What Is Fever?

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Abstract
To determine the normal temperature of healthy infants, we studied 691 infants less than 3 months of age being seen for regularly scheduled well-baby visits. All temperatures were taken rectally with an electronic thermometer. The mean temperature was 37.5°C ± 0.3°C. Using a cutoff of 2 standard deviations (SD) above the mean, fever would be defined as a temperature ≥ 38.1°C. The 95th percentile was 38.0°C, and ≥38.1°C would correspond to values above the 95th percentile. The most widely used definition of fever at present is a temperature ≥38.0°C; by this definition, 6.5% of these well infants would be considered to have a fever. A significant rise in temperature with age was noted. For infants from birth to 30 days old, 2 SD above the mean was 38.0°C; for those 31 to 60 days old, it was 38.1°C; and for those 61 to 91 days old, it was 38.2°C. Similarly, the 95th percentile was 37.9°C, 38.0°C, and 38.1°C, respectively. Temperature also varied significantly with the season of the year, being higher in the summer (2 SD above a mean = 38.3°C) than in the winter (2 SD above a mean = 38.0°C). Fever should be defined as a temperature ≥38.0°C in infants less than 30 days of age, ≥38.1°C in 1-month-olds, and ≥38.2°C in 2-month-olds.