Misclassification of self‐reported smoking in adult survivors of childhood cancer

We investigated misclassification rates, sensitivity, and specificity of self‐reported cigarette smoking through serum cotinine concentration (liquid chromatography tandem mass spectrometry) among 287 adult survivors of childhood cancer. Overall, 2.5–6.7% and 19.7–36.9% of the self‐reported never and past smokers had cotinine levels indicative of active smoking. Sensitivity and specificity of self‐reported smoking were 57.5–67.1% and 96.6–99.2%. Misclassification was associated with younger age (OR = 3.2; 95% CI = 1.4–7.4), male (OR = 2.1; 95% CI = 1.1–4.0), and past (OR = 2.7; 95% CI = 1.3–5.8) or current (OR = 2.6; 95% CI = 1.0–6.6) marijuana use. After adjusting for tobacco‐related variables, current marijuana use remained a significant risk for misclassification. Clinicians/researchers should consider bio‐verification to measure smoking status among survivors.