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No prospective studies are available on the ability of serum cotinine level as a marker of exposure to tobacco to predict lung cancer risk. We analysed serum cotinine level among 1741 individuals enrolled since the 1970s in a prospective study of Norwegian participants in health surveys, who developed lung cancer during the follow-up, and 1741 matched individuals free from lung cancer. Serum cotinine was measured with a competitive immunoassay. Regression dilution was corrected for based on repeated measures on samples from 747 subjects. Mean serum cotinine level was higher in cases then in controls. As compared to subjects with cotinine level below or equal to 5 ng/ml, the odds ratio of lung cancer was increasing linearly, reaching 55.1 (95% confidence interval 35.7-85.0) among individuals with serum cotinine level above 378 ng/ml. There was no suggestion of a plateau in risk at high exposure levels. Eighty-two percent of cases lung cancer are attributable to tobacco smoking as indicated by serum cotinine level above 5 ng/ml. Odds ratios were very similar in men and women. We found no association between serum cotinine level (range 0.1 - 9.9 ng/ml) and lung cancer risk among self-reported non-smokers and long-term quitters (79 cases, 350 controls). Our estimates of lung cancer risk are higher than those obtained in questionnaire-based studies, showing that serum cotinine level is a valid marker of internal dose of tobacco-related lung carcinogens. The plateau in risk at high doses observed in questionnaire-based studies is likely due to artifacts. There is no difference between men and women in the carcinogenicity of tobacco smoking. Serum cotinine level is not a good predictor of lung cancer risk in non-smokers.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]