ALT vs AST: Key Differences Explained

ALT is the more liver-specific enzyme, while AST also comes from the heart, skeletal muscle, and other tissues. See what it means when AST is higher than ALT, when ALT is higher than AST, and how the AST/ALT ratio is read.

Published

April 23, 2026

Last updated

October 6, 2026

Read time

9 min read

Medically reviewed by

Rhonda Collins, FNP-C

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ALT (alanine aminotransferase) and AST (aspartate aminotransferase) are both enzymes that leak into the blood when liver cells are injured. The difference is where else they come from: ALT is found mainly in the liver, while AST is also found in the heart, skeletal muscle, and other tissues. That makes ALT the more important marker for liver injury specifically, and AST most useful read alongside ALT, because the pattern between the two points toward a cause.

Is AST or ALT more important?

For the liver, ALT is the more specific of the two. A family medicine review in American Family Physician notes that ALT elevations are generally more specific for liver injury, while AST can also rise from conditions outside the liver, such as muscle disorders, hemolysis, thyroid disorders, and celiac disease. MedlinePlus describes ALT the same way: because it is mostly found in the liver, an ALT test checks more specifically for liver damage.

That does not make AST unimportant. Clinicians rarely read either one alone. Both are part of a standard liver panel, and the comparison between them (which is higher, and by how much) is often more informative than either number in isolation.

What is ALT?

ALT is an enzyme concentrated in liver cells. Small amounts also exist in the kidneys, heart, and muscle, but a rise in blood ALT most often reflects injury to liver cells. StatPearls lists the most common causes of a raised ALT as alcohol-related liver injury, fatty liver disease, chronic hepatitis B or C, autoimmune hepatitis, and liver injury from drugs or herbal supplements. Mito offers a stand-alone ALT test for tracking this one enzyme.

What is AST?

AST is an enzyme found in the liver and also in the heart, muscles, and other tissues, according to MedlinePlus. Damage to any of these can release AST into the blood, so a high AST on its own does not tell you which tissue it came from. Mito also offers a stand-alone AST test. For the symptoms and causes of a raised AST in more depth, see high AST symptoms.

ALT vs AST at a glance

ALT higher than AST

When ALT is higher than AST, the rise most likely comes from the liver itself. This is the usual pattern of hepatocellular (liver cell) injury. The American College of Gastroenterology (ACG) 2017 guideline lists the main causes to look for: viral hepatitis A, B, and C, fatty liver disease, alcohol-related liver disease, hereditary hemochromatosis, autoimmune hepatitis, Wilson disease, alpha-1 antitrypsin deficiency, and prescribed or over-the-counter medicines.

In primary care, fatty liver disease is the most common reason for a mildly raised ALT or AST without symptoms, accounting for 25% to 51% of cases depending on the study population, according to American Family Physician.

AST higher than ALT

When AST is higher than ALT, two broad explanations are considered: alcohol-related liver disease, or AST coming from outside the liver. In a study cited by American Family Physician, an AST/ALT ratio greater than 2 suggested alcohol-related liver disease rather than nonalcoholic liver disease. If alcohol is not a factor, muscle is the next place to look, because AST is plentiful in skeletal muscle and the heart while ALT is not.

AST high but ALT normal

A raised AST with a normal ALT often points away from the liver. American Family Physician lists hemolysis (red blood cells breaking down), muscle disorders, thyroid disorders, and celiac disease as causes of AST elevation outside the liver, and advises considering hemolysis and strenuous exercise when the clinical picture fits. Heavy alcohol use can also raise AST more than ALT. When there are significant muscle aches, a creatine kinase (CK) test can help show whether the AST is coming from muscle.

Both AST and ALT high

When both are raised together, and more than other liver tests such as alkaline phosphatase (ALP), the ACG 2017 guideline calls this a hepatocellular pattern of injury. How high they are guides how quickly and how broadly the cause is investigated (see the section on concerning levels below). MedlinePlus notes that the ALT number on its own does not tell you how much of the liver is damaged, so the pattern and its trend over time matter as much as one result.

The AST/ALT ratio

The AST/ALT ratio is AST divided by ALT. In the study summarized by American Family Physician, a ratio greater than 2 suggested alcohol-related liver disease, while a ratio below 1 was associated with nonalcoholic fatty liver disease. The ratio is a clue, not a diagnosis, and non-liver sources of AST, such as recent heavy exercise or hemolysis, can push it up.

The fatty liver (MASLD) pattern

In fatty liver disease, ALT is usually higher than AST, giving an AST/ALT ratio below 1. Fatty liver disease not caused by alcohol is now called metabolic dysfunction-associated steatotic liver disease (MASLD). It was formerly known as nonalcoholic fatty liver disease (NAFLD), and the name changed in 2023 after a multisociety consensus process led by the major liver associations, including the AASLD.

Normal enzymes do not rule it out. StatPearls notes that MASLD, and even liver scarring (fibrosis), can be present while ALT and AST are within the lab’s reference range. For the symptoms of fatty liver and the tests that pick it up, see fatty liver disease symptoms and blood tests.

What level of ALT or AST is concerning?

Concern depends on how many times above the upper limit of normal (ULN) a result is, not on a single fixed number, because each lab sets its own reference range. American Family Physician defines a mild elevation as less than five times the ULN. Mild, symptom-free elevations are common: about 10% of the U.S. population has raised transaminases, and fewer than 5% of those people have serious liver disease. Elevations greater than five times the ULN should prompt immediate evaluation, citing the ACG 2017 guideline.

Lab ranges can also be generous. The ACG 2017 guideline puts a truly healthy ALT at 29 to 33 IU/L for men and 19 to 25 IU/L for women, and recommends assessing levels above this, even when a lab range would call them normal. Those figures are for ALT. AST is judged against the lab’s range and alongside ALT.

Exercise, muscle, and other effects on results

Hard training can raise both enzymes for several days. In a small study of 15 healthy men who were not used to weightlifting, one hour of weightlifting significantly raised AST, ALT, and the muscle markers CK and myoglobin, and AST and ALT stayed raised for at least 7 days. MedlinePlus also lists intense exercise, certain medicines and supplements, age, sex, weight, and the menstrual cycle as factors that can affect ALT, and pregnancy, exercise, medicines, age, and sex as factors that can affect AST. If you trained hard in the week before a blood draw, that context belongs with the result.

What to test next

The next tests depend on the pattern, and they usually start with the full liver panel rather than repeating ALT and AST. American Family Physician notes that simply repeating liver enzymes is not necessary in the initial workup, and suggests an initial evaluation that includes:

• markers of metabolic syndrome and insulin resistance: waist circumference, blood pressure, fasting lipids, and fasting glucose or A1C
• a complete blood count with platelets and serum albumin
• iron studies: serum iron, total iron-binding capacity, and ferritin
• hepatitis C antibody and hepatitis B surface antigen
• creatine kinase when muscle aches suggest a muscle source

If the elevation persists after that, a liver ultrasound and testing for less common causes are considered. A hepatic function panel measures ALT and AST together with alkaline phosphatase, bilirubin, albumin, and total protein, which is what the hepatocellular versus cholestatic distinction relies on. For what a liver panel costs and how prices compare, see liver function test cost. For evidence on lifestyle factors linked to liver enzymes, see lowering liver enzymes naturally.

FAQs

Why would AST be elevated but not ALT?

The most common reasons are sources outside the liver, such as muscle injury, strenuous exercise, or hemolysis (red blood cells breaking down), plus thyroid disorders and celiac disease. Heavy alcohol use can also raise AST more than ALT.

Is AST higher than ALT in fatty liver?

Usually not. In fatty liver disease (MASLD, formerly NAFLD), ALT is typically higher than AST, so the AST/ALT ratio is below 1. An AST more than twice ALT is more often associated with alcohol-related liver disease.

What level of AST is concerning?

It depends on the lab’s upper limit of normal. Rises under five times that limit are considered mild and are common, while rises above five times the limit should be evaluated promptly. AST is always read alongside ALT and other liver tests.

What is an alarming ALT level?

An ALT more than five times the upper limit of normal should prompt immediate evaluation. The ACG also recommends assessing ALT above 33 IU/L in men and 25 IU/L in women, even when the lab range is wider.

Can exercise affect AST and ALT?

Yes. In one small study, a single hour of weightlifting raised both AST and ALT in healthy men, and they stayed raised for at least a week.

Sources

• Kwo PY, Cohen SM, Lim JK. ACG Clinical Guideline: Evaluation of Abnormal Liver Chemistries. Am J Gastroenterol. 2017;112(1):18-35
• Oh RC, Hustead TR, Ali SM, Pantsari MW. Mildly Elevated Liver Transaminase Levels: Causes and Evaluation. Am Fam Physician. 2017;96(11):709-715
• StatPearls: Liver Function Tests
• StatPearls: Alanine Aminotransferase (ALT) Test
• MedlinePlus: ALT Blood Test
• MedlinePlus: AST Test
• AASLD: New MASLD Nomenclature
• Rinella ME, et al. A multisociety Delphi consensus statement on new fatty liver disease nomenclature. Hepatology. 2023;78(6):1966-1986
• Pettersson J, et al. Muscular exercise can cause highly pathological liver function tests in healthy men. Br J Clin Pharmacol. 2008;65(2):253-259

Medical disclaimer

This comparison is for informational purposes only and does not provide medical advice, diagnosis, or treatment. It is not a substitute for care tailored to your health history and symptoms.