Conjugated (direct) bilirubin is the water-soluble form of bilirubin that has been processed by the liver — attached to glucuronic acid to make it suitable for excretion in bile. In a healthy person, almost all bilirubin in the blood is unconjugated, as the liver rapidly processes and excretes conjugated bilirubin into the bile.

Elevated conjugated bilirubin is a marker of post-hepatic or hepatic causes of jaundice — including bile duct obstruction (from gallstones, tumours, or strictures), primary biliary cholangitis, and viral or drug-induced hepatitis. Because conjugated bilirubin is water-soluble, it can be excreted in urine when blood levels are high, causing dark-coloured urine — a classic early sign of obstructive jaundice.