Background: The Illness Intrusiveness Ratings Scale (IIRS) measures the extent to which disease or its treatment or both interfere with activities in important life domains. Before comparing IIRS scores within or across groups it is crucial to determine whether a common underlying factor structure exists across patient populations.
Objective: To investigate the factor structure underlying the IIRS and evaluate its stability across diagnoses.
Methods: IIRS responses from 5,671 respondents were pooled from 15 separate studies concerning quality of life in eight patient groups: rheumatoid arthritis; osteoarthritis; systemic lupus erythematosus; multiple sclerosis; end-stage renal disease (maintenance dialysis); renal transplantation; heart, liver, and lung transplantation; and insomnia. Data were gathered by different methods (eg, interview, self-administered, mail survey) and in diverse contexts (eg, individual vs. group).
Results: Exploratory maximum-likelihood factor analysis identified three underlying factors in a randomly selected subset of respondents (n = 400), corresponding to "Relationships and Personal Development," "Intimacy," and "Instrumental" life domains. Confirmatory factor analysis corroborated the stability of this structure in an independent subsample (n = 2100). Complementary goodness-of-fit indices confirmed the consistency of the three-factor solution, corroborating that IIRS scores are uniquely defined across patient populations. Coefficient alpha was high for total and subscale scores.
Conclusions: IIRS scores can be compared meaningfully within and across patient groups. Both total and subscale scores can be used depending on research objectives.