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

INTRODUCTION

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 the MoBa study who delivered infants during the period 2002-2003. Self-reported tobacco use (test variable) and plasma cotinine concentrations (gold standard) were assessed at approximately gestational week 18.

RESULTS

Daily smoking was reported by 9% of the women, occasional smoking by 4%, and nonsmoking by 86% of the women. Sensitivity and specificity for self-reported smoking status were calculated using a cotinine cut-off estimated from the study population (30 nmol/l). Plasma cotinine concentrations ≥30 nmol/l were found in 94% of self-reported daily smokers, 66% of occasional smokers, and 2% of nonsmokers. After the numbers of self-reported nonsmokers with cotinine concentrations above the cut-off limit were added, the daily smoking prevalence increased from 9 to 11%. The sensitivity and specificity for self-reported daily smoking, using 30 nmol/l as the cut-off concentration, were 82 and 99%, respectively.

DISCUSSION

These findings suggest that self-reported tobacco use is a valid marker for tobacco exposure in the MoBa cohort.