Self-reported smoking status and plasma cotinine concentrations among pregnant women in the Norwegian Mother and Child Cohort Study

BACKGROUND: Underreporting of smoking in epidemiologic studies is common and may constitute a validity problem, leading to biased association measures. In this prospective study, we validated self-reported tobacco use against nicotine exposure assessed by plasma cotinine in the Norwegian Mother and Child Cohort Study (MoBa). METHODS: The study was based on a subsample of 2,997 women in MoBa who delivered during the period 2002–2003. Self-reported tobacco use (test variable) and plasma cotinine concentrations (gold standard) were assessed around gestational week 18. RESULTS: Daily smoking was reported by 9%, occasional smoking by 4% and non-smoking by 86% of the women. Sensitivity and specificity for self-reported smoking status were calculated by using a cotinine cut-off estimated from the current material (30 nmol/l). Plasma cotinine concentrations ≥ 30 nmol/l were found for 94% of self-reported daily smokers, 66% of occasional smokers and 2% of non-smokers. Adding the self-reported non-smokers with cotinine concentrations above cut-off increased the daily smoking prevalence from 9% to 11%. The sensitivity and specificity for self-reported daily smoking using 30 nmol/l as cut-off were 82% and 99%, respectively. CONCLUSIONS: These findings suggest that self-reported tobacco use is a valid marker for tobacco exposure in the MoBa cohort.