Fever of unknown origin

Fever of unknown origin is a rare clinical syndrome, that represents a significant diagnostic challenge. There have been described more than 200 potential diseases, that can manifest as a fever of unknown origin. These are classically divided into following categories: infections, non-infectious inflammatory diseases, malignancies, and other miscellaneous disorders. Each of the disease type is associated with rather characteristic symptoms, clinical signs and laboratory findings, which are individually non-specific, but may provide helpful clues for a further focused diagnostic work-up. The clinicians task is to be able to identify these hallmark clinical features and to correctly interpret their significance and limitations in the appropriate differential diagnostic context. The aim of this review is to provide up-to-date clinical research evidence and to propose a  concise clue-oriented diagnostic approach.