Objective: To examine the internal consistency and validity of the Systemic Lupus International Collaborating Clinics/American College of Rheumatology Damage Index (SDI) with respect to disease activity, health status, and medication score.
Methods: A prospective cross sectional study of patients with systemic lupus erythematosus (SLE) attending a specialist lupus outpatient clinic between July 1994 and February 1995. The internal consistency of the SDI components was examined using Cronbach's coefficient alpha. The associations of the SDI components with disease activity measured by the British Isles Lupus Assessment Group (BILAG) index, health status measured by the Medical Outcomes Study (MOS) Short Form 20, and with a medication score were analyzed using Spearman's rank correlation coefficient (p).
Results: 133 women and 8 men ranging in age from 20.1 to 88.7 years (mean 41.1, SD 12.5) were studied. With few exceptions, the components of the SDI that reflect damage in different organ systems were not associated with each other. We found a significant although weak relationship between some related SDI and BILAG components (p 0.25 to 0.28; p < 0.01). While damage to the musculoskeletal system was associated with limitations in physical functioning measured with the MOS Short Form 20 (p-0.30; p < 0.01) and renal damage inversely with fatigue (p-0.23; p < 0.01) there was no significant relationship of other SDI components with the MOS Short Form 20. Renal and neuropsychiatric damage were associated significantly with the medication score (p 0.27 and 0.23; p < 0.01).
Conclusion: The components of the SDI are valid in that they are associated with disease activity in the respective organ systems and some of them with a medication score. However, damage in different organ systems in SLE does not follow a common pattern. It is thus suggested that the SDI profile be used in addition to the SDI total score as an endpoint in clinical and epidemiological studies.