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

Infections

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

Malignancies

• 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

Miscellaneous

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

References:

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]

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