fever of unknown origin

• related: ID

• Definition: fever of 38.3 °C (100.9 °F) or greater for 3 or more weeks that remains undiagnosed after two visits in the ambulatory setting.

• Important history: procedures, surgeries, presence of foreign bodies or implants, immunosuppression, travel, animal and other exposures (including hobbies), dietary habits, and medications (including over-the-counter medications).
• Initial labs: CBC, CMP, LDH, UA, urine culture, ESR, CRP, ANA, RF, HIV, CMV, blood cultures (three sets, each set obtained at least several hours apart), TB, and chest radiography (or chest CT). Q-fever serology should be considered if risk factors exist, and mycobacterial blood cultures should be obtained in HIV-positive patients with CD4 cell counts of 50/µL or less.
• If no cause: abdominal or pelvic CT may be considered to evaluate for intra-abdominal abscess or lymphoproliferative disorders.
• Liver, lymph node, and temporal artery biopsies have a diagnostic yield of about 35%, particularly when performed when infection is unlikely. Posterior cervical, supraclavicular, infraclavicular, epitrochlear, hilar, mediastinal, and mesenteric lymph node biopsies are more likely to provide a diagnosis than that of other lymph nodes. Bone marrow biopsy can be helpful when leukopenia or thrombocytopenia is present.
• FUO lasting more than 1 year is unlikely to be caused by infection or malignancy. Undiagnosed FUO is generally associated with a benign long-term course, particularly when fever is not associated with weight loss or other signs of underlying serious disease.