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Dr Allister J Grant

MB ChB(B'ham) PhD FRCP

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Liver Function Tests
 
Different cells have different enzymes inside them, depending on the function of the cell. Liver cells happen to have lots of AST, ALT, and GGT inside them. When cells die or are damaged, the enzymes leak out causing the blood level of these enzymes to rise; that is why the levels of these enzymes in the blood are considered good indicators of liver cell damage.  GGT and ALP are said to be more specific for evaluating biliary disease since they are made in bile duct cells. In liver disease caused by excess alcohol ingestion, the AST tends to exceed the ALT, while the reverse is true for viral hepatitis. 
 
1. Alanine aminotransferase (ALT) and Aspartate aminotransferase (AST)
These are enzymes that help to process proteins. (An enzyme is a protein that helps to speed up chemical reactions. Various enzymes occur in the cells in the body.) Large amounts of ALT occur in liver cells. When the liver is injured or inflamed (as in hepatitis), the blood level of ALT usually rises.ALT is more specific for liver disease than AST because AST is found in more types of cell (e.g. heart, intestine, muscle). The AST, for instance, will rise after a heart attack or bruised kidney.
 2) Alkaline Phosphatase:
This enzyme level is elevated in a large number of disorders that affect the drainage of bile, such as a gallstone or a tumor blocking the common bile duct, or alcoholic liver disease, or drug-induced hepatitis, blocking the flow of bile in smaller bile channels within the liver. The alkaline phosphatase is also found in other organs, such as bone, placenta, and intestine. For this reason, the GGT is utilized as a supplementary test to be sure that the elevation of alkaline phosphatase is indeed coming from the liver or the biliary tract.
3) Albumin:

Albumin is a major protein which is produced by the liver; and chronic liver disease causes a decrease in the amount of albumin produced. Therefore, in liver disease, and particularly more advanced liver disease, the level of the serum albumin is reduced.

4) Bilirubin:

Bilirubin is the main bile pigment in humans which, when elevated, causes the yellow discoloration of the skin and eyes called jaundice. Bilirubin is formed primarily from the breakdown of a substance in red blood cells called "haem." It is taken up from blood processed through the liver, and then secreted into the bile by the liver. Normal individuals have only a small amount of bilirubin circulating in blood. Conditions which cause increased formation of bilirubin, such as destruction of red blood cells, or conditions which decrease its removal from the blood stream, such as liver disease may result in an increase in the level of serum bilirubin. High levels of bilirubin are usually noticeable as jaundice. The bilirubin may be elevated in many forms of liver or biliary tract disease, and thus it is also relatively nonspecific.

5) Gamma Glutamyl Transpeptidase:

This enzyme level is often elevated in liver disorders. In contrast to the alkaline phosphatase, the GGT tends not to be elevated in diseases of bone, placenta, or intestine.  However the high sensitivity and very low specificity of this test seriously hampers its usefulness. It is elevated in a whole host of liver diseases including those caused by drugs and alcohol but is also elevated in obesity, hyperlipidaemia, diabetes, congestive cardiac failure and diseases of the kidney, pancreas and prostate.

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