Background SLE and MCTD are both chronic immune mediated systemic diseases with similar clinical features. We wanted to compare characteristics and morbidity in addition to mortality prediction models in our large and population based cohorts of SLE and MCTD.

Method 243 SLE patients from the Oslo SLE cohort and 145 patients from the Norwegian MCTD cohort were included in the study. Clinical features were based on questionnaires and medical records in the SLE cohort and examination by protocol in MCTD patients. Vital status at the end of the study was obtained from the National Population Register of Norway. Cox regression analyses were used to find the predictive factors of mortality. Variables at a significant level of P less than 0.25 where considered a candidate in the prediction model by manual backward elimination procedure in addition to known mortality predictors.

Results SLE patients were more often affected by nephritis and leukopenia, while the proportion of Raynaud’s phenomenon and Interstitial Lung Disease (ILD) was larger in MCTD (table 1). More males were diagnosed with MCTD. 25 patients died in the SLE cohort after a mean follow-up of 9 (2) years. 26 patients died in the MCTD cohort after a mean (SD) follow-up of 10 (3) years. Predictors of mortality in multivariable analyses were Lupus Nephritis class III to VI, and having had a myocardial infarct, stroke or Transient Intermittent Attack after adjustments to age, gender and disease duration (table 2). The predictors of mortality in the MCTD cohort were% ILD of Total Lung Volume after age and gender adjustments (table 3). According to the Harrell’s C index, patient outcomes were accurately predicted by the SLE multivariable model 85% of the time and 84% in the MCTD model.

Conclusions SLE and MCTD are similar in many aspects, but differ in disease manifestations that have an impact on mortality, indicating that different follow-up approaches and management is needed.