Bilirubin

Bilirubin is the predominant end-product of haem catabolism, formed when reticuloendothelial cells degrade senescent red blood cells; it circulates as unconjugated (indirect) bilirubin bound to albumin before hepatic uptake, conjugation with glucuronic acid, and biliary excretion. Elevated total bilirubin is classified as pre-hepatic (haemolysis), hepatic (hepatocellular dysfunction), or post-hepatic (biliary obstruction) based on the unconjugated-to-conjugated ratio and accompanying enzymes. Mildly elevated unconjugated bilirubin in isolation — as in Gilbert syndrome — is generally benign and may even confer antioxidant protection, since bilirubin is a lipid-soluble radical scavenger. Severely elevated bilirubin is a marker of advanced liver failure and a component of prognostic scores such as MELD.