Physiological Significance

Bilirubin is a metabolite of haem, derived from haem-containing proteins, including myoglobin, haemoglobin and various P450 enzymes, that serves to coordinate iron in various proteins. Half of it is excreted into bile and the rest contributes to blood bilirubin turnover ¹. It is mainly produced in the spleen and liver but can also occur elsewhere in the body by macrophages and renal tubular cells 2. The internal hydrogen bonding causes it to be water-insoluble and so required enzyme-mediated glucuronidation in the liver for biliary excretion. Under normal conditions, it is mostly unconjugated and bound tightly to albumin ³.

Clinical Significance

Bilirubin is one of the most common diagnostic tests utilised in the diagnosis and monitoring of hepatic dysfunction. It is impaired in cholestatic and parenchymal liver diseases. The most common condition associated with elevated levels is jaundice (a clinical sign of hyperbilirubinaemia), characterised by the yellow colouring of skin, mucous membranes and sclera ³.

Today, it is recognised as being more than just an end-product and considered to be a fundamental substance. Bilirubin is recognised as acting as an antioxidant and anti-inflammatory agent in serum. Evidence exists that states that it can neutralise free radicals, prevent peroxidation of lipids and protects the cardiovascular system, hepatobiliary system, immune system, neuronal system and pulmonary system ⁴.

COVID-19

It has been established that bilirubin levels are significantly elevated in COVID-19 patients ⁵. Patients with abnormal liver function tests were at a significantly higher risk of progressing to a severe disease, such as pneumonia. The presence of abnormal liver function tests became more pronounced during hospitalisation within two weeks, with total bilirubin levels elevated 3 times the upper normal limit ⁶.