To quantify the neonatal mortality/hypothermia relationship and develop evidence-based cutoffs for global definitions of neonatal hypothermia.
Cohort study. Field workers recorded neonatal axillary temperature at home and recorded vital status at 28 days.
Twenty-three thousand two hundred forty infants in Sarlahi, Nepal.
Mortality risk was estimated using binomial regression models. Infants were classified using (1) World Health Organization (WHO) cutoffs for mild, moderate, and severe hypothermia; (2) quarter-degree intervals from 32.0°C to 36.5°C; and (3) continuous temperatures. Estimates were adjusted for age, ambient temperature, and other potential confounders.
Mortality increased among mild (relative risk [RR], 1.70; 95% confidence interval [CI], 1.23-2.35]), moderate (RR, 4.66; 95% CI, 3.47-6.24]), and severe (RR, 23.36; 95% CI, 4.31-126.70]) hypothermia cases. Within the WHO's moderate classification, risk relative to normothermic infants ranged from 2 to 30 times. Adjusted mortality risk increased 80% (95% CI, 63%-100%) for each degree decrease, was strongly associated with temperatures below 35.0°C (RR, 6.11; 95% CI, 3.98-9.38), and was substantially higher among preterm infants (RR, 12.02; 95% CI, 6.23-23.18]) compared with full-term infants (RR, 3.12; 95% CI, 1.75-5.57). Relative risk was highest in the first 7 days, but remained elevated through 28 days.
A new hypothermia classification system should be considered by the WHO for global guidelines. We recommend that grade 1 be equivalent to the current mild category (36.0°C), restricting and splitting the moderate category into grades 2 (35.0°C-36.0°C) and 3 (34.0°C-35.0°C), and expanding severe hypothermia to less than 34.0°C (grade 4). Reducing hypothermia may dramatically decrease the global neonatal mortality burden.