Bilirubin, Direct

The Bilirubin, Direct test (conjugated bilirubin, direct bilirubin) is used to assess liver and biliary tract disorders and to categorize hyperbilirubinemia. Elevations are seen with impaired bile drainage both within the liver and in the extrahepatic ducts, and may also occur with hepatocellular injury such as hepatitis or cirrhosis. Cholestatic drug reactions, Dubin‑Johnson syndrome, and Rotor syndrome can present with increased direct bilirubin; in the latter two hereditary disorders, values are generally less than 5 mg/dL. Results are interpreted alongside total bilirubin to judge whether a bilirubin increase is more consistent with bile duct obstruction versus primary hepatic disease.

Bilirubin is an orange‑yellow bile pigment generated largely from hemoglobin breakdown during clearance of senescent erythrocytes. The unconjugated (indirect) fraction travels to the liver bound to albumin, where it is conjugated to a water‑soluble form (direct, conjugated bilirubin) that is secreted into bile and delivered to the intestine. Direct bilirubin increases when conjugation is intact but biliary excretion is reduced, as occurs in cholestasis or mechanical obstruction of the bile ducts. Concentrations may also rise with hepatocellular injury, including hepatitis and cirrhosis. With impaired bile flow, stools can become pale (acholic) and urine may darken due to enhanced renal excretion of conjugated bilirubin.