King's College London

Background: The PHQ-9 is a self-administered depression screening instrument. Little is known about its utility and accuracy in detecting depression in adults with dissociative seizures (DS). Objectives: Using the Mini – International Neuropsychiatric Interview as a reference, we evaluated the diagnostic accuracy of the PHQ-9 in adults with DS, and examined its convergent and discriminant validity and uniformity. Methods: Our sample comprised 368 people with DS who completed the pre-randomisation assessment of the CODES trial. The uniformity of the PHQ-9 was determined using factor analysis for categorical data. Optimal cut-offs were determined using the area under the curve (AUC), Youden Index, and diagnostic odds ratio (DOR). Convergent and discriminant validity were assessed against pre-randomisation measures. Results: Internal consistency of the PHQ-9 was high (α = 0.87). While the diagnostic odds ratio suggested that a cut-off of ≥10 had the best predictive performance (DOR = 14.7), specificity at this cut off was only 0.49. AUC (0.74) and Youden Index (0.48) suggested a ≥ 13 cut-off would yield an optimal sensitivity (0.81) and specificity (0.67) balance. However, a cut-off score of ≥20 would be required to match specificity resulting from a cut-off of ≥13 in other medical conditions. We found good convergent and discriminant validity and one main factor for the PHQ-9. Conclusions: In terms of internal consistency and structure, our findings were consistent with previous validation studies but indicated that a higher cut-off would be required to identify DS patients with depression with similar specificity achieved with PHQ-9 screening in different clinical and non-clinical populations.