H-Abu-OH, one of the three isomers of aminobutyric acid, is elevated in the plasma of children with with Reye's syndrome, tyrosinemia, homocystinuria, nonketotic hyperglycinemia, and ornithine transcarbamylase deficiency.
References:
[1]. Yudkoff M, et al. On the clinical significance of the plasma alpha-amino-n-butyric acid:leucine ratio. Am J Clin Nutr. 1979 Feb;32(2):282-5.