Abstract
BACKGROUND: Individuals with long-term physical health conditions (LTCs) experience higher rates of depression and anxiety. Conventional self-report measures do not distinguish distress related to LTCs from primary mental health disorders. This difference is important as treatment protocols differ. We developed a transdiagnostic self-report measure of illness-related distress, applicable across LTCs. METHODS: The new Illness-Related Distress (IRD) scale was developed through thematic coding of interviews, systematic literature search, think-aloud interviews with patients and healthcare providers, and expert-consensus meetings. An internet sample (n = 1,398) of UK-based individuals with LTCs completed the IRD scale for psychometric analysis. We randomly split the sample (1:1) to conduct: (1) an exploratory factor analysis (EFA; n = 698) for item reduction, and (2) iterative confirmatory factor analysis (CFA; n = 700) and exploratory structural equation modeling (ESEM). Here, further item reduction took place to generate a final version. Measurement invariance, internal consistency, convergent, test-retest reliability, and clinical cut-points were assessed. RESULTS: EFA suggested a 2-factor structure for the IRD scale, subsequently confirmed by iteratively comparing unidimensional, lower order, and bifactor CFAs and ESEMs. A lower-order correlated 2-factor CFA model (two 7-item subscales: intrapersonal distress and interpersonal distress) was favored and was structurally invariant for gender. Subscales demonstrated excellent internal consistency, very good test-retest reliability, and good convergent validity. Clinical cut points were identified (intrapersonal = 15, interpersonal = 12). CONCLUSION: The IRD scale is the first measure that captures transdiagnostic distress. It may aid assessment within clinical practice and research related to psychological adjustment and distress in LTCs.