Using axillary temperature to approximate rectal temperature in newborns.

Abstract

Various factors have been shown to potentially affect the difference between axillary and rectal temperature measurements in newborns. We aimed to explore their roles and, if possible, to construct a formula that explained the difference. The study was based on a consecutive sample of 175 infants, with a gestational age of 24-42 weeks, whose rectal and axillary temperatures were measured simultaneously at the neonatal unit at Skaraborg Hospital in Sweden. Data were analysed using multiple regressions. Premature infants had a significantly smaller mean difference (0.33°C) between rectal and axillary temperatures than full-term infants (0.43°C). Significant associated factors for premature infants were chronological age (p = 0.025), time of day (p = 0.004) and axillary temperature (p < 0.001). For full-term infants, the only significant associated factor was axillary temperature (p = 0.015). Although it is possible to construct a formula that estimates neonate rectal temperature based on axillary temperature with a slightly higher reliability than simply adding a fixed value like 0.4°C, such a formula would be too complex to apply in practice. Adding 0.3°C or 0.4°C to the measured axillary temperature for premature infants or full-term infants, respectively, yields acceptable approximations in most cases.