Background: The 2004 National Institute for Health and Clinical Excellence (NICE) guidelines highlight the importance of assessing severity of depression in primary care.
Aim: To assess the psychometric properties of the Patient Health Questionnaire (PHQ-9) and the depression subscale of the Hospital Anxiety and Depression Scale (HADS-D) for measuring depression severity in primary care.
Design of study: Psychometric assessment.
Setting: Thirty-two general practices in Grampian, Scotland.
Method: Consecutive patients referred to a primary care mental health worker completed the PHQ-9 and HADS at baseline (n = 1063) and at the end of treatment (n = 544). Data were analysed to assess reliability, robustness of factor structure, convergent/discriminant validity, convergence of severity banding, and responsiveness to change.
Results: Both scales demonstrated high internal consistency at baseline and end of treatment (PHQ-9 alpha = 0.83 and 0.92; HADS-D alpha = 0.84 and 0.89). One factor emerged each for the PHQ-9 (explaining 42% of variance) and HADS-D (explaining 52% of variance). Both scales converged more with each other than with the HADS anxiety (HADS-A) subscale at baseline (P<0.001) and at end of treatment (P = 0.01). Responsiveness to change was similar: effect size for PHQ-9 = 0.99 and for the HADS-D = 1. The HADS-D and PHQ-9 differed significantly in categorising severity of depression, with the PHQ-9 categorising a greater proportion of patients with moderate/severe depression (P<0.001).
Conclusion: The HADS-D and PHQ-9 demonstrated reliability, convergent/discriminant validity, and responsiveness to change. However, they differed considerably in how they catergorised severity. Given that treatment decisions are made on the basis of severity, further work is needed to assess the validity of the scales' severity cut-off bands.