Bilirubinuria

Bilirubinuria is the presence of bilirubin in the urine. In healthy individuals, urine contains little or no bilirubin. Its detection usually indicates that conjugated (direct) bilirubin has entered the bloodstream and is being excreted by the kidneys, or that bile flow to the gut is impaired, allowing conjugated bilirubin to spill into urine.

Biochemically, bilirubin is produced from heme breakdown. Unconjugated bilirubin is not water-soluble and is carried in the blood bound to albumin. It is taken up by the liver and conjugated with glucuronic acid to form water-soluble bilirubin diglucuronide, which is excreted in bile. When there is significant conjugated bilirubin in the serum or cholestasis (obstruction of bile flow), bilirubin can appear in urine. Hepatocellular disease can also cause bilirubinuria by impaired processing or excretion of conjugated bilirubin. Unconjugated bilirubin in urine is uncommon because it is not water-soluble.

Common causes of bilirubinuria include biliary obstruction (gallstones, tumors, strictures), intrahepatic cholestasis from drugs or liver disease, and other forms of hepatocellular dysfunction that disrupt bilirubin excretion. Hemolysis alone usually increases unconjugated bilirubin in serum and may not produce bilirubinuria unless conjugated bilirubin also rises.

Diagnosis relies on urinalysis, where a positive bilirubin test is typically due to conjugated bilirubin. This finding is interpreted in the context of serum bilirubin fractions (direct/directed), liver function tests, and imaging studies to identify the underlying cause. Management targets the underlying biliary or hepatic disorder.