From the Guidelines
No, bilirubin is not a liver enzyme. Bilirubin is a yellowish pigment that is produced when the body breaks down old red blood cells, as stated in the guidelines on the management of abnormal liver blood tests 1. It is processed by the liver and eventually excreted in bile and urine. While bilirubin levels are commonly measured in liver function tests, it is a waste product rather than an enzyme. The actual liver enzymes typically measured include alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), and gamma-glutamyl transferase (GGT) 1.
Key Points to Consider
- Bilirubin exists in two forms, unconjugated and conjugated, and is transported to the liver in its insoluble unconjugated form 1
- Elevations of either fraction will lead to a rise in the measured bilirubin concentration 1
- The most common cause of an isolated elevated bilirubin concentration is Gilbert’s syndrome, which is an inherited disorder of metabolism 1
- Elevated bilirubin levels in the blood can indicate liver dysfunction or other conditions affecting the processing of this pigment, such as hemolytic anemia or bile duct obstruction 1
Clinical Implications
Understanding the distinction between bilirubin and liver enzymes is important for correctly interpreting liver function test results 1. In clinical practice, it is crucial to consider the different forms of bilirubin and their implications for liver function and disease diagnosis. For instance, if the majority of the elevated bilirubin comprises the unconjugated fraction, the cause is virtually always Gilbert’s syndrome, and patients should be fully reassured 1. However, in neonates and infants, a high or prolonged increase in total bilirubin may be pathological and requires urgent assessment 1.
From the Research
Definition of Bilirubin and Liver Enzymes
- Bilirubin is a tetrapyrrolic compound originating from heme catabolism, and it has been found to have various biological functions in the human body 2.
- Liver enzymes, including aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), gamma-glutamyl transpeptidase (GGT), and total bilirubin (TBIL), are conventional biomarkers for liver disease 3.
Relationship Between Bilirubin and Liver Function
- Bilirubin is not typically considered a liver enzyme, but rather a waste product of heme catabolism that is processed by the liver 4.
- Elevated levels of bilirubin can indicate liver dysfunction or disease, such as jaundice or liver cirrhosis 5.
- Low levels of bilirubin, on the other hand, have been associated with metabolic dysfunction and increased risk of cardiovascular complications 4, 2.
Biomarkers for Liver Disease
- Bilirubin, along with other liver enzymes such as AST and ALT, can be used as a biomarker for liver disease 3.
- Other biomarkers, such as HMGB1, cytokeratin 18 (K18), glutathione-S-transferase-α (GST-α), glutamate dehydrogenase (GLDH), malate dehydrogenase (MDH), and microRNAs (miRNA), are also being developed to detect liver disease 3.