Work-up of Fever of Unknown Origin in Adult Patients

known origin (FUO) as persistent fever higher than 38.3°C (101°F) that lasts for 3 weeks with no established diagnosis despite 1 week of inpatient investigations. More recently, the definition of FUO has been updated to reflect developments in medicine and technology; FUO is now defined as persistent fever higher than 101°F for 3 weeks and failure to make a diagnosis with 3 outpatient visits or 3 days of inpatient investigations.2 There are 4 subclasses of FUO: classic FUO, HIV-associated FUO, nosocomial FUO, and FUO in neutropenic patients. Each subclass has a unique differential diagnosis, and therefore each requires a different approach to diagnosis. Given its large differential diagnosis (over 200 disorders; Table 1), FUO can present a diagnostic challenge to physicians. Although the work-up of FUO can be complex, a detailed history and through physical examination, along with judicious use of diagnostic procedures and good clinical judgment, will usually reveal the cause of the fever.