Kim AHJ, et al. - Correlations between blood levels of complement split product iC3b and serum component C3 with clinically meaningful changes in disease activity in patients with systemic lupus erythematosus (SLE) were examined. In CASTLE (Complement Activation Signatures in Systemic Lupus Erythematosus), a prospective observational study, researchers analyzed a total of 159 consecutive patients with SLE, identifying a correlation of the ratio of blood iC3b to serum C3 concentrations with the extent of SLE disease activity and with clinically meaningful changes in disease activity in patients with SLE. Furthermore, they identified a possible utility of iC3b:C3 ratio to discriminate between active and inactive SLE, and between major flares and no active disease.