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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 adiagnosis 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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