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Definition of fever of unknown origin

Accordingly, FUO is now defined as follows:

  • Fever ≥38.3°C (≥101°F) on at least two occasions

  • Illness duration of ≥3 weeks

  • No known immunocompromised state

Diagnosis that remains uncertain after a thorough history-taking, physical examination, and the following obligatory investigations:

  • determination of erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) level;
  • platelet count;
  • leukocyte count and differential;
  • measurement of levels of hemoglobin, electrolytes, creatinine, total protein, alkaline phosphatase, alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, creatine kinase, ferritin, antinuclear antibodies, and rheumatoid factor;
  • protein electrophoresis;
  • urinalysis;
  • blood cultures (n = 3); urine culture; chest x-ray; abdominal ultrasonography; and tuberculin skin test (TST) or interferon γ release assay (IGRA).

IUO noninfectious inflammatory diseases (NIIDs),