Undifferentiated fever and fever without a source in infants and children

Overview & Definition

A febrile child walks through your door. They are hot to touch, irritable, and the parent is worried. There is no runny nose, no obvious rash, no localising story — just fever. This is the single most common paediatric presentation to emergency departments worldwide, and the clinical question is always the same: is this a self-limiting viral illness, or is a serious bacterial infection brewing that will cause harm if I wait? [6] [13]

Fever is defined as a rectal temperature of ≥38 °C (100.4 °F). In clinical practice, axillary, tympanic and oral measurements are used with varying accuracy, but the rectal temperature remains the reference standard in infants and young children. [6] [14]

Fever without a source (FWS) is fever for which history and physical examination do not reveal a clear cause. This is distinct from fever of unknown origin (FUO), which implies prolonged or recurrent fever (typically more than 7–14 days) requiring a different diagnostic workup. The acute undifferentiated fever is what you face in the emergency department: a child who has been febrile for hours to a few days, looks unwell or well, and needs a decision now. [6] [7]

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