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Introduction: The Swedish version of the patient-reported Clinical Outcomes in Routine Evaluation Outcome Measures (CORE-OM) has demonstrated high reliability and acceptable convergent validity in explanatory factor analyses. However, the fundamental scale properties have not yet been validated according to item response theory. The aim of this study was to analyze the measurement properties of the Swedish CORE-OM in a cohort of psychiatric out-patients with depression and anxiety in a multicultural area and to explore combinations of items based on shorter versions of the scale (CORE-10, CORE-6D) to improve measurement properties.

Methods: Data from CORE-OM assessments of 337 patients were analyzed using Rasch analysis. The patients had a mean age of 30 +/- 14 years, the majority were women (72%). Requirements for measurement properties were checked: overall model fit, item fit residuals, targeting, internal consistency, differential item functioning and thresholds. Sensitivity to change was also analyzed.

Results: The CORE-OM showed high internal consistency (person separation index = 0.947) and adequate targeting, but there was overall model misfit (item trait interaction chi(2) = 917.53, p < 0.001), indication of local dependency, and differential item functioning in 9 items. The risk items showed problems with disordered thresholds. The emotional component of the shorter CORE-6D showed the best fit for our sample. Adding 3 items to include depressive and trauma-related content resulted in a unidimensional 8-item set with acceptable reliability, model fit, targeting and sensitivity to change.

Conclusion: For out-patients with diagnosed depression or anxiety in a multicultural area, the Swedish CORE-OM showed high internal consistency, but also validity problems. Based on the shorter CORE-6D version, a unidimensional 8-item set could be an alternative brief measure of psychological distress for this population, but further validity studies are required. Qualitative studies exploring the CORE-OM items in non-native speakers are also warranted.