Background: Depression is a leading cause of disability. International guidelines recommend screening for depression and the Patient Health Questionnaire 9 (PHQ-9) has been identified as the most reliable screening tool. We reviewed the evidence for using it within the primary care setting. Methods: We retrieved studies from MEDLINE, Embase, PsycINFO, CINAHL and the Cochrane Library that carried out primary care-based depression screening using PHQ-9 in populations older than 12, from 1995 to 2018. Results: Forty-two studies were included in the systematic review. Most of the studies were cross-sectional (N=40, 95%), conducted in high-income countries (N=27, 71%) and recruited adult populations (N=38, 90%). The accuracy of the PHQ-9 was evaluated in 31 (74%) studies with a two-stage screening system, with structured interview most often carried out by primary care and mental health professionals. Most of the studies employed a cut-off score of 10 (N=24, 57%, total range 5 – 15). The overall sensitivity of PHQ-9 ranged from 0.37 to 0.98, specificity from 0.42 to 0.99, positive predictive value from 0.09 to 0.92, and negative predictive value from 0.8 to 1. Limitations: Lack of longitudinal studies, small sample size, and the heterogeneity of primary-care settings limited the generalizability of our results. Conclusions: PHQ-9 has been widely validated and is recommended in a two-stage screening process. Longitudinal studies are necessary to provide evidence of long-term screening effectiveness.
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