In this analysis also, serum levels of C4 differed significantly from those of controls (controls: median = 0.029 μmol/L, range = 0.004–0.146 μmol/L; combined MetS and T2DM: median = 0.054 μmol/L, range = 0.003–0.266 μmol/L; p = 0.003). Here, C4A is linked to type 2 diabetes mellitus.