

Hematology Department Hospital Vall d'Hebron Barcelona Spain

Hematology Oncology and Palliative Care Johannes Wesling Academic Medical Center University of Hannover Teaching Hospital Germany

Hematology Section Uddevalla Hospital NU Hospital Group Sweden

Hematotherapy and Hemostasis Department Hospital Clínic Barcelona Spain

Institute of Hematology and Blood Transfusion Prague Czech Republic

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