Bilirubin is a yellow-orange pigment produced when haemoglobin from aged or damaged red blood cells is broken down in the spleen and liver. It is transported to the liver, where it is conjugated (made water-soluble) and excreted in bile. Bilirubin gives bile its yellow-green colour and is responsible for the yellow colour of urine and the brown colour of faeces.

Elevated bilirubin (hyperbilirubinaemia) causes jaundice — yellowing of the skin, whites of the eyes, and mucous membranes. The source of the elevation helps identify the underlying cause: pre-hepatic (excessive red blood cell breakdown), hepatic (liver disease), or post-hepatic (bile duct obstruction). Total bilirubin is routinely measured as part of liver function testing.