Measurement invariance of assessments of depression (PHQ-9) and anxiety (GAD-7) across sex, strata and linguistic backgrounds in a European-wide sample of patients after Traumatic Brain Injury

Background: The Patient Health Questionnaire-9 (PHQ-9) and the Generalized Anxiety Disorder (GAD-7) are two widely used instruments to screen patients for depression and anxiety. Comparable psychometric properties across different demographic and linguistic groups are necessary for multiple group comparison and international research on depression and anxiety. Objectives and Method: We examine measurement invariance for the PHQ-9 and GAD-7 by: (a) the sex of the participants, (b) recruitment stratum, and (c) linguistic background. This study is based on non-randomized observational data six months after Traumatic Brain Injury (TBI) that were collected in 18 countries. We used multiple methods to detect Differential Item Functioning (DIF) including Item Response Theory, logistic regression, and the Mantel-Haenszel method. Results: At the 6-month post-injury, 2137 (738 [34.5%] women) participants completed the PHQ-9 and GAD-7 questionnaires: 885 [41.4%] patients were primarily admitted to the Intensive Care Unit (ICU), 805 [37.7%] were admitted to hospital ward, and 447 [20.9%] were evaluated in the Emergency Room and discharged. Results supported the invariance of PHQ-9 and GAD-7 across sex, patient strata and linguistic background. For different strata three PHQ-9 items and one GAD-7 item and for different linguistic groups only two GAD-7 items were flagged as showing differences in two out of four DIF tests. However, the magnitude of the DIF effect was negligible. Limitation: Despite high number of participants from ICU, patients have mostly mild TBI. Conclusion: The findings demonstrate adequate psychometric properties for PHQ-9 and GAD-7, allowing direct multigroup comparison across sex, strata, and linguistic background.