Finally, individual variation in nicotine metabolism may also be assessed by using a phenotypic measure, namely the ratio of the nicotine metabolites derived from cigarette smoking (3-HC/cotinine) (Benowitz et al., 2003; Dempsey et al., 2004). The 3-HC/cotinine ratio can be measured reliably in saliva or plasma (Dempsey et al., 2004), has minimal diurnal variation (Lea et al., 2006), and is independent of smoking patterns or time since last cigarette (Levi et al., 2007), at least among relatively regular smokers. Null or reduced activity CYP2A6 alleles (*2, *4, *9, and *12) are associated with lower 3-HC/cotinine ratios and slower metabolism (Dempsey et al., 2004; Johnstone et al., 2006; Malaiyandi et al., 2006a). The nicotine metabolite ratio may be optimal for assessing individual differences in nicotine-metabolism rate, because it accounts for environmental factors such as ethnicity, sex, and age, which may alter the nicotine metabolic rate (Benowitz et al., 2006a; Johnstone et al., 2006; Mwenifumbo and Tyndale, 2007).
