Urinary creatinine excretion and estimation of muscle mass in infants of 25-34 weeks gestation.

The urinary excretion rate of creatinine was measured over 89 days in 31 infants of gestational ages 25-34 weeks in the first week of life. Creatinine excretion was shown to have a significant positive correlation with weight and postconceptional age. Creatinine excretion factored by body weight was also positively correlated with weight and postconceptional age. Pooled analysis of published data together with our own, confirms that this correlation is important. Muscle mass was estimated from creatinine excretion rate. A regression equation was derived predicting muscle mass from birth-weight and gestational age. Muscle mass increases from 12% of birthweight at 25 weeks gestation to 19% at 34 weeks and 24% at 40 weeks. This is in agreement with classic dissection studies which showed muscle mass to be 25% of body weight at term.