VARIABILITY OF PLASMA-CORTISOL LEVELS IN EXTREMELY LOW-BIRTH-WEIGHT INFANTS

Cortisol is secreted by children and adults in a pulsatile pattern of 15-30 peaks and nadirs each day with a circadian rhythm. Newborns are known to lack the circadian pattern, leading to uncertainty about the appropriate time for blood sampling for assessment of adrenal function . Because extremely low birth weight (ELBW) infants may manifest signs of adrenal insufficiency, knowledge of the pattern of cortisol levels is necessary to guide the appropriate timing of blood sampling. To de fine the pattern of plasma cortisol levels in 14 ELBW infants, we obta ined blood specimens every 20 min over a 6-h period at 4-6 days of lif e. Although cortisol levels in the 14 infants ranged from 2.0-54.5 mu g/dL, each infant's cortisol levels varied little from his or her own mean cortisol level. The sns calculated from each infant's mean cortis ol level were small, ranging from 0.37-4.12 mu g/dL. Cluster analysis was applied to the data; only 0.6 cortisol pulses/infant.6-h period we re detected. Each infant's plasma cortisol levels were plotted against time, and regression analysis was performed. The slopes of the result ing lines of regression ranged from -0.0284 to 0.0221. Our data indica te that ELBW infants show little variability in their plasma cortisol levels over time; therefore, a single random measurement provides an a dequate reflection of the adrenal status of the ELBW infant.