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Fever of Unknown Origin

Fever of unknown origin (FUO), pyrexia of unknown origin (PUO) or febris e causa ignota (febris E.C.I.) refers to a condition in which the patient has an elevated temperature and despite investigations by a physician no explanation can be found.

If a cause is determined it is usually is a diagnosis of exclusion, that is, by eliminating all possibilities until only one explanation remains, and taking this as the correct one.

In 1961 Petersdorf and Beeson suggested the following criteria:

Fever higher than 38.3°C (101°F) on several occasions

  • Persisting without diagnosis for at least 3 weeks
  • At least 1 week's investigation in hospital

Current medical practice criteria is broader and stipulates the following criteria:

  • 3 outpatient visits or
  • 3 days in the hospital without elucidation of a cause or
  • 1 week of "intelligent and invasive" ambulatory investigation.

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