C4A and primary biliary cholangitis: However, serum C4 concentrations in patients with cirrhotic PBC were significantly lower compared to healthy control subjects (6.1 ± 2.6 vs. 18.0 ± 3.0 ng/ml; p < 0.05) and to patients with non-cirrhotic PBC (6.1 ± 2.6 vs. 15.7 ± 3.8 ng/ml; p < 0.05) (Fig 1B).