C4A and irritable bowel syndrome: Additionally, significant differences in the serum levels of C4 were found between IBS patients (median, [27.40 mg/dL]; range, [11.20 mg/dL–61.80 mg/dL]) and HC group (median, [23.80 mg/dL]; range, [13.70 mg/dL–34.20 mg/dL]; p < 0.001).