Is the Fagerström test for nicotine dependence invariant across secular trends in smoking? A question for cross-birth cohort analysis of nicotine dependence

Background: The Fagerström Test for Nicotine Dependence (FTND), a derivation of the Fagerström Tolerance Questionnaire, was first published in 1991. The FTND remains one of the most widely used measures of nicotine dependence for studying genetic and epidemiological risk factors and the likelihood of smoking cessation. However, it is unclear whether secular trends in patterns of smoking alter the psychometric properties of the FTND and its interpretation. Methods: We examined measurement invariance in the lifetime and current FTND scores across birth cohorts using participants drawn from six study samples (N = 13,775). Results: We found significant (p < 0.05) measurement non-invariance in means and factor loadings of most FTND items by birth cohort, but effect sizes, ranging from r² = 0.0001 to r² = 0.0035, indicated that less than 0.5% of the model variance was explained by the measurement non-invariance for each factor loading. To assess its impact, we regressed the lifetime FTND latent variable on well-established factors associated with nicotine dependence (quitting smoking and the nicotinic acetylcholine receptor gene [CHRNA5] variant rs16969968, separately), and we observed that the regression coefficients were unchanged between models with and without adjustment for measurement non-invariance. Conclusions: These findings suggest that possible FTND non-invariance that occurs across study samples of various birth years has a negligible impact on study results.