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McCarthy LK, O'Donnell CPF. Comparison of rectal and axillary temperature measurements in preterm newborns. Arch Dis Child Fetal Neonatal Ed. 2021 Feb 8:fetalneonatal-2020-320627. doi: 10.1136/archdischild-2020-320627. Epub ahead of print. PMID: 33558215.

Objective: To compare rectal and axillary temperatures in preterm newborns on admission to the neonatal intensive care unit (NICU).

Design: Secondary analysis of data collected in a randomised controlled trial (RCT).

Setting: Maternity hospital, level 3 NICU.

Patients: Seventy-two newborns <31 weeks who were enrolled in the BAMBINO RCT (A randomised trial of exothermic mattresses to prevent heat loss in preterm infants at birth, ISRCTN31707342).

Interventions: Newborns were placed in polyethylene bags and were randomised to placement on exothermic mattresses, or not in the delivery room. All infants had rectal and axillary temperatures measured in immediate succession using a digital thermometer on NICU admission.

Outcome measures: Admission rectal and axillary temperatures.

Results: Mean (SD) gestational age was 28 (2) weeks and birth weight was 1138 (374) g. Mean rectal-axillary temperature difference was 0.1 (0.5°C) (range -1.4°C to +1.5°C). Rectal and axillary temperatures differed by ≥0.5°C in 18/72 (25%) infants; axillary temperature was higher than rectal in 6 (8%) and lower in 12 (17%). There was a positive linear relationship between rectal and axillary measurements (Pearson’s correlation R=0.84). Applying the Bland-Altman technique, the width of 95% prediction interval was 1.8°C (-0.8°C to 1.0°C) implying that rectal and axillary measurements may vary by up to 1.0°C. Axillary temperature had a sensitivity of 65% when used to detect rectal hyperthermia and 100% sensitivity for hypothermia.

Conclusion: Paired rectal and axillary temperature measurements in preterm newborns on NICU admission vary significantly. Axillary temperature was sensitive at detecting rectal hypothermia but not hyperthermia. Axillary temperature may not be an accurate proxy for rectal temperature measurement in all preterm newborns on NICU admission.

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