INTRODUCTION

Fever is a common presenting complaint in children, accounting for nearly one-third of pediatric outpatient visits in the United States [1]. The specific entity of "fever of unknown origin" (FUO), as opposed to "fever without a source" (FWS), has occupied a special place within infectious diseases since the first definition of and series about FUO by Petersdorf and Beeson in 1961 [2]. Although the original definition has been modified, the assessment of broad categories of illness (including infections, connective tissue disease, and malignancy) as a cause of FUO remains useful.

An approach to FUO in children will be reviewed here. Etiologies of pediatric FUO, FWS, and fever in unique host groups (eg, newborns, neutropenic children, or those with human immunodeficiency virus [HIV] infection) are discussed separately. (See "Etiologies of fever of unknown origin in children" and "Fever without a source in children 3 to 36 months of age".)

DEFINITIONS

The term fever of unknown origin (FUO) initially was reserved for adults with fever >38.4ºC (101.2ºF) on at least several measurements over three or more weeks without an established cause after at least one week of investigation in the hospital. This exacting definition probably was never rigorously applied in pediatrics.

We apply the following definitions for FUO and fever without a source (FWS):

• FUO – Children with fever >101ºF (38.3ºC) of at least eight days' duration, in whom no diagnosis is apparent after initial outpatient or hospital evaluation that includes a careful history and physical examination and initial laboratory assessment.
• FWS – Children with fever lasting for one week or less without adequate explanation after a careful history and thorough physical examination. (See "Evaluation and management of fever in the neonate and young infant (less than three months of age)" and "Fever without a source in children 3 to 36 months of age".)