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Table 3 Subtypes of fever of unknown origin as defined by Durack and Street (1991) [6]

From: Undifferentiated epithelioid sarcoma presenting as a fever of unknown origin: a case report

FUO subtype Definition Major causes
Classical FUO - Temperature of 38.3 °C
- > 3 weeks’ duration of illness
- Failure to reach diagnosis with 1 week of in-patient investigations
Malignancies, infections, inflammatory diseases (non-infectious)
Hospital acquired FUO - Temperature of 38.3 °C
- Patient hospitalized 24 hours, fever not present or incubating on admission
- Evaluation for 3 days
HAIs, postoperative complications, drug-induced
Immunocompromised or neutropenic FUO - Temperature of 38.3 °C
- Absolute neutrophil count 500 per mm3
- Evaluation for 3 days
Various bacterial, viral, and fungal infections
HIV-related FUO - Temperature of 38.3 °C
- Out-patient duration > 4 weeks; >  3 days for in-patients
- Confirmed HIV infection
HIV defining infections (Mycobacteria, CMV, Toxoplasma, Cryptococcus), lymphoma, IRIS
  1. CMV cytomegalovirus, FUO fever of unknown origin, HAIs hospital acquired infections, HIV human immunodeficiency virus, IRIS immune reconstitution inflammatory syndrome. (Adapted from table presented in Hayakawa et al., 2012 [5])