Can severity of substance use be measured across drug classes? Estimating differential item functioning by drug class in two general measures of substance use severity

Background: Substance use severity is frequently measured using generic (i.e., non-drug specific) items. Yet, the measurement properties of these items must be evaluated for measurement invariance across inidividuals who use differing substances to ensure total scores can be compared across groups. Method: This study used data from two independent samples (n₁ = 474; n₂ = 5183) and two measures of general substance use severity with generic items, the Patient Reported Outcomes Measurement Information System (PROMIS) Severity of Substance Use and DAST-10, to examine for differential item functioning (DIF) across substances (i.e., sedatives, opioids, amphetamines, cocaine, and cannabis). We utilized moderated nonlinear factor analysis to estimate DIF. Finally, we compared factor scores across estimation methods with and without accounting for DIF to examine the impact of DIF. Results: A minority of items showed statistically significant DIF in each scale (Items with DIF: PROMIS Sample 1: 5/37; PROMIS Sample 2: 7/20; DAST-10 Sample 2: 3/10). Factor scores across scoring methods showed extremely high correlations (0.994 – 0.999), estimates of mean differences across substance groups did not vary considerably across scoring methods, but measurement differences were correlated with factor scores. Discussion: These findings suggest that these two measures of substance use severity can be used across individuals using different substances. Factor scores appear similar across scoring methods and mean differences do not appear to be substantially biases. Measures with generic items may offer a parsimonious alternative to measures with drug specific items but more research is needed to evaluate the robustness of these findings.