The factor structure of the PHQ-9 in palliative care.
OBJECTIVES
The Primary Care Evaluation of Mental Disorders Patient Health Questionnaire (PRIME-MD PHQ-9) is a common screening tool designed to facilitate detection of depression according to DSM-IV criteria. However, the factor structure of the PHQ-9 within the palliative care population has not been evaluated.


METHODS
300 participants completed the PHQ-9 within one week of referral to a palliative care service. Participants completed the PHQ-9 again four weeks later (n=213). Confirmatory factor analysis (CFA) and multiple-group CFA were undertaken to test the factor structure of the PHQ-9 and evaluate model invariance over time.


RESULTS
A two-factor model comprising somatic and cognitive-affective latent factors provided the best fit to the data. Multiple-group CFA suggested model invariance over time. Structural equation modelling revealed that follow-up (time 2) cognitive-affective and somatic symptoms were predicted by their baseline (time 1) factors.


CONCLUSIONS
The PHQ-9 measures two stable depression factors (cognitive-affective and somatic) within the palliative care population. Studies are now required to examine the trajectories of these symptoms over time in relation to clinical intervention and events.