rythrocytosis is defined as a concentration of red blood cells higher than age-and sex-specific reference ranges, most commonly measured as an increase in the hemoglobin level or hematocrit. Since the identification of the JAK2 V617F mutation in patients with polycythemia vera in 2005, 1 molecular testing of the JAK2 gene has become part of the standard diagnostic workup for patients presenting with erythrocytosis. The increasing availability of molecular diagnostics and a decrease in hemoglobin thresholds (> 160 g/L for women, > 165 g/L for men) required to diagnose polycythemia vera in the revised 2016 World Health Organization (WHO) classification 2 are 2 potential factors driving increased use of molecular testing in patients with erythrocytosis. 3 Secondary causes of erythrocytosis -such as smoking, hypoxic lung diseases and medications -are more common than polycythemia vera, and current literature supports first excluding these causes by taking a thorough medical history and by focused investigations, including serum erythropoietin levels, before performing molecular testing. 4 Despite these recommendations, in practice, this work-up is often performed concurrently with molecular testing, which can carry substantial health care costs.The primary objective of this study was to characterize practice patterns in the investigation of erythrocytosis at our centre, focusing on the use of molecular testing for JAK2