In obstruction, conjugated bilirubin cannot reach the gut, so no pigment is made there and the stools lose their brown colour. The same conjugated bilirubin backs up into the blood instead. Because conjugation has made it water-soluble, the kidneys can filter it, and it appears in the urine and turns it dark. One blockage, two opposite colours, both explained by where the bilirubin goes when it cannot go forwards.
Step by step
Normally, conjugated bilirubin leaves the liver in bile and reaches the intestine. There, bacteria convert it to urobilinogen. Most of that becomes stercobilin, the pigment that makes stool brown. A small fraction is reabsorbed, and a little of that is excreted by the kidneys as urobilin, which gives normal urine its yellow colour.
Block the bile duct and two things happen at once:
- Nothing reaches the gut. No urobilinogen is made, no stercobilin is formed, and the stool is pale, putty-coloured and may be greasy and hard to flush — because bile acids are missing too, so fat is not being absorbed.
- Bilirubin backs up into the blood. It is already conjugated, so it is water-soluble, not bound to albumin, and freely filtered at the glomerulus. It spills into the urine and darkens it to the colour of strong tea.
The key fact underneath both
Unconjugated bilirubin never appears in the urine. It is fat-soluble and travels tightly bound to albumin, so it cannot be filtered. Only the conjugated form can. That single fact is what lets a urine dipstick separate haemolysis from obstruction at the bedside.
What the dipstick tells you
| Urine bilirubin | Urine urobilinogen | Stool | |
|---|---|---|---|
| Haemolysis | Negative | Increased | Normal or dark |
| Hepatocellular | Positive | Normal or increased | Normal |
| Complete obstruction | Positive | Absent | Pale |
In haemolysis more bilirubin than usual reaches the gut, so more urobilinogen is made and reabsorbed, and the urine urobilinogen rises — while the urine bilirubin stays negative because the excess in the blood is unconjugated. In complete obstruction nothing reaches the gut at all, so urobilinogen disappears from the urine entirely. Those two rows are mirror images, and examiners like them for exactly that reason.
Ward pearl
Patients notice the urine before they notice the yellow eyes, because dark urine appears at a lower bilirubin level than visible scleral icterus. "When did your urine change colour?" often dates the illness more accurately than "when did you go yellow?"
What else goes with it
- Itch, from retained bile acids and other pruritogens, often before the jaundice is visible.
- Steatorrhoea and deficiency of vitamins A, D, E and K, because without bile acids in the gut there are no micelles to absorb fat.
- A rising INR that usually corrects with vitamin K — the reasoning is here.