Fever of unknown origin: a diagnostic approach to this vexing problem.

Fever is a common complaint leading families to seek medical attention. Its routine management is the bread and butter of pediatric practice. When fever is seen as prolonged beyond the expected time course (eg, 10 days for a presumed viral respiratory tract infection or 3 weeks for mononucleosis), concern for fever of unknown origin (FUO) may ensue. This diagnosis is among the most challenging for health care providers to approach and often involves referral to subspecialists. Generally, the pace of the evaluation should be guided by the severity of the disease, rather than the anxiety of the family or of the health care providers. It is useful to recognize that uncommon manifestations of common diseases are more likely than are rare diseases. Furthermore, clues to the diagnosis are frequently present in the history and physical examination but are not elicited or unappreciated (perhaps due to time constraints). Therefore, thoroughness and repetition are vitally important. Although the differential diagnosis of FUO is vast, a thoughtful, focused approach based on information gleaned from a thorough history and physical examination (together with any laboratory or other study results) is preferable to a "shotgun" or "running the list" one. Finally, FUO in special populations, including children in the hospital, those with HIV infection or other immunocompromise, and those in the developing world, require special consideration. Most children do well, compared to adults with FUO, but true FUO is not always a benign condition, necessitating the best care a health care provider can offer.