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