Bilirubin

Bilirubin is a breakdown product of haem (heme in American English). Haem is a special ring shaped molecule that is found in haemoglobin. The haem ring holds the iron atoms of haem and is essential to the oxygen carrying capacity of blood.

Bilirubin is essentially a waste product, formed when haemoglobin is broken down. It is not soluble in water and is carried to the liver bound onto albumin. Bilirubin is made water soluble in the liver by conjugation with glucuronic acid[?]. The soluble bilirubin then passes through the biliary tract[?] to the gallbladder and then through the common bile duct into the gut. Some of the excreted bilirubin may be reabsorbed (entero-hepatic circulation). Bacteria in the intestines modify bilirubin, causing the brown color of feces.

Bilirubin blood tests

Bilirubin is found in blood either bound to albumin ("indirect") or in the soluble form ("direct"). The terms "direct" and "indirect" refer to the fact that soluble bilirubin can be measured directly, whereas insoluble, or indirect, bilirubin must be solubilised before measurement. Bilirubin is broken down by UV light, so blood tubes (especially serum tubes) should be protected from light.

Although both direct and indirect bilirubin can be measured separately, it is more common to just measure total bilirubin. When we try to further elucidate the causes of jaundice or increased bilirubin it is usually simpler to look at other liver function tests (especially the enzymes ALT, AST, GGT[?], Alk Phos[?]), blood film[?] examination (haemolysis etc.) or evidence of infective hepatitis (e.g. Hepatitis A, B, C, delta E etc).

Bilirubin is basically an excretion product and the body does not control levels. Bilirubin levels reflect the balance between production and excretion. Thus strictly speaking there is not a normal level of bilirubin.

The reference range for total bilirubin is 2 - 14 μmol/L. For direct bilirubin, it is 0 - 4 μmol/L.

Mild rises in bilirubin:

• Haemolysis or increased breakdown of blood.
• Gilberts syndrome[?] - slightly increased bilirubin due to an inherited protein (trasport protein?) deficiency. This has no clinical significance.

Moderate rise in bilirubin:

• Drugs (especially anti-psychotic[?], some sex hormones, and a wide range of other drugs).
• Hepatitis (levels may be moderate or high).
• Cirrhosis (may be normal, moderate or high, depending on exact features of the cirrhosis)

Very High levels:

• Neonatal[?] hyperbilirubinaemia (see jaundice).
• Usually large bile duct obstruction, eg stone in common bile duct, tumour obstructing common bile duct etc.
• Severe liver failure with cirrhosis
• Severe hepatitis.