Screening for depression: Rasch analysis of the dimensional structure of the PHQ-9 and the HADS-D.

Background: Both the depression modules of the Hospital Anxiety and Depression Scale (HADS-D) and the Patient Health Questionnaire (PHQ-9) are widely used for the screening of depression. We analyzed the dimensionality and the item fit of both scales individually and across the scales. Moreover, we sought to identify items which evidenced item response bias associated with age and gender.

Methods: The depression subscales HADS-D and the PHQ-9 were administered to 1271 patients (mean age 67.2; 22.5% women) undergoing coronary artery bypass graft surgery (CABG). Rasch analyses were performed to assess the overall fit of the model, individual item fit and differential item functioning (DIF).

Results: Rasch analysis revealed that the HADS-D and the PHQ-9 feature a common core construct containing six items of the HADS-D and three items of the PHQ-9. Two of these items are identical with the 2-item short form of the PHQ-9. In addition, fatigability was the only somatic item that fitted the model. No substantial DIF was observed.

Limitations: The generalizability of these results might be restricted to patients awaiting CABG.

Conclusions: The short form of the PHQ-9 seems to be an economic and valid instrument for the screening of depression, which indicates the same latent construct that is captured by six items of the HADS-D. Further studies are needed to evaluate whether the addition of fatigability might enhance the validity of the PHQ-2 in this patient population.