conjugated bilirubin

Bilirubin, a breakdown product of heme (from senescent RBCs via macrophages), is initially unconjugated (indirect), lipid-soluble, and bound to albumin for transport to the liver. In hepatocytes, UDP-glucuronosyltransferase (UGT1A1) conjugates bilirubin with glucuronic acid to form conjugated (direct) bilirubin, which is water-soluble. This conjugated form is actively secreted into bile canaliculi (via MRP2 transporter) and excreted into the intestine, where gut bacteria convert it to urobilinogen (further oxidized to stercobilin, giving stool its brown color; some urobilinogen is reabsorbed and excreted in urine).

Clinical significance: Elevated conjugated bilirubin (>50% of total, or absolute >0.3-0.4 mg/dL) suggests post-hepatic or intrahepatic cholestasis/obstruction, or defective excretion, since the liver has already processed bilirubin but cannot excrete it into bile. Causes include: - Extrahepatic obstruction: gallstones, pancreatic/cholangiocarcinoma, strictures - Intrahepatic cholestasis: primary biliary cholangitis, primary sclerosing cholangitis, drug-induced cholestasis, sepsis - Hepatocellular dysfunction: viral hepatitis, cirrhosis (mixed pattern) - Genetic defects in excretion: Dubin-Johnson syndrome (MRP2 mutation, black liver) and Rotor syndrome (impaired storage/transport, no liver pigmentation)

Because conjugated bilirubin is water-soluble, it can be excreted in urine, causing dark (tea-colored) urine and is measurable via urine dipstick—useful in distinguishing conjugated (bilirubinuria present) from unconjugated hyperbilirubinemia (bilirubinuria absent, as seen in hemolysis or Gilbert/Crigler-Najjar syndromes).

Lab interpretation: Total bilirubin = conjugated + unconjugated. Predominantly unconjugated hyperbilirubinemia suggests increased production (hemolysis) or impaired conjugation (Gilbert syndrome, Crigler-Najjar). Predominantly conjugated hyperbilirubinemia suggests impaired excretion/cholestasis as above.