Fever of unknown origin caused by adult juvenile rheumatoid arthritis: the diagnostic significance of double quotidian fevers and elevated serum ferritin levels.

Fever of unknown origin (FUO) in adults is a commonly encountered clinical problem. Treatable causes of FUO in the adult should be the primary focus of the diagnostic workup. Neoplasms have replaced infectious diseases as being the most common cause of FUO in adults, and collagen vascular diseases are now relatively rare. The most important collagen vascular diseases presenting as an FUO include Takayasu's arteritis, Kikuchi's disease, polymyalgia rheumatica, and adult juvenile rheumatoid arthritis (JRA) (adult Still's disease). There are no specific diagnostic tests for these disorders, which commonly present as prolonged fevers that are not easily diagnosed (i.e., FUO). Adult JRA is a rare but important cause of FUO in adults. Typically, patients with adult Still's disease present with liver/spleen involvement, posi-articular arthritis, ocular involvement, and evanescent salmon-colored truncal rash. An important diagnostic finding in adult JRA is the presence of a double quotidian fever, which occurs in few other disorders. Only visceral leishmaniasis and adult JRA are causes of FUO in adults associated with double quotidian fevers. Highly elevated serum ferritin levels are the most important nonspecific diagnostic finding associated with adult JRA. We present a case of FUO caused by adult JRA presenting with diffuse polyarticular migrating arthritis, evanescent rash, and splenomegaly. The diagnosis of adult JRA was suggested by these findings in association with a double quotidian fever and a highly elevated serum ferritin level. Clinicians should appreciate the diagnostic significance of fever patterns and the diagnostic significance of elevated serum ferritin levels in patients with FUO.