A statistical analysis of the interrelationships between disease activity in different systems in systemic lupus erythematosus

Objectives. To develop models for disease activity in patients with systemic lupus erythematosus (SLE) and to examine the hypothesis that possible subsets exist within the disease, notably renal disease and little else, mucocutaneous and musculoskeletal disease in isolation and more multisystem disease. Methods. Four hundred and forty patients with SLE were followed for a period of 10 yr. Socio-demographic data were obtained at the first visit with disease activity being recorded at subsequent visits and damage scores at 6-monthly intervals. Prognostic factors for active disease in each of the mucocutaneous, musculoskeletal and renal systems were examined statistically. The results were then validated using data collected over 5 yr on a further 295 SLE patients from a different centre. Results. Logistic regression analyses indicated that for all three systems studied a patient known to have an involvement in that system is more likely to present with active disease in that same system than a patient with no known prior involvement. Patients with a higher frequency of clinic visits with active disease in a system are more likely to represent with active disease than those with fewer visits. The results suggest that renal disease is most likely to occur on its own. Associations between activity in the mucocutaneous and musculoskeletal systems support the suggestion that patients with musculoskeletal and mucocutaneous disease alone represent a possible subset of SLE. None of the associations identified were modified by the medication a patient received. Conclusions. Previous disease history and involvement of other systems determine a patient's chance of developing further episodes of active disease in SLE.