How is fever defined in fever of unknown origin (FUO)?

Updated: May 17, 2021
  • Author: Sandra G Gompf, MD, FACP, FIDSA; Chief Editor: Michael Stuart Bronze, MD  more...
  • Print

A baseline definition of "fever" is important in determining whether a patient's report of an elevated temperature warrants an FUO workup. Most temperatures are measured orally for both practical and physiologic purposes. A "normal" core (internal) body temperature ranges from 96º Fahrenheit (F) (35.6º Celsius [C]) to 100.8ºF (38.2ºC) in healthy persons, with a mean of 98.2ºF (36.8ºC). Core temperature in the afternoon is about 1ºF higher later in the day and may be a bit higher in women.

The temperature of the sublingual fossa correlates most closely, and changes most consistently, with core body temperature, which is fairly constant; the rectum and axilla do not, especially during sepsis. The tympanic membrane also correlates with core body temperature and is nearest to the hypothalamic center that regulates temperature, but accuracy depends on user technique and whether the ear canal is obstructed (eg, by wax); cold weather also cools the tympanic membrane. [10]

For the purposes of this article, the term FUO refers to the classic category, which focuses on the adult population. The definition of FUO in the pediatric age group varies, with a time frame ranging from 1-3 weeks in the literature. In this age group, the differential diagnoses are led by infections, followed by collagen vascular diseases; malignancy is typically not heralded by fever alone in children. [14] This article excludes FUO in the setting of impaired immunity such as HIV disease, solid-organ and bone marrow transplantation, and neutropenia. Disease-specific diagnostic algorithms in these conditions are described elsewhere. Regardless of age group, most clinicians define FUO as a persisting conundrum with few or no objective clues.

Realistically, it is difficult to define a set time frame or defined list of examinations to be performed before declaring "FUO". The duration of unsuccessful diagnosis varies widely because the diagnostic approach to fever is highly dependent upon the tools accessible in a given healthcare setting, including socioeconomic, and other disparities in healthcare. Similarly, local geography and epidemiology factor into diagnostics.

Did this answer your question?
Additional feedback? (Optional)
Thank you for your feedback!