Measuring Depression-Severity in Critically Ill Patients' Families with the Patient Health Questionnaire (PHQ): Tests for Unidimensionality and Longitudinal Measurement Invariance, with Implications for CONSORT

Abstract Context Families of intensive care unit patients are at risk for depression and are important targets for depression-reducing interventions. Multi-item scores for evaluating such interventions should meet criteria for unidimensionality and longitudinal measurement invariance. The Patient Health Questionnaire (PHQ), widely used for measuring depression severity, provides standard nine-, eight-, and two-item scores. However, published studies often report no (or weak) evidence of these scores' unidimensionality/invariance, and no tests have evaluated them as measures of depression severity in intensive care unit patients' families. Objectives To identify multi-item PHQ constructs with promise for evaluating change in depression severity among family members of critically ill patients. Methods Structural equation models with rigorous fit criterion (χ2 , P ≥ 0.05) tested the standard nine-, eight-, and two-item PHQ, and other item subsets, for unidimensionality and longitudinal invariance, using data from a trial evaluating an intervention to reduce depressive symptoms in family members. Results Neither the standard nine-item nor the eight-item PHQ construct showed longitudinal invariance, although the standard two-item construct and other item subsets did. Conclusion The longer eight- and nine-item PHQ scores appear inappropriate for assessing depression severity in this population, with constructs based on smaller subsets of items being more promising targets for future trials. The Consolidated Standards of Reporting Trials requirement for prespecified trial outcomes is problematic because unidimensionality/invariance testing must occur after trial completion. Consolidated Standards of Reporting Trials could be strengthened by endorsing rigorous assessment of composite scores and encouraging use of the most appropriate substitute, should trial-based evidence challenge the legitimacy of prespecified multi-item scores.