The distribution of cotinine levels between smoking cases and controls were compared using the Kruskal-Walis test. The associations between SNPs and cotinine levels or CPD were investigated in current smokers using multivariate linear regression models with smoking variables (CPD or cotinine) as outcomes, adjusting for study center, gender and case-control status. The mean cotinine levels adjusted for study center, gender and case-control status were calculated for each genotype.
