OBJECTIVESThe reliability of surveys on smoking habits based on questionnaires was investigated, using the urinary cotinine content as an objective index.

METHODSThe subjects tested were 2,849 office workers of middle age, who responded to questions concerning their smoking status, and also their urinary cotinine was measured by the HPLC method.

RESULTSThe boundary value between smokers and non-smokers, determined by the histogram independent of the questionnaire, was 63.1 and 79.4 ng/mg of creatinine for males and females, respectively. The rate of misclassification of the non-smokers and former smokers as smokers was 1.3% for males and 1.8% for females, whereas that of current smokers as non-smokers was 6.3% and 2.1%. We also assessed the effect of smoke inhalation on the urinary cotinine value, and found a significant difference for males in the cotinine value by the presence of inhalation and also its depth.

CONCLUSIONSThe rate of misclassification in this study was considered to be comparatively low. Several studies have also assessed the reliability of the questionnaire on smoking habits, and found different misclassification rates, indicating the dependence on the race and number of subjects tested. To our knowledge, there were only a few surveys on smoking among large groups, particularly in Japan, such as this one, therefore the results obtained in this study are meaningful.