Anthropometric determinants of creatinine excretion in preterm infants.

Creatinine output per unit of body weight increases throughout childhood. this reflects the increase in percent muscle mass observed during growth. Standard values for creatinine excretion are not available for the preterm infant. Accordingly, urinary creatinine output was measured in timed 24-hour specimens obtained in the second week of life of 15 preterm infants (gestational age 26 to 33 weeks). Three highly significant regression equations were derived that relate creatinine excretion to birth weight, length, and gestational age. Creatinine output correlated best with birth weight. Stepwise regression yielded no linear combination of these variables that improved the estimate based on weight alone. In a similar manner, consideration of other variables, including weight and length for age (expressed as percent of 50th percentile), did not improve the estimate based on weight alone. These data demonstrate that creatinine output in milligrams per kilogram of body weight increases by 16% between birth weight of 0.7 and 1.5 kg. This increase parallels that in body protein content per kilogram obtained from published fetal body composition data in infants of similar age. Creatinine output in preterm infants, therefore, can serve as a measure of growth.