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Tree analysis modeling of the associations between PHQ-9 depressive symptoms and doctor diagnosis of depression in primary care

Published online by Cambridge University Press:  26 April 2018

Weng-Yee Chin*
Department of Family Medicine & Primary Care, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong
Eric Yuk Fai Wan
Nuffield Department of Population Health, University of Oxford, Oxford, UK
Christopher Dowrick
Institute of Psychology Health and Society, University of Liverpool, Liverpool, UK
Bruce Arroll
Department of General Practice and Primary Health Care, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand
Cindy Lo Kuen Lam
Department of Family Medicine & Primary Care, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong
Author for correspondence: Weng-Yee Chin, E-mail:



The aim of this study was to explore the relationship between patient self-reported Patient Health Questionnaire-9 (PHQ-9) symptoms and doctor diagnosis of depression using a tree analysis approach.


This was a secondary analysis on a dataset obtained from 10 179 adult primary care patients and 59 primary care physicians (PCPs) across Hong Kong. Patients completed a waiting room survey collecting data on socio-demographics and the PHQ-9. Blinded doctors documented whether they thought the patient had depression. Data were analyzed using multiple logistic regression and conditional inference decision tree modeling.


PCPs diagnosed 594 patients with depression. Logistic regression identified gender, age, employment status, past history of depression, family history of mental illness and recent doctor visit as factors associated with a depression diagnosis. Tree analyses revealed different pathways of association between PHQ-9 symptoms and depression diagnosis for patients with and without past depression. The PHQ-9 symptom model revealed low mood, sense of worthlessness, fatigue, sleep disturbance and functional impairment as early classifiers. The PHQ-9 total score model revealed cut-off scores of >12 and >15 were most frequently associated with depression diagnoses in patients with and without past depression.


A past history of depression is the most significant factor associated with the diagnosis of depression. PCPs appear to utilize a hypothetical-deductive problem-solving approach incorporating pre-test probability, with different associated factors for patients with and without past depression. Diagnostic thresholds may be too low for patients with past depression and too high for those without, potentially leading to over and under diagnosis of depression.

Original Articles
Copyright © Cambridge University Press 2018 

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