The liver is the largest internal organ in the body. It is involved in processing dietary amino acids, carbohydrates, lipids, and vitamins. It also metabolizes cholesterol and toxins, produces clotting factors, and stores glycogen. As well, the liver is the principal site for the synthesis of all circulating proteins (except gamma globulins).
Four biomarkers are routinely measured on a liver function panel:
- Liver enzymes
- Prothrombin and partial thromboplastin time
Let’s look at each of these tests in a bit more detail.
Bilirubin is a waste product that is produced from the breakdown of red blood cells. Heme (derived from the breakdown of hemoglobin) ultimately becomes unconjugated bilirubin, which is a water-insoluble molecule that is bound to serum albumin in the blood.
Unconjugated bilirubin is taken up by the liver, where it is converted to conjugated bilirubin. Conjugated bilirubin is water-soluble and can be excreted in bile.
Six liver enzymes are commonly measured in a liver function panel:
- Alanine aminotransferase (ALT)
- Aspartate aminotransferase (AST)
- Alkaline phosphatase (ALP)
- Gamma-glutamyl transpeptidase (GGT)
- Lactate dehydrogenase (LDH)
Prothrombin and partial thromboplastin time
The coagulation cascade is divided into the extrinsic, intrinsic, and common pathways. Prothrombin time (PT) is one way of measuring how long it takes a patient’s blood to form a clot. It is a common measurement included in a liver function panel and assesses the integrity of the extrinsic and common coagulation pathways.
Partial thromboplastin time (PTT) is another coagulation test. It measures the integrity of the intrinsic and common coagulation pathways.
Except for factor VIII, all blood clotting factors are produced in the liver—including prothrombin (which is also called factor II). Therefore, when liver function is significantly impaired (in the acute or chronic setting), clotting factor levels will be reduced.
Albumin is the most common protein found in the blood. It is used by the body for growth and tissue repair. When liver function is impaired over a prolonged period, albumin synthesis is also impaired, which results in low levels of albumin in the blood. For this reason, hypoalbuminemia is a common finding in chronic (but not acute) liver failure.
What do these liver function tests tell us?
Biomolecules such as ALT, AST, ALP, and bilirubin are markers of liver injury. So, a change in their levels suggests a pathological process causing liver damage or injury.
In contrast, albumin, bilirubin, PT, and PTT are markers of hepatocellular function. This means that changes in these biomarkers suggest an altered or reduced function of the liver cells.
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