What Are the Liver Enzymes ALT and AST?

How Their Relationship Helps Diagnose Disease

Stages of Liver Disease
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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, either acute (occurring suddenly and progressing rapidly) or chronic (long-standing or persistent).

Two of the enzymes central to the investigation are aspartate aminotransferase (AST) and alanine aminotransferase (ALT). When used comparatively, AST and ALT can help identify liver toxicity, liver disease, or liver damage.

Roles of AST and ALT

Aminotransferases are chemicals that the liver uses to make glycogen. Glycogen is the stored form of glucose, a sugar that the body uses for energy. Any glucose not immediately used will be converted into glycogen and stored in cells for future use. Most will be stored in the liver, while the remainder will be warehoused in skeletal muscles, glial cells of the brain, and other organs.

Aspartate aminotransferase, or 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 indicative of a tissue injury, it is not specific to the liver per se.

By contrast, alanine aminotransferase, or ALT, is found primarily in the liver. Any elevation of the ALT is a direct indication of a liver injury, whether minor or severe. Occasional increases may occur in association with a short-term infection or illness. Sustained increases are more problematic as they suggest an underlying disease and a greater likelihood 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) as well as the individual lab's reference value. 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 is number 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 exceed 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.

Think of it this way: if the liver sustains an acute injury, you can expect to see a sudden spike in the ALT. On the other hand, if a liver disease is slowly progressing, the damage incurred by the liver will gradually affect other organs as well. As these organs are damaged, the AST will begin to rise.

We see this with diseases like hepatitis C in which long-term liver damage will trigger an ever-expanding array of symptoms involving the kidneys, brain, eyes, skin, and joints (referred to as extra-hepatic symptoms).

This enzyme relationship can be described diagnostically with the AST/ALT ratio. This is a calculation which compares the level of AST and ALT in your blood. Depending on which value is elevated and the extent of that elevation, doctors can often get a pretty strong indication as to what disease is involved.

What the AST/ALT Ratio Tells Us

The AST/ALT ratio is important insofar as the pattern of elevation can tell us a lot about the condition involved. Among the general guidelines used to diagnose liver disease:

However, a disease cannot be diagnosed by the pattern of elevation alone. We also need to evaluate the magnitude of elevation described in multiples of the ULN. It is only when the magnitude is above a certain threshold that the ratio can be considered diagnostic.

Among the examples:

  • With alcoholic fatty liver disease, the AST would generally be more than 8 times the ULN and the ALT more than 5 times the ULN.
  • With non-alcoholic fatty liver disease, the AST and ALT would both be more than 4 times the ULN.
  • With acute viral hepatitis, the AST and ALT would both be 25 times the ULN.
  • With chronic hepatitis C, the AST and ALT could be anywhere from 2 to 10 times the ULN.
  • With ischemic hepatopathy (also known as shock liver), the AST and ALT would be over 50 times the ULN.

The diagnoses would also be supported with other liver enzyme test results, including albumin, bilirubin, and gamma-glutamyl transpeptidase (GGT).

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 assess your response to a liver treatment

Even beyond the scope of liver disease, an LFT can assess whether a drug is causing liver injury. This not includes prescription drugs but over-the-counter and herbal remedies as well.

While the blood test requires no preparation per se, your doctor may ask that you stop certain medications or foods prior to testing. 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.

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