Axillary Temperature Fever Definition for Newborns
Fever in newborns is defined as a rectal temperature ≥38.0°C (100.4°F), and rectal measurement—not axillary—is the mandatory gold standard method for fever detection in this population. 1
Why Rectal Temperature is Required
Rectal temperature is the only acceptable method for diagnosing fever in newborns because it provides the closest approximation to core body temperature and maximizes sensitivity for detecting serious bacterial infections, which occur in 8-13% of young febrile infants. 1
Alternative measurement methods (axillary, oral, tympanic, temporal artery) are explicitly not acceptable in newborns due to lower reliability and the high stakes of missing serious bacterial infection. 1, 2
The American College of Radiology specifically warns against using axillary, tympanic, or temporal artery thermometers for diagnostic purposes in newborns. 1, 2
The Problem with Axillary Temperature in Newborns
While axillary temperature measurement is commonly used for screening and monitoring, it systematically underestimates core body temperature and has limited specificity for detecting fever:
Axillary readings are always lower than rectal readings by a mean of 1.15°C, with wide limits of agreement (0.32 to 1.98°C difference). 3
Axillary thermometry has good sensitivity (95-98%) for detecting fever but limited specificity (75%), meaning it can miss true fever or give false reassurance. 3, 4
In one study, axillary temperature had only 47% sensitivity for detecting fever in children beyond the neonatal period, though it performed better in neonates specifically (98% sensitivity). 4
When Axillary Temperature May Be Used
Axillary measurement is acceptable only for screening or monitoring in specific contexts, not for diagnostic fever determination:
The Italian Pediatric Society recommends axillary measurement using a digital thermometer for children aged <4 weeks in healthcare settings and for home monitoring by parents. 5
If axillary screening suggests fever, this must be confirmed with rectal temperature before making clinical decisions about sepsis workup or antibiotic therapy. 3
For monitoring temperature stability in preterm infants (<34 weeks' gestation), axillary temperature is used to assess hypothermia (<36.0°C) or hyperthermia (>38.0°C) during resuscitation and NICU admission. 6
Critical Age-Based Context
Infants ≤28 days (neonates) are at highest risk for invasive bacterial infections and require the lowest threshold for full sepsis workup. 1, 7
Infants ≤90 days remain at elevated risk due to perinatal bacterial pathogen exposure and lack of vaccine-based immunity. 1
Fever is often the only sign of serious illness in newborns, making accurate temperature measurement critical since clinical appearance alone cannot differentiate benign viral illness from invasive bacterial infection. 1
Common Pitfalls to Avoid
Never rely on axillary temperature alone to rule out fever in a newborn presenting with concern for infection—always obtain rectal temperature for diagnostic purposes. 1
Do not assume a normal axillary temperature excludes serious bacterial infection, as the wide limits of agreement mean significant fever could be missed. 3
Account for recent antipyretic use, as this may mask fever severity and serious infection. 1
Never delay sepsis workup in neonates <28 days based on clinical appearance alone—the threshold for full evaluation is appropriately low in this age group. 7