Measuring Distress in the Presence of Substance Use: A Differential Item Functioning Analysis of the Kessler 10 Psychological Distress Scale

When comparing scores on psychological distress scales across people who use substances versus those who do not, it is essential that the psychometric characteristics of the measure across both groups are comparable. The current study investigated the presence of differential item functioning (DIF) in the Kessler 10 psychological distress scale (K10) among people who do and do not regularly use alcohol and other drugs. Data were from the 2019 National Drug Strategy Household Survey, a stratified, multistage random sample of Australian private households. Those aged between 18 and 79 with complete data were included in the analysis (n = 19339). A multiple indicator multiple cause (MIMIC) modelling approach was used to determine uniform and non-uniform DIF. The analysis identified indicators of tiredness, restlessness, psychomotor agitation, depression, and sadness that demonstrated significant levels of either uniform or non-uniform DIF across substance use groups. The overall impact of the identified DIF across groups was minimal. While there was minimal impact of DIF on overall K10 scores, the presence of DIF might reflect differences in the fundamental experience of psychological distress among those who use substances regularly. These differences could prove to be important when implementing universal prevention or early interventions for psychological distress.