Bilirubin is the orange-yellow pigment responsible for the physical appearance of jaundice and gives bile it’s yellow/green colour. It is made from the haemoglobin in red blood cells and is formed when old red blood cells break down as part of a normal healthy process. Around 300mg of bilirubin is produced per day as a breakdown product of haem (iron containing molecule) from blood haemoglobin.
Bilirubin is transported from the blood to liver bound to a protein called albumin. The liver is responsible for extracting bilirubin from the blood and it is excreted into bile duct and stored in the gallbladder. The main function of the bile is to facilitate fat digestion and absorption in the small intestine and is ultimately excreted in the stool, where they are responsible for the the brown colouration of the faeces.
A normal liver is able to process and excrete bilirubin. It is measured to assess liver function and check for liver disease, such as hepatitis or cirrhosis, or to monitor the effects of medicines that can damage the liver. It is also useful in identifying if something is blocking the bile ducts such as gallstones. Bilirubin measurements also allow the severity of jaundice to be quantified.
A raised blood level of bilirubin occurs in various liver and bile duct conditions. Increases in bilirubin levels result in the classic jaundice symptoms of yellowing of the skin and whites of the eyes. The most common cause of jaundice is a blockage in bile flow (cholestasis) for example by gallstones, where the bilirubin levels will be particularly high as it cannot be excreted into the urine or stool.
Bilirubin can also be raised with when the liver is damaged as by hepatitis, liver injury, or long-term alcohol abuse. Another cause of raised bilirubin levels is excessive breakdown of red blood cells (haemolysis) for example, in haemolytic anaemia. A mildly raised bilirubin can also occur in people with Gilbert's syndrome which is a common, harmless, genetic condition at least 5% of the UK population. Gilbert’s syndrome doesn’t cause an increased risk of liver disease and the jaundice experienced with this condition is usually mild and short lived. The jaundice in this condition can be triggered by fasting, dehydration, physical exertion, menstruation, surgery or infection.