Iatrogenic and transient hyperglycinemia in patients with phenylketonuria.

Two patients with phenylketonuria detected by newborn screening for inborn errors of metabolism were treated with low phenylanine formulae. Serum phenylalanine levels were well controlled, but serum glycine levels were elevated until 4 or 5 months of age. This was probable due to the high content of glycine in the formulae. Glycine level returned to be normal in these patients, even though they were kept on the same formula, suggesting, immature metabolism of glycine during this period. No clinical problems were encountered in either patient.