Fever of unknown origin as the primary manifestation of extrapulmonary sarcoidosis with bone marrow involvement: a rare clinical scenario.

Isolated bone marrow involvement in sarcoidosis is a rare condition, mostly manifested by cytopenia, hypercalcemia and/or hypercalciuria. We present the case of a 48-year-old man with a prolonged fever meeting the criteria for fever of unknown origin, in whom a suspicion of sarcoidosis only emerged after non-necrotizing granulomas were identified in the bone marrow, one year after the onset of the disease. The patient exhibited only mild anemia, moderate leukocytosis, thrombocytosis, and increased erythrocyte sedimentation rate, C-reactive protein levels. Other potential causes of fever of unknown origin and bone marrow non-necrotizing granulomas were ruled out. Although serum angiotensin-converting enzyme levels were elevated, they did not reach diagnostic thresholds. The diagnosis of sarcoidosis was confirmed by newly detected intra-abdominal lymphadenopathy on computed tomography and whole-body 18F-fluorodeoxyglucose positron emission tomography with computed tomography, along with the presence of multiple sarcoid-type non-necrotizing granulomas in the intra-abdominal lymph nodes. Treatment with prednisolone normalized the patient's body temperature, improved laboratory-inflammatory profile, and enhanced the patient's quality of life. In cases where fever of unknown origin is present and sarcoidosis is suspected, a bone marrow biopsy may be considered, even in the absence of cytopenia, hypercalcemia, or hypercalciuria.