Urine bilirubin testing helps differentiate which condition?

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Urine bilirubin testing looks for direct (conjugated) bilirubin in the urine. Conjugated bilirubin is water-soluble, so it appears in the urine when bilirubin is reaching the bloodstream due to cholestasis. If the bile duct is completely blocked, bile cannot reach the gut, so conjugated bilirubin builds up in the blood and is excreted by the kidneys, giving a positive urine bilirubin test. If the obstruction is incomplete, some bile still reaches the gut and is converted to urobilinogen, so there may be little or no bilirubin in the urine. This is why the test helps distinguish complete versus incomplete duct obstruction.