The present study examines the concept of illness identity, the degree to which a chronic illness is integrated into one's identity, in adults with a chronic illness by validating a new self-report questionnaire, the Illness Identity Questionnaire (IIQ). Self-report questionnaires on illness identity, psychological, and physical functioning were assessed in two samples: adults with congenital heart disease (22-78 year old; n = 276) and with multisystem connective tissue disorders (systemic lupus erythematosus or systemic sclerosis; 17-81 year old; n = 241). The IIQ could differentiate four illness identity states (i.e., engulfment, rejection, acceptance, and enrichment) in both samples, based on exploratory and confirmatory factor analysis. All four subscales proved to be reliable. Rejection and engulfment were related to maladaptive psychological and physical functioning, whereas acceptance and enrichment were related to adaptive psychological and physical functioning. The present findings underscore the importance of the concept of illness identity. The IIQ, a self-report questionnaire, is introduced to measure four different illness identity states in adults with a chronic illness.
Keywords: Chronic illness; Congenital heart defects; Illness identity; Multisystem connective tissue disorders; Psychological functioning.