IN THEIR classic description of unexplained fevers, Petersdorf and Beeson1 defined fever of unknown origin (FUO) as an illness of at least 3 weeks' duration whose cause could not be determined after inpatient evaluation. This syndrome has fascinated clinicians and is periodically reviewed (with special attention to its causes) in prestigious medical journals.1-10 The entity has also been adapted to the economic realities of contemporary health care; consequently, the workup is now routinely accomplished outside the hospital.7-9
The definition of classic FUO excludes short-lived febrile illnesses; in fact, the duration criterion was proposed primarily to eliminate self-limited diseases, such as common viral respiratory tract infections, which are difficult to diagnose precisely and/or have little therapeutic impact.1
A more frequent but less precisely defined entity is new fever in a hospitalized patient for which the cause is not immediately apparent. This syndrome might be termed acute FUO
DiNubile MJ. Acute Fevers of Unknown Origin: A Plea for Restraint. Arch Intern Med. 1993;153(22):2525–2526. doi:10.1001/archinte.1993.00410220021002
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