Agreement between self-reported smoking and cotinine concentration in adolescents: a validation study in Brazil

To validate self-reported data on smoking in adolescents in comparison with cotinine concentration.

Two thousand two hundred nine seventh- and eighth-grade students from 32 public schools in Pelotas, Brazil. Adolescents were contacted twice--before and after an educational intervention--and samples of urine for cotinine analyses were taken. In this paper, only data from the baseline phase are presented. High-performance liquid chromatography was used for cotinine analysis. Two cutoff points for cotinine were used: 10 ng/mL and 30 ng/mL. Two self-reported smoking variables were used: at least one cigarette smoked in the previous 30 days; and daily smoking.

The self-reported prevalence of smoking in the previous 30 days was 7.4%, and 0.9% of the adolescents reported to be daily smokers. Those who reported smoking in the previous 30 days presented mean cotinine values 10 times greater than those who reported to be nonsmokers. Using a cutoff of 10 ng/mL for cotinine, sensitivity of self-reported smoking was 16.3% (95% confidence interval [CI] 11.7; 20.9) and specificity was 93.6% (95% CI 92.8; 95.0). Using a cutoff of 30 ng/mL, sensitivity was 22.6% (95% CI 15.6; 29.6) and specificity was 93.7% (95% CI 92.6; 94.8).

Self-reported smoking presents low agreement with cotinine concentration, suggesting that adolescents underestimate tobacco consumption.