Definition of Fever in Newborns
Fever in newborns is defined as a rectal temperature ≥38.0°C (≥100.4°F), and rectal measurement is the gold standard method that must be used in this age group due to the critical need for accuracy in detecting serious bacterial infections. 1
Temperature Threshold and Measurement Method
- The universal definition of fever in newborns is rectal temperature ≥38.0°C (100.4°F) 1, 2, 3
- Rectal temperature measurement is mandatory in neonates because it provides the closest approximation to core body temperature and maximizes sensitivity for detecting fever 1, 4
- Alternative measurement methods (axillary, oral, tympanic) are not acceptable in newborns due to lower reliability and the high stakes of missing serious bacterial infection 1, 4
Critical Age-Based Risk Stratification
The definition remains the same across newborn age groups, but the clinical implications differ dramatically:
Neonates <28 Days of Age (Highest Risk)
- This age group carries approximately 12% risk of invasive bacterial infection when fever is present 5
- Fever is rarely seen in this age group, but when present, it is more frequently serious 5
- Any fever ≥38.0°C in infants <28 days mandates full sepsis workup (blood culture, urine culture, cerebrospinal fluid culture, chest radiography) plus immediate empiric intravenous antibiotics and hospital admission 5, 6
Infants 28-90 Days of Age (High Risk)
- Infants ≤90 days remain at high risk due to perinatal bacterial pathogen exposure and lack of vaccine-based immunity 1
- The risk of serious bacterial infection is 8-13% in young febrile infants, predominantly urinary tract infections 1
- Infants <28 days have higher risk than those 28-90 days 1
Important Clinical Context
- Fever occurring on the third day of postnatal life has significantly higher association with bacterial disease compared to other days in the first four days of life 7
- Temperature ≥39.0°C carries significantly higher incidence of bacterial disease than temperatures <39.0°C 7
- Fever is often the only sign of illness in young infants, making clinical differentiation between benign viral illness and invasive bacterial infection extremely difficult 1
Common Pitfalls to Avoid
- Never use alternative temperature measurement methods (axillary, tympanic, temporal artery) in newborns—only rectal temperature is acceptable 1, 4
- Do not assume environmental causes without thorough evaluation; while 17% of fevers in the first hour of life may relate to maternal fever or radiant warmers, this cannot be assumed without proper assessment 7
- Never rely on clinical appearance alone, as many infants with serious bacterial infections may appear well initially 2
- Account for recent antipyretic use, as this may mask fever severity and serious infection 2