Bilirubin

Bilirubin is an orange-yellow pigment, a waste product primarily produced by the normal breakdown of haem, which is a component of a protein called haemoglobin. Haemoglobin is found in red blood cells and gives them their characteristic red colour and is used to carry oxygen round the body. Bilirubin is ultimately processed by the liver to allow its elimination from the body. This test measures the amount of bilirubin in the blood to evaluate a person's liver function or to help diagnose anaemia caused by excessive red blood cell destruction.

The normal life-span of red blood cells in the circulation is 120 days. When they are broken down the haem is initially released from the haemoglobin and then converted to unconjugated bilirubin. The unconjugated bilirubin is not water-soluble so it is carried by proteins in the blood to the liver. Within the liver, the unconjugated bilirubin has sugars attached to form water soluble conjugated bilirubin. This is secreted into bile and carried to the intestine where bacteria break it down, eventually producing the brown pigment that colours normal stools.

A small amount (approximately 250 to 350 milligrams) of bilirubin is produced daily in a normal, healthy adult. Normally, small amounts of unconjugated bilirubin are found in the blood, but virtually no conjugated bilirubin is present. Both forms can be measured by the laboratory tests but total bilirubin result (a sum of these) is usually reported.

How is the sample collected for testing?

In adults, blood is collected by needle from a vein in the arm. In newborns, a few drops of blood are usually collected from a heel-prick. Sometimes in newborns bilirubin is estimated by placing an instrument on the skin (a transcutaneous bilirubin meter).

Is any test preparation needed to ensure the quality of the sample?

No test preparation is necessary. The blood sample should ideally be protected from bright light before analysis.