aminotransferase (AST) is an enzyme that’s present in various parts of your body. An enzyme is a protein that helps trigger chemical reactions that your body needs to function.
AST is found in the highest concentrations in your muscles, heart, red blood cells, and liver. A small amount of AST is typically in your bloodstream. Higher-than-normal amounts of this enzyme in your blood may be a sign of a health problem. Any AST level below 40 units per liter (U/L) in adults isn’t uncommon.
AST levels increase when there’s damage to the tissues and cells where the enzyme is found. Elevated levels indicate that there is a certain amount of damage in that area. AST levels can rise as soon as 6 to 10 hours after damage occurs and remain high for months, depending on the cause. It’s normal for AST levels to be elevated in children from birth up to 2 years old. They generally decrease with age into adulthood, and then increase slightly in older adults.
The AST test measures the amount of AST in your blood. The test is also known as a serum glutamic-oxaloacetic transaminase test.
Doctors commonly use the AST test to check for liver diseases, such as hepatitis. It’s usually measured together with alanine aminotransferase (ALT). The AST-to-ALT ratio can help your doctor diagnose liver disease. This ratio can determine if there’s been damage to your liver as opposed to damage to your heart or muscles.
Your doctor may order an AST test for several reasons:
You’re experiencing the symptoms of liver disease
The symptoms of liver disease that may cause your doctor to order an AST test include:
- a loss of appetite
- swelling of your abdomen
- yellow skin or eyes, which is called jaundice
- dark urine
- severe skin itching, or pruritus
- bleeding difficulties
- abdominal pain
You’re at risk for liver conditions
Your doctor may order this test if you’re at high risk of developing liver problems. Your liver plays important roles in your body, including making proteins and removing toxins. You can have mild liver damage and not show any signs or symptoms. Your doctor may order the AST test to help them find any hidden issues if you’re at an increased risk for liver disease.
Factors that increase your risk of having liver problems include:
- exposure to viruses that cause hepatitis
- heavy alcohol or drug use
- a family history of liver disease
- being overweight
Your doctor wants to monitor an existing liver condition
Your doctor can use the AST test to check the status of a known liver disorder. They can use it to check the effectiveness of treatment as well. If it’s being used to monitor liver disease, your doctor may order it periodically while you’re being treated. This will help them determine whether your treatment is working or not.
Your doctor wants to check that medications aren’t causing liver damage
Your doctor can use AST testing to make sure the medications you’re taking aren’t causing liver damage. If the AST test results suggest liver damage, your doctor may tweak your medications to prevent any further damage.
Your doctor wants to check if other health conditions are affecting your liver
The AST level may be abnormal if you have any of these conditions:
The AST test is performed on a blood sample. A healthcare provider usually takes the sample from a vein in your arm or hand using a small needle. They collect the blood in a tube and send it to a lab for analysis. Your doctor will inform you about your results when they become available.
Although no special preparations are necessary for the AST test, you should always tell your doctor about any medications you’re taking prior to a blood draw.
The risks of the AST test are minimal. You may experience some discomfort when the blood sample is drawn. You may have pain at the puncture site during or after the test.
Other potential risks of a blood draw include:
- difficulty obtaining a sample, resulting in multiple needle sticks
- excessive bleeding at the needle site
- fainting due to the needle stick
- an accumulation of blood under your skin, or a hematoma
- an infection at the puncture site
AST test results will vary based on the laboratory completing the analysis. The ranges for normal levels also differ depending on your sex and age. Your doctor will talk to you about your results and what they mean. Elevated AST levels could be a sign of liver damage.
People with acute viral hepatitis can have AST levels that are more than 10 times the normal amount. Acute viral hepatitis is sudden inflammation of the liver due to infection by one of the several viruses that cause hepatitis. Acute viral hepatitis over time can become chronic hepatitis. In these cases, AST continues to stay abnormal but at much lower levels.
Some of the other conditions that cause AST levels in your liver to be abnormal are:
- liver cancer
- autoimmune diseases
- certain genetic disorders
- non-alcoholic fatty liver disease (NAFLD)
- liver trauma in physical injury
Other possible reasons for an increased AST level that aren’t related to the liver include:
- a recent heart attack
- strenuous activity
- an injection of medicine into your muscle
Levels of AST may also be extremely elevated as a result of exposure to drugs or other substances that are toxic to your liver.
Depending on the reason for the test and your results, your doctor may recommend additional tests. If your AST test result shows elevated levels, your doctor may compare it with the results of other tests to help determine which form of liver disease you may have. These include tests for levels of alkaline phosphatase, total protein, and bilirubin. Your doctor may also recommend an ultrasound or CT scan of your liver to help identify other reasons for abnormal tests.
Once you know which form of liver disease is causing the damage to your liver, you and your doctor can work together to come up with a treatment plan that meets your needs.