Fever of unknown origin caused by late-onset rheumatoid arthritis.

Fever of unknown origin (FUO) is always a diagnostic challenge. The causes of FUO are legion and may be due to malignancy, infection, collagen vascular disease, and a variety of other unusual disorders. Currently, malignancies-followed by infectious etiologies-are the most common cause of FUO. We present an elderly female patient with an FUO who was thought to have subacute bacterial endocarditis because of an antecedent history of recent dental work. Subacute bacterial endocarditis was ruled out on the basis of negative cultures and negative transesophageal echocardiography. No evidence for an infectious disease or neoplastic etiology could be demonstrated in this patient. The diagnosis of FUO is most difficult when there is a paucity of clues from the history and physical examination, as was the case in this patient. Nonspecific laboratory tests included highly increased erythrocyte sedimentation rate (>or=100 mm/h), highly increased C-reactive protein, relative lymphocytopenia, and chronic thrombocytosis. These findings are compatible with a variety of infectious and inflammatory disorders. No evidence could be found for vasculitis. The only laboratory diagnostic findings present in her case were a highly increased rheumatoid factor titer and perinuclear antineutrophilic cytoplasmic antibody level. Polymyalgia rheumatica/temporal arteritis, systemic lupus erythematosus, and adult Still's disease were ruled out. The patient's FUO was best explained by the finding of late-onset rheumatoid arthritis (LORA), which is characterized by acute onset in elderly patients without the usual musculoskeletal manifestations of rheumatoid arthritis. Both the highly increased rheumatoid factor titer and perinuclear antineutrophilic cytoplasmic antibody level in the absence of an alternate explanation indicate that the FUO in this patient was caused by LORA.