Plasma amino acid concentrations in premature infants: effect of sampling site

Amino acid concentrations in plasma obtained from infants by heel puncture differ from those in venous plasma primarily because of skin contamination but data from the two sites might agree more closely in premature infants because of their poorly developed sweat glands. To evaluate the importance of sampling site, amino acids were analyzed by a sensitive high-performance liquid chromatographic method in plasma obtained both by heel puncture and venipuncture in 14 premature infants. Histidine, tryptophan, alanine, aspartic acid, glycine, and serine were higher in capillary plasma. Tryptophan and glycine levels in capillary plasma correlated strongly with those in venous plasma. An additional wash with soap and water done in five infants did not eradicate the differences. Thus the lack of agreement between data from the two sites may be due to factors other than contamination.