What is aspartate transaminase

Aspartate transaminase also known as aspartate aminotransferase (AST), is an enzyme found in cells throughout the body but mostly in the heart and liver and, to a lesser extent, in the kidneys and muscles. In healthy individuals, levels of aspartate transaminase in the blood are low. When liver or muscle cells are injured, they release aspartate transaminase into the blood. This makes aspartate transaminase a useful test for detecting or monitoring liver damage.

The liver is a vital organ located in the upper right-hand side of the abdominal area just beneath the rib cage. It is involved in many important functions in the body. The liver helps to process the body’s nutrients, manufactures bile to help digest fats, produces many important proteins such as blood clotting factors, and breaks down potentially toxic substances into harmless ones that the body can use or excrete.

A number of conditions can cause injury to liver cells and may cause increases in aspartate transaminase. The test is most useful in detecting liver damage due to hepatitis, drugs toxic to the liver, cirrhosis, or alcoholism. aspartate transaminase, however, is not specific for the liver and may be increased in conditions affecting other parts of the body.

An aspartate transaminase test is often performed along with an alanine aminotransferase (ALT) test. Both are enzymes found in the liver that become elevated in the blood when the liver is damaged. A calculated aspartate transaminase/alanine aminotransferase ratio is useful for differentiating between different causes of liver injury and in recognizing when the increased levels may be coming from another source, such as heart or muscle injury.

Aspartate transaminase test

The blood test for aspartate aminotransferase (AST) is usually used to detect liver damage. It is often ordered in conjunction with another liver enzyme, alanine aminotransferase (ALT), or as part of a liver panel or comprehensive metabolic panel (CMP) to screen for and/or help diagnose liver disorders.

Aspartate transaminase and alanine aminotransferase are considered to be two of the most important tests to detect liver injury, although alanine aminotransferase is more specific for the liver than is aspartate transaminase and is more commonly increased than is aspartate transaminase. Sometimes aspartate transaminase is compared directly to alanine aminotransferase and an aspartate transaminase/alanine aminotransferase ratio is calculated. This ratio may be used to distinguish between different causes of liver damage and to distinguish liver injury from damage to heart or muscle.

Aspartate transaminase levels are often compared with results of other tests such as alkaline phosphatase (ALP), total protein, and bilirubin to help determine which form of liver disease is present.

Aspartate transaminase is often measured to monitor treatment of persons with liver disease and may be ordered either by itself or along with other tests for this purpose.

Sometimes aspartate transaminase may be used to monitor people who are taking medications that are potentially toxic to the liver. If aspartate transaminase levels increase, then the person may be switched to another medication.

When is aspartate transaminase test ordered?

Aspartate transaminase may be ordered as part of a comprehensive metabolic panel (CMP) when someone has a routine health examination.

An aspartate transaminase test may be ordered along with several other tests when a person has signs and symptoms of a liver disorder. Some of these may include:

• Weakness, fatigue
• Loss of appetite
• Nausea, vomiting
• Abdominal swelling and/or pain
• Jaundice
• Dark urine, light-colored stool
• Itching (pruritus)
• Swelling in the legs and ankles
• Tendency to bruise easily

Aspartate transaminase may also be ordered, either by itself or with other tests, for people who are at an increased risk for liver disease since many people with mild liver damage will have no signs or symptoms. Some examples include:

• Persons who might have been exposed to hepatitis viruses
• Persons who are heavy drinkers
• Persons who have a history of liver disease in their family
• Persons taking drugs that can damage the liver
• Persons who are overweight and/or have diabetes

When aspartate transaminase is used to monitor treatment of persons with liver disease, it may be ordered on a regular basis during the course of treatment to determine whether the therapy is effective.

Aspartate transaminase levels

Aspartate transaminase normal range

Males

• 0-11 months: not established
• 1-13 years: 8-60 U/L
• > or =14 years: 8-48 U/L

Females

• 0-11 months: not established
• 1-13 years: 8-50 U/L
• > or =14 years: 8-43 U/L

Aspartate transaminase low

Low levels of aspartate transaminase in the blood are expected and are normal.

Aspartate transaminase high

Very high levels of aspartate transaminase (more than 10 times normal) are usually due to acute hepatitis, sometimes due to a viral infection. With acute hepatitis, aspartate transaminase levels usually stay high for about 1-2 months but can take as long as 3-6 months to return to normal. Levels of aspartate transaminase may also be markedly elevated (often over 100 times normal) as a result of exposure to drugs or other substances that are toxic to the liver as well as in conditions that cause decreased blood flow (ischemia) to the liver.

With chronic hepatitis, aspartate transaminase levels are usually not as high, often less than 4 times normal, and are more likely to be normal than are alanine aminotransferase levels. aspartate transaminase often varies between normal and slightly increased with chronic hepatitis, so the test may be ordered frequently to determine the pattern. Such moderate increases may also be seen in other diseases of the liver, especially when the bile ducts are blocked, or with cirrhosis or certain cancers of the liver. aspartate transaminase may also increase after heart attacks and with muscle injury, usually to a much greater degree than alanine aminotransferase.

Aspartate transaminase is often performed together with the alanine aminotransferase test or as part of a liver panel.

In most types of liver disease, the alanine aminotransferase level is higher than aspartate transaminase and the aspartate transaminase/alanine aminotransferase ratio will be low (less than 1). There are a few exceptions; the aspartate transaminase/alanine aminotransferase ratio is usually increased in alcoholic hepatitis, cirrhosis, hepatitis C virus-related chronic liver disease, and in the first day or two of acute hepatitis or injury from bile duct obstruction. With heart or muscle injury, aspartate transaminase is often much higher than alanine aminotransferase (often 3-5 times as high) and levels tend to stay higher than alanine aminotransferase for longer than with liver injury.

Elevated aspartate transaminase values may also be seen in disorders affecting the heart, skeletal muscle, and kidney.

Mild to moderate aspartate transaminase increases may be seen with vigorous exercise and skeletal muscle injury or in conditions such as acute pancreatitis and heart attacks.

Pregnancy, a shot or injection of medicine into muscle tissue, or even strenuous exercise may increase aspartate transaminase levels. Acute burns, surgery, and seizures may raise aspartate transaminase levels as well.

In rare instances, some drugs can damage the liver or muscle, increasing aspartate transaminase levels. This is true of both prescription drugs and some “natural” health products. Be sure to tell your healthcare practitioner about all of the drugs and/or health supplements that you are taking.

What other tests may be used to help determine the cause of liver damage?

After a thorough physical exam and evaluation of a person’s medical history, there are several other tests that may be performed as follow up depending on what is suspected to be the cause of liver damage. Some of these include:

• Tests for hepatitis A, hepatitis B, and hepatitis C
• Testing for exposure to drugs and other substances toxic to the liver
• Ethanol level
• Copper and ceruloplasmin for Wilson disease
• Iron tests and genetic tests for hereditary hemochromatosis

A liver biopsy may be performed to help determine the cause of liver injury and to evaluate the extent of liver damage.