Equivalence of measurement properties of the Dermatology Life Quality Index (DLQI) across 13 European countries

The Dermatology Life Quality Index (DLQI) is the most widely used tool to measure burden of skin diseases and assess intervention effectiveness. It is embedded in national guidelines and disease registries in > 45 countries and available in > 138 translations. This study examined psychometric properties of the DLQI across 13 European languages. 3,408 patients had completed the DLQI questionnaire with no missing data. Confirmatory factor analysis (CFA) was performed using datasets of each country and multigroup CFA (MGCFA) across all countries. Known-group analysis was performed of DLQI sum-score against physician-assessed severity by country. All countries’ DLQI data showed essential unidimensionality with good fits to a 1-factor CFA model with CFI, TLI unscaled values all > 0.97 (COSMIN criteria). MGCFA indicated different country translations of the DLQI conceptualize the same construct and showed equality of factor loadings. Although, scalar invariance was not seen, meaning that direct comparison of mean DLQI scores across 12 countries is not supported by the current results, partial scalar invariance was achieved after constraining one threshold for DLQI item 7. Known-group analysis of DLQI versus disease severity was significant for every country (all p < 0.025). Good psychometric properties were present across all 13 languages examined with essential unidimensionality confirmed. The DLQI differentiated between levels of disease severity and EQ-5D-3 L levels across all these languages. This adds further confidence in the appropriateness of using the DLQI with this range of languages.

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