Liver Function Tests
Liver function tests (liver tests) are laboratory markers of liver function and liver damage. Basic liver tests include determination of four liver enzymes that may be supplemented with evaluation of bilirubin. Liver enzymes are substances occurring in the liver cells, and we are interested in the following:
- AST (aspartate transaminase)
- ALT (alanine transaminase)
- GGT (gamma-glutamyl transpeptidase)
- ALP (alkaline phosphatase)
We are interested in concentration of these enzymes in blood. This concentration is expressed in international units per liter [IU/ Liter]. Elevated levels of these enzymes in the blood may indicate liver damage and failure to properly drain bile from the liver. According to the elevation of enzymes, we can approximately estimate the severity of liver damage.
Generally, the values of AST, ALT and GGT should be approximately below 50 IU per liter. The exact values are not as important as every laboratory has slightly different ranges. More accurate values are stated below:
- AST: under 40 IU/Liter
- ALT: under 40 IU/Liter
- GGT: under 85 IU/Liter in males and 55 IU/L in females
- ALP: under 140 IU/L
Method of detection
The enzymes can be evaluated from a sample of venous blood, which is sent to a biochemical laboratory. The examination is quick, easy and cheap.
Elevated liver function tests indicate that something is wrong with the liver tissue. The most common cause is a liver inflammation or liver damage by toxic substances. The possible causes include chronic alcoholism, infectious hepatitis, gallstones complications, liver cancer, liver metastases, liver congestion in right-side heart failure, autoimmune hepatitis, primary biliary cirrhosis, primary sclerosing cholangitis, hemochromatosis, Wilson's disease and many, many more. Liver damage usually causes elevation of all four main enzymes, but some may be elevated more than the others. Let's say some information about particular enzymes.
GGT (with ALP) is elevated mainly in obstructions of biliary tract and impaired bile outflow. In addition, GGT elevation is also typical for chronic alcoholics. When such patient is admitted to hospital, one's GGT value is high, but later it decreases (because the patient does not have usually access to alcohol when hospitalized).
Along with GGT, ALP enzyme is increased in disorders of the bile outflow. When we find only isolated elevation of ALP, we should be careful, because it may mean other pathology than liver damage. A form of alkaline phosphatase is also present in bones and its elevation may mark their diseases (including bone metastases).
ALT and AST
These are two basic enzymes that inform us about the damage of the liver cells. The ratio of these enzymes are also important - If ALT elevation is higher than AST, it suggests rather slighter damage and on the contrary, higher AST elevation is more commonly present in severe liver damage.
Bilirubin is not a liver enzyme, but it is an important supplement to liver function tests. Bilirubin is a part of blood pigment hemoglobin, which is present in red blood cells. When the red blood cells break up, the bilirubin is released, metabolized by liver ("conjugated") and excreted by bile into the intestine. The level of bilirubin increases in liver damage when the liver tissue is unable to metabolize or excrete the bilirubin. High level of bilirubin causes its accumulation in tissues, which manifests with jaundice. Normal level of bilirubin in blood is below 2.1 mg/dL. The visible yellowish discoloration of tissues appears in level of bilirubin above 3.0 mg/dL.