Equating the PHQ-9 and GAD-7 to the HADS depression and anxiety subscales in patients with major depressive disorder

Objective: The present study aimed to equate the 9-item Patient Health Questionnaire (PHQ-9) and 7-item Generalized Anxiety Disorder Scale (GAD-7) to the Hospital Anxiety and Depression Scale (HADS) depression and anxiety subscales (HADS-D,HADS-A) respectively in patients with major depressive disorder (MDD) and generate crosswalks of raw scores. Methods: As it is a single group design that adopts common-person equating method, a total of 460 patients with MDD completed the PHQ-9, GAD-7 and HADS at the same time. Rasch analysis was used to filter out invalid participants, investigate the psychometric properties of test items and participants, link the PHQ-9 and HADS-D as well as GAD-7 and HADS-A, and produce conversion tables respectively. The differences between original scores and converted scores were analyzed to validate the crosswalks. Result: 401 samples of depression part and 396 samples of anxiety part were left for final samples. Both the PHQ-9 / HADS-D and GAD-7 / HADS-A combined analysis adequately fit the unidimensional Rasch model, demonstrated acceptable reliability and item-person targeting and showed no disordering category. Slight differential item functioning across gender was found in item PHQ9 and item GAD6. The crosswalks were generated and verified to be validity. Limitations: The results might be restricted to patients with MDD recruited in a single mental health center. Conclusion: The PHQ-9, GAD-7 and HADS depression and anxiety subscales were successfully linked, producing conversion tables that could be used for directly converting raw score from one instrument to the other.