1 Lady Davis Institute for Medical Research, Jewish General Hospital, Montréal, Québec, Canada; Department of Epidemiology, Biostatistics and Occupational Health, McGill University, Montréal, Québec, Canada.
2 Department of Epidemiology, Biostatistics and Occupational Health, McGill University, Montréal, Québec, Canada; Respiratory Epidemiology and Clinical Research Unit, McGill University Health Centre, Montréal, Québec, Canada; Department of Medicine, McGill University, Montréal, Québec, Canada.
3 Stanford Prevention Research Center, Department of Medicine, Stanford University School of Medicine, Stanford, CA, USA; Department of Health Research and Policy, Stanford University School of Medicine, Stanford, CA, USA; Department of Biomedical Data Science, Stanford University School of Medicine, Stanford, CA, USA; Department of Statistics, Stanford University School of Humanities and Sciences, Stanford, CA, USA.
4 Lady Davis Institute for Medical Research, Jewish General Hospital, Montréal, Québec, Canada.
5 Lady Davis Institute for Medical Research, Jewish General Hospital, Montréal, Québec, Canada; Department of Epidemiology, Biostatistics and Occupational Health, McGill University, Montréal, Québec, Canada; Department of Psychiatry, McGill University, Montréal, Québec, Canada.
6 Lady Davis Institute for Medical Research, Jewish General Hospital, Montréal, Québec, Canada; Department of Psychology, McGill University, Montréal, Québec, Canada.
7 Lady Davis Institute for Medical Research, Jewish General Hospital, Montréal, Québec, Canada; Department of Mental Health, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD, USA.
8 Schulich Library of Physical Sciences, Life Sciences, and Engineering, McGill University, Montréal, Québec, Canada.
9 Department of Clinical, Neuro and Developmental Psychology, Amsterdam Public Health Research Institute, Vrije Universiteit Amsterdam, Amsterdam, the Netherlands.
10 Hull York Medical School and the Department of Health Sciences, University of York, Heslington, NY, UK.
11 Library, Concordia University, Montréal, Québec, Canada.
12 Department of Mental Health, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD, USA.
13 Department of Community Health Sciences, University of Calgary, Calgary, Alberta, Canada; Hotchkiss Brain Institute and O'Brien Institute for Public Health, University of Calgary, Calgary, Alberta, Canada.
14 Lady Davis Institute for Medical Research, Jewish General Hospital, Montréal, Québec, Canada; Department of Epidemiology, Biostatistics and Occupational Health, McGill University, Montréal, Québec, Canada; Department of Family Medicine, McGill University, Montréal, Québec, Canada.
15 Department of Medicine, Johns Hopkins University School of Medicine, Baltimore, MD, USA.
16 Faculty of Medicine, King Abdulaziz University, Jeddah, Makkah, Saudi Arabia.
17 Department of Rehabilitation Medicine, University of Washington, Seattle, WA, USA.
18 Louis and Gabi Weisfeld School of Social Work, Bar Ilan University, Ramat Gan, Israel.
19 Endocrine Research Center, Institute of Endocrinology and Metabolism, Iran University of Medical Sciences, Tehran, Iran; Ageing Clinical & Experimental Research Team, Institute of Applied Health Sciences, School of Medicine, Medical Sciences and Nutrition, University of Aberdeen, Aberdeen, Scotland, UK.
20 Kbo-Lech-Mangfall-Klinik Garmisch-Partenkirchen, Klinik für Psychiatrie, Psychotherapie & Psychosomatik, Lehrkrankenhaus der Technischen Universität München, Munich, Germany.

Objectives: Depression symptom questionnaires are not for diagnostic classification. Patient Health Questionnaire-9 (PHQ-9) scores =10 are nonetheless often used to estimate depression prevalence. We compared PHQ-9 =10 prevalence to Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders (SCID) major depression prevalence and assessed whether an alternative PHQ-9 cutoff could more accurately estimate prevalence.

Study design and setting: Individual participant data meta-analysis of datasets comparing PHQ-9 scores to SCID major depression status.

Results: A total of 9,242 participants (1,389 SCID major depression cases) from 44 primary studies were included. Pooled PHQ-9 =10 prevalence was 24.6% (95% confidence interval [CI]: 20.8%, 28.9%); pooled SCID major depression prevalence was 12.1% (95% CI: 9.6%, 15.2%); and pooled difference was 11.9% (95% CI: 9.3%, 14.6%). The mean study-level PHQ-9 =10 to SCID-based prevalence ratio was 2.5 times. PHQ-9 =14 and the PHQ-9 diagnostic algorithm provided prevalence closest to SCID major depression prevalence, but study-level prevalence differed from SCID-based prevalence by an average absolute difference of 4.8% for PHQ-9 =14 (95% prediction interval: -13.6%, 14.5%) and 5.6% for the PHQ-9 diagnostic algorithm (95% prediction interval: -16.4%, 15.0%).

Conclusion: PHQ-9 =10 substantially overestimates depression prevalence. There is too much heterogeneity to correct statistically in individual studies.