What Do AST and ALT Test Result Mean? (2024)

Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are two enzymes measured in a blood test to check the health of your liver. High AST and ALT levels are a general sign of a liver problem.

Based on which enzyme is elevated—or if both are elevated—healthcare providers can make an educated guess as to the underlying cause of the liver problem and order additional tests. This may help diagnose diseases ranging from hepatitis and cirrhosis to liver cancer and liver failure.

This article explains what ALT and AST liver enzymes are, why they are tested, and what the results of the blood tests mean.

When ALT and AST Testing Is Used

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

• If you have symptoms of liver disease, including jaundice (yellowing of the skin or eyes), dark urine, pale stools, nausea, vomiting, and fatigue
• To monitor the progression of a liver disease
• To determine when drug treatments should be started
• To check your response to treatment

The test measures ALT and AST, enzymes produced by the liver. These enzymes are used by the body to facilitate metabolism (the conversion of food into energy).

When the liver is damaged, ALT and AST can leak into the bloodstream. High levels of ALT and AST in the blood are reliable markers for liver disease.

Why ALT and AST Are Important

High ALT and AST are strong indications of liver disease, but what they tell us individually and together can be very different. This is because ALT is produced mainly in the liver, while AST is produced in the liver, brain, pancreas, heart, kidneys, lungs, and muscles.

As such, a high ALT is a pretty clear indication of liver disease. On the other hand, a high AST may be a sign of liver disease or conditions affecting the brain, pancreas, heart, kidneys, lungs, or muscles.

This shouldn't suggest that the AST is any less important in diagnosing liver disease. This is because other organs are commonly affected by liver disease, and a high ALT accompanied by a high AST can tell us how advanced the liver disease is.

Results of ALT and AST Testing

ALT and AST are measured in international units per liter (U/L). The normal range of values (meaning the values between which levels are normal) are:

• ALT: Between 4 and 36 U/L
• AST: Between 8 and 33 U/L
  

The high end of the reference range is referred to as the upper limit of normal (ULN).

High ALT and AST levels may indicate a problem with your liver, but it doesn't necessarily mean that you have a condition requiring treatment. This is because there is a wide gap between the ULN and what is considered to be dangerously high levels.

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

Very elevated ALT and AST levels can indicate:

• Fatty liver disease (liver injury caused by the buildup of fat in the liver)
• Alcoholic liver disease (fatty liver disease caused by alcohol)
• Viral hepatitis (liver inflammation caused by a virus)
• Cirrhosis (loss of liver function due to severe liver scarring)
• Hepatoxicity (liver injury caused by medications or toxins)
  
• Liver cancer (most commonly a type called hepatocellular carcinoma)
• Liver failure (the inability of the liver to service the body's needs)

It may also be due to any number of other medical conditions, such as infectious mononucleosis, heart problems, or pancreatitis.

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 tell whether the condition is acute (sudden and severe) or chronic (persistent).

For instance, if the liver experiences an acute injury, you can expect a sudden spike in the ALT and a relatively normal (or only mildly elevated) AST.

On the other hand, if liver disease is chronic, the damage will gradually affect other organs. As these organs are damaged, the AST will begin to rise.

The relationship is described with the AST/ALT ratio (also known as the De Ritis ratio).

The AST/ALT ratio is important because the pattern can tell much about the underlying condition. By way of example:

• An AST/ALT ratio of less than 1 (where the ALT is higher than the AST) is a general indication of fatty liver disease.
• An AST/ALT ratio equal to 1 (where the ALT is equal to the AST) may indicate acute viral hepatitis or hepatotoxicity.
• An AST/ALT ratio higher than 1 (where the AST is higher than ALT) indicates cirrhosis.
• An AST/ALT ratio higher than 2:1 (where the AST is more than twice as high as the ALT) is a common sign of alcoholic liver disease.
  

What Can Affect AST/ALT Results?

High or low ALT or AST results do not always indicate a health problem. Certain factors can cause a temporary increase or decrease in levels, such as:

• Recent heavy alcohol use
• Extreme physical activity
• Pregnancy
• Recent cocaine use (which is hepatotoxic)
• Recent fasting or extreme weight loss

Obesity, sex, and age can also influence ALT or AST levels, meaning that what is considered "normal" overall may need to be adjusted if you are overweight or underweight, male or female, or older or younger. Labs will generally take these factors into account.

Summary

ALT and AST are liver enzymes produced by the liver. If you have high levels of ALT and AST in your blood, it could be a sign of liver disease. The ratio of ALT to AST can help diagnose the underlying cause.

By Charles Daniel
Charles Daniel, MPH, CHES is an infectious disease epidemiologist, specializing in hepatitis.