Factors affecting the serum protein binding of salicylic acid in newborn infants and their mothers.

The protein binding of salicylic acid in serum of 49 full-term newborn infants and their mothers immediately after delivery was determined, and potential variables affecting the results obtained were examined. The free fraction of salicylic acid added in vitro at an initial drug concentration of 20 mg/100 ml ranged from 0.104 to 0.242 in the infants and from 0.180 to 0.406 in the mothers. The ratio of free fraction values, infant:mother, was 0.67 +/- 0.13 (mean +/- SD). The more extensive serum protein binding of salicylic acid in the newborns' serum explains why serum salicylate concentrations at the time of delivery are usually higher in newborn infants than in their mothers. A cascade of patient characteristics (induction of labor by oxytocin, serum albumin concentration, duration of labor, mother's age, type of anesthesia, use of analgesics, and nonalbumin protein concentration) explained 61% and 37% of the variability of the salicylic acid free fraction values in the serum of infants and their mothers, respectively.