C4A and immunoglobulin G4-related sclerosing disease: Similar to the correlations observed in patients with elevated serum IgG4 concentrations in our study, a previous study consisting of 72 patients with IgG4-RD (68 [94.4%] patients with elevated serum IgG4 concentrations) showed that serum IgG4 concentration, serum IgG concentration, C3, and C4 correlate with IgG4-RD RI (17).