Secondary polycythemia in acutely ill COVID-19 patients is associated with higher mortality but not markedly higher thrombotic risk.

Secondary polycythemia is commonly observed among patients with chronic pulmonary diseases. However, its significance in the context of Coronavirus disease 2019 (COVID-19) is unknown. We retrospectively evaluated a total of 5872 hospitalized COVID-19 patients with mostly severe and critical symptoms, and without prior or subsequently diagnosed myeloproliferative neoplasm. Patients were stratified based on admission hemoglobin into four subgroups: anemia (hemoglobin <120 g/L for females and 130 g/L for males), normal hemoglobin, mild (hemoglobin 160-165 g/L for females and 165-185 g/L for males) and severe polycythemia (hemoglobin >165 g/L for females and >185 g/L for males). Among 5872 patients, a total of 158 (2.7%) had mild and 25 (0.4%) severe polycythemia. Polycythemia was significantly associated with higher respiratory and functional impairment, reduced plasma volume, higher serum osmolarity and comorbidity burden specific to the degree of polycythemia. Patients presenting with mild (odds ratio (OR) = 1.63, p = .003) and severe polycythemia (OR = 4.98, p < .001) had increased risk of death in comparison to patients with normal hemoglobin, whereas no significant associations with venous thromboembolism, arterial thrombosis nor major bleeding were observed. Anemia was associated with higher risk of death (OR = 1.42, p < .001), venous thromboembolism (OR = 1.34, p < .006) and major bleeding (OR = 2.27, p < .001) in comparison to normal hemoglobin. Associations of polycythemia and anemia with mortality diminished, and anemia with venous thromboembolism and major bleeding persisted, after multivariate adjustments for age, sex, comorbidities, COVID-19 severity and functional status. Secondary polycythemia in hospitalized COVID-19 patients without prior of subsequently diagnosed myeloproliferative neoplasm is rare and is associated with high mortality, increasing with degree of polycythemia, but not markedly higher thrombotic risk.