International Journal of Epidemiology, Vol 26, 321-327, Copyright © 1997 by International Epidemiological Association
P Suadicani, HO Hein and F Gyntelberg
OBJECTIVE: Misclassification of smokers as non-smokers may bias estimates
of the excess morbidity and mortality associated with smoking. The issue
has been given little, if any, attention in prospective epidemiological
studies. This study examined characteristics of potentially misclassified
smokers with respect to mortality, morbidity, and risk factors. 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 threshold for active smoking. In all,
3270 males aged 53-74 years who reported their previous and current tobacco
habits, including the use of chew tobacco and snuff, were included.
Incidence of all causes of mortality (ACM) during 9 years and death due to
ischaemic heart disease (IHD) during 8 years of follow-up were the main
outcome measures. RESULTS: Overall cumulative incidence rates of 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 levels < 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 prevalence of tobacco-related cancer, and a highly significant
higher prevalence of myocardial infarction, P < 0.001. Compared to
non-tobacco users 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, for IHD. CONCLUSIONS: Potentially misclassified smokers
deviated strongly 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.
ARTICLES
Mortality and morbidity of potentially misclassified smokers
Epidemiological Research Unit, Copenhagen University Hospital, Denmark.
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