Validation and internal consistency of Patient Health Questionnaire-9 for major depression in Parkinson's disease

Objective: we assessed the discriminant validity of the PHQ-9 in order to establish the best cut-off score for the diagnosis of major depression in PD patients. Method: one hundred and ten patients with a diagnosis of PD without dementia were evaluated with the Structured Clinical Interview for DSM-IV (SCID), considered as the gold standard for the diagnosis of major depression. Eighty-four PD patients completed the PHQ-9, the 15-item Geriatric Depression Scale (GDS-15) and the Zung Self-rating Depression Scale (SDS). Results: the prevalence of current depression in the sample of PD patients was 25.5%. Maximal discrimination between depressed and non-depressed patients was reached with a cut-off score of 9 in the PHQ-9 (sensitivity of 100% and specificity of 83.1%). The internal consistency of the scale was 0.83 and, when used as a diagnostic instrument, the PHQ-9 had a sensitivity of 52.6% and specificity of 95.4%. The correlation coefficient between the PHQ-9 and the other two scales was 0.63. Conclusions: the PHQ-9 is an adequate instrument for the screening-but not diagnosis-of depression in PD patients, with optimal sensitivity and specificity attained with a cut-off score of 9.