Single nucleotide polymorphisms (SNPs) in the gene encoding gamma glutamyl hydrolase (GGH) associated with reduced GGH activity are disclosed. The primary SNP is a change from a cytosine to a thymine at a position corresponding to nucleotide 511 of Genbank sequence accession no. NM 003878. Methods and kits for detecting these SNPs are provided, along with primers useful in detecting these SNP and for amplifying portions of the GGH gene containing these SNPs.