Pyrexia of unknown origin.

Classical pyrexia of unknown origin (PUO), defined as fever of >38 °C on several occasions for greater than three weeks despite investigation in hospital (>3 days) or out of hospital (>2 visits), is an uncommon but challenging problem. The incidence and aetiology vary according to the geographic region, the age structure of the population, and the immune status of the patient; alternative definitions of PUO exist for immune compromised individuals. Preliminary investigations should be determined by detailed history and repeated examination. Biopsy of abnormal tissues should be performed early. If uncertainty persists, abdominal computerised tomography (CT), radiolabeled white cell scans, and the Duke endocarditis criteria carry the highest diagnostic yield. Blind bone marrow biopsy is probably only useful in immunocompromised patients.