Overview of ALT and AST Liver Enzymes

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Liver enzymes are substances produced by the liver that can be measured with a blood test. Any elevation in an enzyme level may be a sign of a liver problem.

Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are two of the enzymes doctors look at when trying to figure out if you have a problem with your liver. When compared to each other, AST and ALT can help identify toxins in the liver, liver disease, or liver damage.

This article will discuss ALT and AST liver enzymes. It will talk about what they do inside your body and what happens when you have elevated levels of ALT or AST.

When is a liver function test recommended?

Verywell / Elise Degarmo

Roles of AST and ALT

Aminotransferases are chemicals that the liver uses to make glycogen. Glycogen is the stored form of glucose, the sugar that the body uses for energy.

Any glucose not immediately used will be changed into glycogen. It is then stored in cells for future use. Most will be stored in the liver. The leftover amount will be warehoused in the:

  • Skeletal muscles
  • Glial cells of the brain
  • Other organs

Aspartate aminotransferase (AST) is found in a variety of tissues, including the liver, brain, pancreas, heart, kidneys, lungs, and skeletal muscles. If any of these tissues are damaged, AST will be released into the bloodstream. While increased AST levels are signs of a tissue injury, it doesn't always relate to the liver.

By contrast, alanine aminotransferase (ALT) is found mainly in the liver. If your ALT level is high, this may be a sign of a liver injury. It could be minor or severe.

Occasional increases in ALT may occur when you have a short-term infection or illness. Sustained increases are more serious. That's because this may mean there's an underlying disease and a greater chance of liver damage.

Normal Lab Values

AST and ALT are measured in international units per liter (IU/L). The normal levels vary based on a person's body mass index (BMI) It also depends on the lab's reference value, or typical results. 

Generally speaking, the normal reference value for adults is:

  • AST: 8 to 48 IU/L
  • ALT: 7 to 55 IU/L

The high end of the reference range is referred to as the upper limit of normal (ULN). This number is used to establish how elevated your liver enzymes are.

Mild elevations are generally considered to be two to three times the ULN. With some liver diseases, the level can be more than 50 times the ULN. Levels this high are described as deranged.

AST/ALT Ratio

While it may seem that a high ALT is all that is needed to diagnose liver disease, its relationship to AST can provide valuable clues as to what exactly is going on. It will also tell you whether the issue is acute (occurring suddenly and progressing rapidly) or chronic (long-standing or persistent).

If the liver experiences an acute injury, you can expect to see a sudden spike in the ALT. On the other hand, if liver disease is slowly progressing, the damage in the liver will gradually affect other organs too. As these organs are damaged, the AST will begin to rise.

This occurs with diseases like hepatitis C. It causes long-term liver damage that triggers symptoms involving:

  • Kidneys
  • Brain
  • Eyes
  • Skin
  • Joints

These are referred to as extra-hepatic symptoms.

The relationship between these enzymes is described using the AST/ALT ratio. This is a calculation that compares the levels of AST and ALT in your blood. Depending on which value is elevated and the amount of elevation, doctors can often get a pretty strong indication as to what disease is involved.

What the AST/ALT Ratio Reveals

The AST/ALT ratio is important because its pattern can tell a lot about the condition involved. Here are the general guidelines used to diagnose liver disease:

  • An AST/ALT ratio of less than one (where the ALT is significantly higher than the AST) means you may have non-alcoholic fatty liver disease.
  • An AST/ALT ratio equal to one (where the ALT is equal to the AST) may be a sign of acute viral hepatitis or drug-related liver toxicity.
  • An AST/ALT ratio higher than one (where the AST is higher than ALT) means you may have cirrhosis.
  • An AST/ALT ratio higher than 2:1 (where the AST is more than twice as high as the ALT) is a sign of alcoholic liver disease.

However, a disease cannot be diagnosed by the pattern of elevation alone. How elevated the levels are also needs to be measured. This is described in multiples of the ULN. It is only when the levels are above a certain threshold that the ratio can be considered diagnostic.

Recap

The AST/ALT ratio is a comparison of the levels of the two enzymes. This measurement can point to liver diseases, but only if the enzyme levels are elevated several times above normal.

When Testing Is Recommended

AST and AST are part of a comprehensive testing panel known as the liver function test (LFT). An LFT may be ordered:

  • If you have symptoms of liver disease, including jaundice, dark urine, nausea, vomiting, and fatigue
  • To monitor the progression of a liver disease
  • To determine when certain drug treatments should be started
  • To check your response to a liver treatment

An LFT can also determine whether a drug (prescription or over-the-counter) or an herbal remedy is causing liver injury.

If the lab test is processed on-site, the results can be returned within hours. Otherwise, your doctor will usually receive the results in anywhere from one to three days.

Summary

ALT and AST liver enzymes are produced by the liver. Doctors can test these levels with a blood test. If you have elevated liver enzymes, it could be a sign that you have liver disease.

AST is found in the liver, brain, pancreas, heart, kidneys, lungs, and skeletal muscles. ALT is found mainly in the liver.

If your AST levels are too high, it might be a sign of an injury affecting tissues other than the liver. High ALT levels may mean you have a liver injury. It could also be a sign that you have an infection or short-term illness.

The ratio of the two enzymes can help diagnose specific liver diseases.

Frequently Asked Questions

  • What does a high AST level mean?

    A high AST (aspartate aminotransferase) level can indicate a problem with your liver. However, it does not usually mean you have a medical condition that needs treatment. It could be a side effect of medication. Very elevated AST levels can indicate hepatitis, cirrhosis, mononucleosis, heart problems, or pancreatitis. 

  • What AST level is considered high?

    The upper limit of normal for AST is 48 IU/L. Levels that are double to triple the upper limit of normal are considered mildly elevated. In liver disease, AST levels can be 50 times the upper limit of normal. The medical term for this is deranged levels.

  • What does a high ALT level mean?

    High levels of alanine transaminase (ALT) can indicate a liver problem but do not necessarily mean you have a health condition. Very elevated levels of ALT may be suggestive of liver damage from hepatitis, infection, liver cancer, or liver disease. High ALT levels may also be a side effect of certain medications.

  • What ALT level is considered high?

    The upper limit of normal for ALT is 55 IU/L. When an ALT level is double to triple the upper limit of normal, it is considered mildly elevated. Severely elevated ALT levels found in liver disease are often 50 times the upper limit of normal.

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5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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