bilirubin
Meaning of bilirubin: The same pigment gives bile its color, stool its brown and a fading bruise its yellow, all from one breakdown product.
Definition of bilirubin
Bilirubin is a yellow-orange pigment formed when the body dismantles hemoglobin from red blood cells that have reached the end of their life. The liver removes it from the blood, converts it into a water soluble form and passes it into bile, which drains to the intestine. A buildup in the blood turns skin and eyes yellow.
Where does it come from and where does it go?
Red blood cells last a few months. When one is broken down, the iron in its hemoglobin is salvaged and reused, and the ring shaped molecule that held the iron is converted, in two steps, into bilirubin. That form does not dissolve in water, so it travels through the blood attached to a carrier protein. The liver takes it up, links it to a sugar derivative to make it water soluble, and secretes it into bile. Bile drains into the intestine, where bacteria transform it further into the compounds that give stool its brown color; a small share is reabsorbed and leaves in the urine, which is why urine is yellow.
Why does a report show two figures?
Laboratories measure both the water soluble form the liver has already processed and the total, then work out the unprocessed portion by subtraction. Which of the two is raised localizes the problem along the path above, and that is the single most useful thing the test does.
| Pattern | Where the problem lies | Examples |
|---|---|---|
| Mostly unprocessed | Too much pigment being made, or the liver not conjugating it fast enough | Rapid red cell breakdown; the immature liver of a newborn; Gilbert syndrome, a common and harmless inherited variation |
| Mixed, with liver enzymes raised | The liver cells themselves are damaged | Viral hepatitis, alcohol related liver disease, drug injury |
| Mostly processed, with pale stool and dark urine | Bile cannot drain out of the liver | A stone in the bile duct, or a tumor pressing on it |
What does jaundice look like, and when does it matter?
Yellowing appears first in the whites of the eyes and then in the skin, and jaundice becomes visible only once the level is well above normal, so a modest rise is detected by testing rather than by looking. In newborns the pattern has a particular importance: the unprocessed form can cross into the brain, which the processed form cannot, and that is the reason newborn jaundice is measured rather than judged by eye. Itching often accompanies the obstructive pattern, and pale stools with dark urine together point toward blocked drainage rather than a problem inside the liver cells.
Used in a sentence
Laboratory testing showed an elevated bilirubin level with a predominance of the conjugated fraction.
Good to know
Distinguishing whether elevated bilirubin is mostly unconjugated or conjugated helps indicate whether the underlying problem lies in the rate of red blood cell breakdown, the liver's processing of bilirubin, or drainage of bile from the liver.
Word origin
Where “bilirubin” comes from
Formed from bili-, from Latin bilis, meaning bile, and rubin, from Latin ruber, meaning red.