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Approach to the adult with fever of unknown origin

David H Bor, MD
Section Editor
Peter F Weller, MD, MACP
Deputy Editor
Anna R Thorner, MD


Clinicians commonly refer to a febrile illness without an initially obvious etiology (sometimes called fever without localizing signs) as fever of unknown origin (FUO). This usage is not accurate. Most febrile illnesses either resolve before a diagnosis can be made or develop distinguishing characteristics that lead to a diagnosis. FUO refers to a prolonged febrile illness without an established etiology despite intensive evaluation and diagnostic testing.

Large case series of FUO applying this definition have been collected over a number of decades; these facilitate an approach to patients with FUO and an understanding of the changing patterns of FUO with time and newer diagnostic techniques.

The definitions and special populations with FUO as well as the diagnostic approach to the adult with this problem are reviewed here. A specific discussion of the common and uncommon entities causing FUO and the approach to children with FUO is presented separately. (See "Etiologies of fever of unknown origin in adults" and "Fever of unknown origin in children: Evaluation".)


The definition of FUO derived by Petersdorf and Beeson in 1961 from a prospective analysis of 100 cases has long been the clinical standard [1]:

Fever higher than 38.3ºC on several occasions

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Literature review current through: Nov 2017. | This topic last updated: Apr 27, 2016.
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