The factor structure of the patient health questionnaire-9 in stroke: a comparison with a non-stroke population

Background There are concerns that the measurement of depression by the Patient Health Questionnaire-9 (PHQ-9), a self-report screening questionnaire, is biased by comorbid stroke sequelae. We, therefore, aimed to investigate these concerns in stroke, benchmarked against a non-stroke comparison sample, using factor analysis. Methods The secondary data sample constituted 787 stroke and 12,016 non-stroke participants, in a cross-sectional design. A subsample of 1,574 non-stroke participants was selected via propensity score matching. Dimensionality was assessed by comparing fit statistics of one-factor, two-factor, and bi-factor models. Between-group differences in factor structure were identified using measurement invariance. Results A two-factor model, consisting of somatic and cognitive-affect factors, had a superior fit to a unidimensional model (CFI = .984 versus CFI =.974, p<.001), but the high correlation between the factors indicated unidimensionality (r = .866). Configural invariance between stroke and non-stroke was supported (CFI = .983, RMSEA = .080), as were invariant thresholds (p = .092) and loadings (p = .103) for all items. Strong invariance was violated (p < .001, ΔCFI = -.003), indicating non-invariant item intercepts. Partially invariant models indicated responsibility of the tiredness and appetite intercepts, and latent depression severity was significantly overestimated in stroke, relative to the general population, using a summed score approach (Cohen’ s d= .434). Conclusions The findings suggest that the PHQ-9 measures a single latent factor in stroke. However, the presence of non-invariant intercepts means that PHQ-9 total scores may be disproportionately influenced by fatigue in post-stroke vs. non-stroke patients and that total scores are incomparable between groups. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement No external funding was provided ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Not Applicable The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This study was approved by the University of East Anglia Faculty of Medicine and Health Research Ethics Committee (UEA FMH REC) on 11th August 2021 (approval number: 2020/21-046). Participation consent had been provided to primary authors. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Not Applicable I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Not Applicable I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Not Applicable Secondary data were acquired under specific authorisation for the primary author (data usage agreements). Thus, we do not own the data, and any request for data would first need to be approved by all co-authors, and this may require additional data protection approval.