MORTALITY AND MORBIDITY OF POTENTIALLY MISCLASSIFIED SMOKERS

Objective. Misclassification of smokers as non-smokers may bias estima tes of the excess morbidity and mortality associated with smoking. The issue has been given little, ii any, attention in prospective epidemi ological studies. This study examined characteristics of potentially m isclassified smokers with respect to mortality, morbidity, and risk fa ctors. Method. A prospective study (within The Copenhagen Male Study, Denmark) used serum cotinine as an objective marker of use of tobacco. A serum concentration of 100 ng/ml was regarded as a relevant thresho ld for active smoking. In all, 3270 males aged 53-74 years who reporte d their previous and current tobacco habits, including the use of chew tobacco and snuff, were included. Incidence of all causes of mortalit y (ACM) during 9 years and death due to ischaemic heart disease (IHD) during 8 years of follow-up were the main outcome measures. Results. O verall cumulative incidence rates oi ACM and IHD were 19.1% and 4.3%, respectively, Of 1405 men who reported being non-tobacco users, i.e no current smoking and no use of chewing tobacco or snuff, 1377 had leve ls <100 ng/ml, 28 men (2%) had levels equal to or above this threshold value and were considered potentially misclassified smokers. They had significantly higher mortality rates, 35.7% versus 14.7%, P < 0.001, than other self-reported non-tobacco users, and a slightly higher prev alence of tobacco-related cancer, and a highly significant higher prev alence of myocardial infarction, P < 0.001. Compared to non-tobacco us ers with low cotinine, age-adjusted relative risks (95% CI) were 2.4 ( 1.3-4.5), P < 0.01, for ACM, and 5.3 (95% CI : 2.1-13.4), P < 0.001, f or IHD. Conclusions. Potentially misclassified smokers deviated strong ly from other non-smokers with respect to mortality and morbidity, The importance of this reporting bias when estimating the risk associated with active or passive smoking is discussed.