C4A and irritable bowel syndrome: The median level of C4 in active IBD patients was 53.1 ng/mL, IBD remission was 52.2 ng/mL, IBD after surgery was 85.7 ng/mL, IBS-D was 7.5 ng/mL, microscopic colitis was 69.3 ng/mL, and healthy controls was 3.7 ng/mL.