Fever of unknown origin due to preleukemia/myelodysplastic syndrome: the diagnostic importance of monocytosis with elevated serum ferritin levels.

Fever of unknown origin (FUO) is a common clinical diagnostic dilemma. In the elderly, causes of FUO most commonly include malignancy or infection, and less commonly include collagen vascular diseases. Among the collagen vascular diseases causing FUO in the elderly, polymyalgia rheumatica/temporal arteritis, and adult Still's disease (adult juvenile rheumatoid arthritis) are difficult diagnoses to prove. Among the infectious causes of FUO in the elderly are subacute bacterial endocarditis, intra-abdominal abscesses, and extrapulmonary tuberculosis. In the elderly, neoplastic causes of FUO include lymphomas, hepatomas, renal cell carcinomas, and hepatic or central nervous system metastases. Acute leukemias, particularly during "blast" transformation, may present as acute fevers in the absence of infection, but are rare causes of FUO. Preleukemia/myelodysplastic syndromes are exceedingly rare causes of FUO. We present a case of an elderly man who presented with findings that initially suggested adult Still's disease. Prolonged and profound monocytosis provided the key clue to his subsequent diagnosis of preleukemia/myelodysplastic syndrome. In this patient, a positive Naprosyn test result also suggested a neoplastic cause for his FUO. After months of prolonged fevers, myelocytes/metamyelocytes were eventually demonstrated in his peripheral smear during hospital evaluation. These findings, in concert with the persistent monocytosis, highly elevated ferritin levels, polyclonal gammopathy on serum protein electrophoresis, and eventual presence of myelocytes/metamyelocytes on peripheral smear, prompted a bone marrow test that demonstrated blast cells confirming the diagnosis of preleukemia myelodysplastic syndrome as the cause of this patient's FUO.