Unifying Concepts

Classification of Fever of Unknown Origin (FUO)

FUO is body temperature  38.3° C (101° F) rectally that does not result from transient and self-limited illness, rapidly fatal illness, or disorders with clear-cut localizing symptoms or signs or with abnormalities on common tests such as chest x-ray, urinalysis, or blood cultures.

Category of FUO Definition Common etiologies
Classic Temperature >38.3°C (100.9°F)
Duration of >3 weeks
Evaluation of at least 3 outpatient visits or 3 days in hospital
Infection, malignancy, collagen vascular disease
Nosocomial Temperature >38.3°C
Patient hospitalized >=24 hours but no fever or incubating on admission
Evaluation of at least 3 days
Clostridium difficile enterocolitis, drug-induced, pulmonary embolism, septic thrombophlebitis, sinusitis
Immune deficient (neutropenic) Temperature >38.3°C
Neutrophil count <=500 per mm3
Evaluation of at least 3 days
Opportunistic bacterial infections, aspergillosis, candidiasis, herpes virus
HIV-associated Temperature >38.3°C
Duration of >4 weeks for outpatients, >3 days for inpatients
HIV infection confirmed
Cytomegalovirus, Mycobacterium avium-intracellulare complex, Pneumocystis carinii pneumonia, drug-induced, Kaposi’s sarcoma, lymphoma

HIV = human immunodeficiency virus.

Common Etiologies of Fever of Unknown Origin


  • Tuberculosis (especially extrapulmonary)
  • Abdominal abscesses
  • Pelvic abscesses
  • Dental abscesses
  • Endocarditis
  • Osteomyelitis
  • Sinusitis
  • Cytomegalovirus
  • Epstein-Barr virus
  • Human immunodeficiency virus
  • Lyme disease
  • Prostatitis
  • Sinusitis


  • Chronic leukaemia
  • Lymphoma
  • Metastatic cancers
  • Renal cell carcinoma
  • Colon carcinoma
  • Hepatoma
  • Myelodysplastic syndromes
  • Pancreatic carcinoma
  • Sarcomas

Autoimmune conditions

  • Adult Still’s disease
  • Polymyalgia rheumatica
  • Temporal arteritis
  • Rheumatoid arthritis
  • Rheumatoid fever
  • Inflammatory bowel disease
  • Reiter’s syndrome
  • Systemic lupus erythematosus
  • Vasculitides


  • Drug-induced fever
  • Complications from cirrhosis
  • Factitious fever
  • Hepatitis (alcoholic, granulomatous, or lupoid)
  • Deep venous thrombosis
  • Sarcoidosis



  1. Durack DT, Street AC. Fever of unknown origin–reexamined and redefined. Curr Clin Top Infect Dis. 1991;11:35-51. [Medline]
  2. Hirschmann JV. Fever of unknown origin in adults. Clin Infect Dis. 1997 Mar;24(3):291-300 [Medline]


Created: July 04, 2006

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