Validation of self-reported smoking status by measuring serum cotinine levels: An Indian perspective

Background. Serum cotinine levels are a reliable marker of tobacco use. Few studies have validated questionnaires assessing smoking and exposure to environmental tobacco smoke (ETS) against serum levels. We undertook such a study in industrial workers in India. Methods. We chose 426 individuals by stratified random sampling from a database of 3397 individuals surveyed at New Delhi for the cardiovascular disease surveillance programme in a large industrial setting. Questionnaires assessing details of smoking practices and duration of exposure to ETS (if any) were administered. Cotinine levels were measured in the blood samples of these individuals. Results. The study population comprised 142 nonsmokers not exposed to ETS, 142 non-smokers exposed to ETS and 142 active smokers. Cotinine levels among nonsmokers not exposed to ETS were non-detectable; and for non-smokers exposed to ETS and active smokers, the median (interquartile range) levels were non-detectable (non-detectable to 46.1 ng/ml) and 336 ng/ml (204-500 ng/ml), respectively. The best combined sensitivity (91%) and specificity (87.2%) yielded a cotinine cut-off level of 40.35 ng/ml to differentiate active smokers from non-smokers not exposed to ETS and those exposed to ETS (area under the curve 0.902). The cut-off cotinine level was estimated at 10.95 ng/ml using a similar analysis (sensitivity 43%, specificity 82%; area under the curve 0.64) to distinguish non-smokers not exposed to ETS from those exposed to ETS. The misclassification rate was estimated at 19% and 57.1% among self-reported non-smokers not exposed to ETS and those exposed to ETS, respectively. Conclusions. Obtaining a history of tobacco use is an accurate method of detecting smokers in epidemiological studies whereas serum cotinine levels accurately differentiate smokers from non-smokers. However, a brief questionnaire assessing passive exposure to smoke has poor sensitivity in distinguishing non-smokers exposed to ETS from those not exposed to ETS.