Validity of the Fagerström test for nicotine dependence among persons with intellectual disabilities: Cross-sectional evidence from the POWER PID study

Introduction: Tobacco use and the screening of nicotine dependence using the Fagerström test among adults with intellectual disabilities (ID) is an understudied public health issue. Despite lower reported prevalence relative to the general population, methodological challenges - particularly the suitability of self-reported tools - limit reliable measurement. This study assesses the convergent validity of the Fagerström test for nicotine dependence (FTND) by comparing it with exhaled carbon monoxide (CO), an objective biomarker of recent smoking, in a sample of adults with ID. Methods: Data were collected from an observational cross-sectional validation study during 2024, which investigates addictive behaviors, health indicators, and physical activity among adults with ID in northern France. Nurses administered a standardized questionnaire, including the FTND, and measured exhaled CO using a portable analyzer. Associations were examined using correlation analyses and univariable and multivariable linear regressions controlling for sociodemographic and health variables. Results: Among 35 people who smoke, FTND scores were positively and strongly correlated with exhaled CO (r=0.540, p<0.001). In adjusted regression models, FTND remained a significantly associated factor of CO levels (0.60; 95% CI: 0.17-1.02), alongside age (0.065; 95% CI: 0.00-0.13), employment in sheltered workshops (1.86; 95% CI: -0.07-3.78), and cognitive support needs (-1.62; 95% CI: -3.51-0.26). Analyses conducted on the full sample, including 68 people who do not smoke, yielded consistent results, with FTND remaining the strongest predictor of exhaled CO. Conclusions: The results suggest evidence of validity of the FTND in adults with ID, suggesting that self-report instruments can reliably estimate nicotine dependence when administered with appropriate support. Nevertheless, the interpretation of CO levels in adults with ID should account for age, cognitive support needs, and contextual factors.