Baseline factors defining immune complex detection in systemic lupus erythematosus: development of a three-factor score in a retrospective cohort study

Objective Immune complexes (ICs) play a central role in the pathogenesis of systemic lupus erythematosus (SLE). We previously showed that detectable ICs predicted clinical remission off corticosteroids (CR off C), also referred to as glucocorticoid-free clinical remission. Because the C1q-based IC assay is no longer commercially available in Japan, we aimed to identify clinical and serological predictors of IC detection, develop a simple score, and explore the association between this score and CR off C. Methods We retrospectively evaluated two cohorts derived from patients with active SLE treated at a single center. The evaluation cohort included patients followed for ≥ 1 year to assess CR off C, whereas the derivation cohort consisted of patients who had circulating ICs measured. Logistic regression with Firth correction was used to identify independent baseline predictors of IC detection. These factors were then used to develop an exploratory score for IC detection. The association between the score and CR off C was assessed using Kaplan–Meier analysis and Cox proportional hazards modeling. Results Among the evaluation cohort ( n = 70), 34% achieved CR off C during a median follow-up of 4.4 years. In the derivation cohort ( n = 61), three independent predictors of IC detection were identified: anti-dsDNA antibody ≥ 57.5 IU/mL, low C3 (<68.5 mg/dL), and male sex. A three-factor score (0–3 points) showed increasing frequencies of IC detection (0: 6.3%; 1: 11.8%; 2: 91.7%; 3: 100%) and was associated with achievement of CR off C (log-rank p = 0.04; crude hazard ratio per factor 1.71, 95% CI 1.04–2.80). Conclusion A simple three-factor score was associated with IC detection and a higher likelihood of achieving CR off C in our cohort of patients with SLE. In settings where direct measurement of ICs is unavailable, this score may serve as a potential surrogate for identifying patients likely to have detectable ICs and for assessing their likelihood of achieving glucocorticoid-free clinical remission.