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Liver Enzymes

Liver enzymes are chemical substances produced by the liver in the course of doing its job to support the body's health. An enzyme is a catalyst that assists and strengthens a chemical reaction. The body depends on enzymes, many of which are produced by the liver, to allow its chemical processes to run strongly enough to support life. The liver, therefore, is an enormously complex and important organ, helping to regulate and augment many different processes of digestion and metabolism. The name of the organ comes from a very old recognition that it is extremely important for life; it means "that which lives." Measuring the amount of various liver enzymes in the blood can serve as an important diagnostic tool for recognizing liver disease, as well as other problems such as kidney disease. For purposes of conducting blood tests, there are a few of the enormous number of enzymes produced by the liver that are especially important.

Alanine Transaminase (ALT)

Alanine transaminase or ALT is an important liver enzyme for use in liver disease diagnosis, perhaps the single most important enzyme. Significant elevation of levels of ALT in the blood indicates damage to the cells of the liver from many different possible causes: physical or traumatic damage to the liver itself, hepatitis (viral or non-viral), diabetes, congestive heart failure, problems with the bile duct, infectious mononucleosis, fatty liver disease, and early signs of serious liver problems such as cirrhosis of the liver or liver cancer. Because elevated ALT can indicate so many different problems, it's always necessary to perform follow-up tests to identify the specific reason for the elevated enzyme and hence the correct course of treatment. Also, ALT levels can vary over the course of the day, and so an elevated level of ALT from one blood test is not a sure sign that anything is wrong at all; repeat testing may be indicated to rule out normal diurnal variation. Continued below....

Aspartate Transaminase (AST)

Aspartate transaminase or AST is another important liver enzyme but not as often used as a stand-alone diagnostic test. In addition to the liver, AST is also found in the heart, skeletal muscles, kidneys, brain, and red blood cells. Elevation of AST is therefore not as clear a sign of liver disease as elevated ALT. Elevated AST may also indicate a heart problem and this enzyme is a major heart marker.

ratio of AST to ALT can be used to help diagnose specific liver problems. An AST/ALT ratio above 2.0 indicates a likelihood of viral hepatitis, alcoholic hepatitis, or liver cancer. When the AST/ALT ratio is between 1.0 and 2.0, it's likely associated with cirrhosis of the liver. When it's less than 1.0, the probable indication is that other liver disorders are present. Of course, this test is used as a follow-up given elevated liver enzymes in the first place.

Alkaline Phosphatase (ALP)

Alkaline phosphatase is an enzyme occurring in the cells lining the liver's bile ducts. As such, elevated ALP can indicate bile duct obstruction, as well as other reasons for bile being unable to flow from the liver to the duodenum, and some other liver diseases. ALP is also produced by the bones and placenta, so it's expected to be higher in growing children, pregnant women, and elderly patients suffering from Paget's disease, which causes breakdown and reformation of the bones.

Gamma Glutamyl Transpeptidase (GGT)

GGT or gamma glutamyl transpeptidase is actually a more specific indicator of liver disease than ALT. However, it is not used as a first-line diagnostic test because it may be elevated with even minor levels of liver dysfunction that are considered non-clinical (in other words, it's too sensitive). This test is most often used to determine more information about the specific liver condition when ALT elevation is detected. GGT is often elevated in chronic alcohol abuse.

5' Nucleotidase

Another secondary-diagnostic enzyme, 5' nucleotidase can help identify whether elevated ALT indicates a problem with the bile ducts or with some other part of liver functioning. It's sometimes used instead of a test for GGT, or the two may both be used as a secondary diagnosis.

Lactate Dehydrogenase (LDH)

Lactate dehydrogenase or LDH is produced in tissue breakdown, not just in the liver but in the rest of the body as well. A test for LDH has many medical uses. It can be an indicator of many different kinds of cancer, as cancer cells break down more readily than normal cells; it can also indicate hemolysis, a condition of rupture of the red blood cells, and it can indicate certain complications of HIV infection. In diagnosis of liver disease, this is again a follow-up test and elevated LDH is a marker of liver cancer where elevated ALT is already detected.

In addition to liver enzyme tests, liver disease is diagnosed using examination of lifestyle factors such as alcoholism or obesity. However, elevation of liver enzymes is usually the first indication of problems with the liver, as liver disease is typically asymptomatic.

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