Related tests: AST, ALT, Alkaline phosphatase, GGT
What is it?
Bilirubin is a breakdown product of haemoglobin, formed when red blood cells are destroyed by the spleen. Bilirubin itself is insoluble (it is called indirect bilirubin) in the watery environment of the blood, so to be carried to the liver it must bind to albumin. Once taken up by the liver, bilirubin is conjugated with two molecules of glucuronic acid, becoming soluble and so able to be excreted in the bile. In the intestine, conjugated (or direct) bilirubin is converted by bacteria into bilinogen, which helps give stools their colour. However, some bilinogen is reabsorbed into the circulation and taken up by the liver to be excreted again in the bile (enterohepatic circulation of bilinogen), or is excreted in the urine as urobilinogen (see urine, chemical and physical examination).
A rise in serum indirect bilirubin can be caused by overproduction (haemolytic diseases) or by a defect in uptake by the liver (physiological neonatal jaundice, Gilbert’s syndrome type I, intrahepatic or extrahepatic hepatocellular damage) or, more rarely, by a defect in the conjugation of bilirubin with glucuronic acid (e.g. Crigler-Najjar jaundice types I and II).
A rise in serum direct bilirubin can be due, rarely, to a defect in excretion into the bile ducts (Dubin-Johnson jaundice, Rotor jaundice) or, more often, to cholestasis of intrahepatic origin (hepatitis, cirrhosis) or extrahepatic origin (bile ducts blocked by gallstones or compressed by pancreatic cancer).
Clinical indications
Jaundice, monitoring of liver disorders.
Sample type
The patient must have a blood sample taken.
Preparation
You must fast for at least 8 hours; a small amount of water is allowed. You must have been standing or sitting upright for at least 30 minutes.
Reference values
Total bilirubin: 0.10 – 1.30 mg/dL
Direct bilirubin: < 0.3 mg/dL
Indirect bilirubin: < 1.2 mg/dL
Notes
Direct bilirubin is measured only when total bilirubin is raised.