Case Report: Polycythemia vera presenting as persistent unexplained low-grade fever

Persistent fever as an initial manifestation of polycythemia vera (PV) is uncommon and may lead to diagnostic delay. We report a 72-year-old woman with recurrent low-grade fever for 1 month, with a maximum temperature not exceeding 38.0 °C. Although this presentation did not meet the classic fever-of-unknown-origin temperature threshold, it prompted an FUO-like diagnostic evaluation because routine infectious and autoimmune investigations were unrevealing and empirical antimicrobial therapy was ineffective. Laboratory evaluation revealed elevated hemoglobin and hematocrit levels, accompanied by leukocytosis and initially high-normal platelet counts that later progressed to thrombocytosis. Chest CT/PET-CT showed diffuse ground-glass/interstitial pulmonary changes; active bacterial pneumonia was considered unlikely based on the absence of respiratory symptoms, procalcitonin at the upper limit of the normal range, negative cultures and sputum smears, and absence of FDG-avid pulmonary consolidation, although a concomitant viral or noninfectious inflammatory pulmonary process could not be completely excluded. Bone marrow biopsy showed hypercellular marrow with panmyelosis and megakaryocytic proliferation, and molecular testing identified a JAK2 V617F mutation with a high allele burden. The diagnosis of PV was established based on the 5th edition of the World Health Organization (WHO) diagnostic criteria. The patient was treated with hydroxyurea and aspirin, resulting in gradual resolution of fever and improvement in hematological parameters. This case highlights that PV should be considered in the differential diagnosis of persistent unexplained low-grade fever when erythrocytosis and other myeloproliferative features are present, while alternative pulmonary causes should be assessed carefully.