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AST and ALT in the Liver - Levels and Significance

Scot Henry, Artem Cheprasov
  • Author
    Scot Henry

    Scot Henry's career spans 2 decades of study and research including an undergraduate degree in Environmental Sciences (UNBC, 2000), a PhD in Molecular Virology (EUMC, 2008), and work at Columbia University in cutting edge medical sciences (through to 2015). With 21 scientific articles, local and federal grants, 9 review articles and 1 book chapter, Scot continues to consult and remains a reviewer for 14 scientific magazines.

  • Instructor
    Artem Cheprasov

    Artem has a doctor of veterinary medicine degree.

What are AST & ALT? Read about AST/ALT lab values, their levels, and how overall ratios allow doctors to diagnose health issues. Also see when tests to detect liver function are required. Updated: 09/20/2021

Table of Contents


The Liver and Its Functions

The human liver, the largest internal organ, weighs approximately 1.25 kilograms or approximately 2.75 pounds (in humans). The liver is located just under the rib cage in the upper right portion of the abdomen. The liver's primary function is the detoxification of the blood. In addition, it aids in the breakdown of food through the production of stomach bile and produces a variety of proteins and chemicals that drive protein and fat metabolism for use in digestion and providing energy sources (as glycogen). The liver can be viewed as a large filtering system for the body; anything that is ingested gets filtered through the extensive network of liver cells (hepatocytes), which break down and remove dangerous materials and yield building blocks for metabolism and energy sources. Excess glucose can be built into long-chain energy storage as glycogen that can be broken down for utilization at a later time when available energy sources are scarce.

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  • 0:03 Different Ways to…
  • 0:26 The Albumin Clue
  • 1:28 Using AST and ALT as a Clue
  • 3:13 Using ALP as a Clue
  • 6:00 Important Parting Notes
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Human liver location

a transparent bust image showing the location of the human liver on the upper right abdomen, under the rib cage.

Liver Enzymes: What are AST & ALT?

The liver produces a large number of catalytic and metabolic enzymes that aid in the breakdown of foods, the removal of toxins, and the synthesis of molecules such as glycogen (a storage version of the energy-rich glucose). Two key liver enzymes are AST (asparatate aminotranferase) and ALT (alanine aminotransferase). As transaminase enzymes, they catalyze the reversible conversion of amino acids by the transference of amino groups. This reaction is a key step to breaking down proteins into amino acid building blocks, readying them for further metabolism. Alkaline phosphatase (ALP) is another key liver enzyme that can be used to indicate liver damage as it specifically increases due to blockages of bile ducts. These enzymes are easy to detect in routine blood samples and as such can be used in health diagnostics. When levels change from normal, to either high or low, information can be inferred about possible liver damage, improper function, or viral infections.

Significance of ALT & AST

AST and ALT hold great clinical significance as their increase (or rapid decrease) in blood serum can indicate problems within the liver. These enzymes are used as an indicator of the level of liver damage, as they are released into the bloodstream as liver cells (hepatocytes) and die and break apart due to a variety of damage. A mixture of clinical information can be obtained through the levels of these enzymes, ranging from the individual levels, the correlations between their levels, and the direct ratios between them.

Within a healthy adult, Columbia University Medical Center (a large transplant center in New York, NY) defines normality as AST within a range of 12-38 International Units (IU) per milliliter (ml) of serum and ALT between 7-41 IU/ml. Often used in diagnosis, a level limit is set as the Upper Limit of Normal (ULN), though there seems to be no consensus on what that limit is as it varies up to twofold from lab to lab and center to center. This ULN indicates to a doctor that an issue may be present if the levels of AST or ALT are beyond (either too high or too low) this limit. A mild increase of ALT and AST is described as less than 5 times the ULN and can be an indication of chronic Hepatitis C or B, acute viral or autoimmune hepatitis, non-alcoholic fatty liver disease, alcohol-related liver disease, or from a variety of medications. A moderate increase in AST and ALT levels (at 5-15 times the ULN) indicates a progressive liver disease from a variety of sources. At 15 times the ULN, severe liver injury has or is occurring, stemming again from acute liver disorders.

Taken individually, the serum levels of these two enzymes can indicate specific issues, but their direct ratio is also an important indication of specific issues. As ALT is at its highest concentration in the liver, it is used predominately as an indication for a variety of liver issues. AST, on the other hand, is present in other organs including the heart, muscles, kidneys, brain, pancreas, and lungs. Therefore, the relationship between these two enzymes can reveal more information about the state of disease of the patient. For example, if the ALT level is greater than the AST level, this could indicate that the damage that is causing the elevated levels in the bloodstream is coming from the liver and not other parts of the body. Whereas if the AST level is greater than the ALT blood serum levels, the damage is coming from other sources, such as the heart or pancreas. These values help doctors narrow down the possible causes towards a full diagnosis.

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Frequently Asked Questions

What does AST and ALT measure?

A test for AST and ALT measures the levels of each of the enzymes present in the blood. Levels above or below a suggested 'normal' level can give indications of liver damage, viral infections or loss or non-functioning livers.

What does it mean when ALT and AST are high?

If your AST and ALT levels are above the upper limit of normal (ULN), you are likely experiencing symptoms of abdominal pain, darkened urine, jaundicing of the skin or eyes, all of which could be attributed to a lower liver function, a viral infection or liver damage.

What AST and ALT levels indicate liver disease?

Any level of AST and ALT above the upper limit of normal (ULN) has an indication of possible liver problems, stemming from a variety of sources.

What do you do if AST and ALT are high?

This question is not really relevant. The correct answer would be listen to your doctor and follow his advice. I would like to have this FAQ removed, or replaced with something more relevant to the asset

What is normal AST ALT ratio?

A normal AST/ALT ratio should be around 1.15 in healthy adults, with slightly more AST than ALT in blood levels. Levels above this 'normal' can reveal information about liver damage and the possible causes.

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