How is the syndrome of fever of unknown origin (FUO) defined?

Updated: May 17, 2021
  • Author: Sandra G Gompf, MD, FACP, FIDSA; Chief Editor: Michael Stuart Bronze, MD  more...
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The syndrome of fever of unknown origin (FUO) was defined in 1961 by Petersdorf and Beeson as the following: (1) a temperature greater than 38.3°C (101°F) on several occasions, (2) more than 3 weeks' duration of illness, and (3) failure to reach a diagnosis despite one week of inpatient investigation. [3, 4]  It is important to allow for flexibility in this definition, however. "Normal" core temperature in studies in developed nations has declined since the Industrial Revolution and may be inferred to peak at 99.9º F (37.7º C). [5, 6]  The emergence of the human immunodeficiency virus (HIV) and the expanding use of immunomodulating therapies prompted Durack and Street to propose differentiating FUO into 4 categories: classical FUO (Petersdorf definition), hospital-acquired FUO, immunocompromised or neutropenic FUO, and HIV-related FUO. [7]

Emerging techniques such as molecular diagnostics, expanding use of immunocompromising therapies and organ transplantation, and the advent of globally mobile populations demand an evolving approach to defining and evaluating FUO. [7, 8, 9] Modern imaging techniques (eg, ultrasonography, computed tomography [CT] scanning, magnetic resonance imaging [MRI], positron emission tomography [PET]) enable early detection of abscesses and solid tumors that were once difficult to diagnose.

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