To assess the extent to which smoking cessation among cohort members during the follow-up affects the estimated association between smoking and coronary heart disease (CHD) risk in an observational study with a single determination of the smoking status at baseline.
Design
Cross-sectional trend and prospective cohort analyses.
Settings
North Karelia and Kuopio provinces in eastern Finland.
Subjects
Men (n = 13 542) aged 30–59 years who participated in risk factor surveys between 1972 and 1992. Of these, 3937 men belonged to the 1972 cohort with the prospective follow-up.
Main outcome measurements
Smoking prevalence, CHD mortality, non-CHD mortality, and total mortality.
Results
Smoking prevalence in the population decreased from 53% to 37% between 1972 and 1992. In the follow-up of the 1972 study cohort, the association between baseline smoking status and CHD weakened markedly when the follow-up time from the baseline measurement was extended. The risk ratio of cumulative CHD mortality associated with smoking was 6.96 in the first 2-year follow-up and it decreased gradually to 2.06 at the 20-year follow-up. A similar decrease in the risk ratio was observed when the analysis was performed during periods of the follow-up. The association between smoking and non-CHD mortality weakened only slightly when the follow-up time became longer.
Conclusion
The results from prospective observational studies conducted in populations with a decreasing smoking prevalence may be biased by the misclassification of study subjects during the follow-up as a result of smoking cessation. This results in an underestimation of the risk of CHD caused by smoking. Thus, smoking may be even more harmful for health than the estimates from prospective studies suggest.
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