Effect of sampling site on plasma amino acid concentrations of infants: effect of skin amino acids

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Abstract
Plasma taurine, aspartate, threonine, serine, glycine, alanine, valine, leucine, tyrosine, phenylalanine, tryptophan, lysine, histidine, and ornithine concentrations are significantly greater (p < 0.05, “Student's” t test) in blood samples obtained by conventional heel skin puncture techniques from 1-yr-old infants than values in venous plasma. Differences in plasma concentrations of taurine, aspartate, serine, glycine, and ornithine were particularly striking, with levels in plasma collected from the heel being 1.6 to 6.7 times higher than levels in venous plasma. These increased plasma amino acid concentrations were shown to result primarily from contamination of the plasma with amino acids present on the skin surface. Thorough washing and stimulation of blood flow to the heel by warming prior to skin puncture reduced observed differences. Plasma amino acid concentrations of blood samples obtained by conventional heel skin puncture procedures can be “normalized” to venous values through the use of data on the amino acid composition of heel skin washings.