Assessing smoking status in children, adolescents and adults: cotinine cut‐points revisited

Abstract

Aims To reassess saliva cotinine cut‐points to discriminate smoking status. Cotinine cut‐points that are in use were derived from relatively small samples of smokers and non‐smokers 20 or more years ago. It is possible that optimal cut‐points may have changed as prevalence and exposure to passive smoking have declined. Design Cross‐sectional survey of the general population, with assessment of self‐reported smoking and saliva cotinine. Participants A total of 58 791 respondents aged 4 years and older in the Health Survey for England for the years 1996–2004 who provided valid saliva cotinine specimens. Measures Saliva cotinine concentrations, demographic variables, self‐reported smoking, presence or absence of smoking in the home, a composite index of social disadvantage derived from occupation, housing tenure and access to a car. Findings A cut‐point of 12 ng/ml performed best overall, with specificity of 96.9% and sensitivity of 96.7% in discriminating confirmed cigarette smokers from never regular smokers. This cut‐point also identified correctly 95.8% of children aged 8–15 years smoking six or more cigarettes a week. There was evidence of substantial misreport in claimed ex‐smokers, especially adolescents (specificity 72.3%) and young adults aged 16–24 years (77.5%). Optimal cut‐points varied by presence (18 ng/ml) or absence (5 ng/ml) of smoking in the home, and there was a gradient from 8 ng/ml to 18 ng/ml with increasing social disadvantage. Conclusions The extent of non‐smokers' exposure to other people's tobacco smoke is the principal factor driving optimal cotinine cut‐points. A cut‐point of 12 ng/ml can be recommended for general use across the whole age range, although different cut‐points may be appropriate for population subgroups and in societies with differing levels of exposure to secondhand smoke.