Age Variations in the Concentration of Non-Protein Nitrogen, Creatinine and Urea in Blood of Infants and Children

Non-protein nitrogen as well as serum urea and creatinine concentrations were determined in 113 healthy children ranging in age from newborn to six years. The results were compared with those from corresponding analyses of adult blood.
NPN and creatinine levels at birth were found to approximate those in adults but to decrease within five or six days to values significantly lower than the adult ones. This confirmed reports published much earlier. It was found, in addition, that at an age between about five and seven months the NPN concentration rose rather abruptly to the adult range. The question as to whether this phenomenon resulted from the child's natural development or whether it stemmed from the increased protein intake, is discussed.
The serum concentration of urea at birth was found to be significantly lower than the adult value. That this was attributable to a low urea concentration in the maternal blood is a possibility which cannot be ruled out. As with NPN and creatinine, the concentration of urea increased to the adult range at an age of five to seven months.
The normal means and concentration ranges for NPN, creatinine, and urea-N in blood from children of different age groups are tabulated.