Validation of the Danish translation of the illness intrusiveness rating scale in patients with rheumatic disease

Rheumatic diseases significantly affects quality of life, making self-management critical. The Illness Intrusiveness Rating Scale (IIRS) measures the extent to which illness interferes with various life domains, but until now it has not been translated into Danish. This study aimed to translate the IIRS into Danish and assess its psychometric properties in Danish patients with a rheumatic disease. Following COSMIN guidelines, the IIRS was translated and culturally adapted through a multi-step process, including forward and backward translations and cognitive interviews. Psychometric testing included assessments of internal construct validity by confirmatory factor analysis, internal consistency by Cronbach’s α, reliability by test–retest, standard error of measurement, and responsiveness by minimal detectable change. The final Danish version was well-understood, though minor issues arose, such as the relevance of “religious expression.” The scale demonstrated high internal consistency (Cronbach’s α = 0.92). Confirmatory factor analysis confirmed the original three-factor structure with an acceptable model fit (comparative fit index = 0.94, the Root Mean Square Error of Approximation = 0.10). Strong correlations were found within “Relationships and Personal Development” and “Instrumental” domains, but the “Diet” item did not meet factor assignment criteria. Test–retest reliability was acceptable (intraclass correlation coefficients ≥ 0.70 for most items). The Danish IIRS showed strong psychometric properties, making it a reliable and valid tool for assessing illness impact and self-management interventions in Danish patients with rheumatic disease.