For all 16 studies, the allele counts, that is the number of A (wild-type) and G (variant) alleles, of the case and control groups were ascertained, either directly from the article or, if not indicated there, were calculated from the genotype counts. Three studies did not report the genotype counts of their subjects (the number of subjects with A/A, A/G, and G/G genotypes), but they did report allele counts.16,17,19 For each study that reported allele counts (all 16 studies), an odds ratio (OR) was calculated to compare the frequency of the G variant allele between case (opioid-dependent) and control subjects. Similarly, for each study that reported genotype counts (13 studies), an OR (with 95% CI) was calculated to compare the frequency of the homozygous mutant genotype (G/G) between opioid-dependent and control subjects.
