ALT vs AST: What's the Difference?

ALT vs AST at a Glance

This table summarizes the key differences between ALT (alanine aminotransferase) and AST (aspartate aminotransferase), two enzymes commonly included in liver function testing.

Normal ranges may vary by lab, age, sex, and testing method. Some laboratories use narrower ranges.

Key Differences Between ALT and AST

The most important distinction is where each enzyme is concentrated in the body. ALT is found almost exclusively in liver cells (hepatocytes), which means an elevated ALT level strongly suggests liver damage or inflammation. AST, on the other hand, is present in the liver, heart muscle, skeletal muscle, kidneys, brain, and red blood cells. This broader distribution means that elevated AST does not automatically point to a liver problem.

For this reason, ALT is considered the more liver-specific enzyme. When a healthcare provider sees elevated ALT with normal AST, the first consideration is usually a liver-related cause. When AST is elevated but ALT is normal, non-liver causes such as muscle injury, strenuous exercise, or cardiac events may be investigated.

Both enzymes are released into the bloodstream when cells that contain them are damaged or destroyed. In many liver conditions — including viral hepatitis, fatty liver disease, and drug-induced liver injury — both ALT and AST are elevated together. The pattern of elevation (which one is higher, and by how much) provides important diagnostic clues.

It is worth noting that normal ALT and AST levels do not completely rule out liver disease. Some chronic liver conditions, including early-stage cirrhosis, can be present even when enzyme levels are within the normal range. Additional tests such as albumin, bilirubin, and imaging studies may be needed for a full assessment.

The AST/ALT Ratio (De Ritis Ratio)

The AST/ALT ratio, also known as the De Ritis ratio, is a simple calculation (AST divided by ALT) that helps healthcare providers distinguish between different types of liver disease. It was first described by Fernando De Ritis in 1957 and remains a useful clinical tool today.

In non-alcoholic fatty liver disease (NAFLD), ALT is typically higher than AST, producing a ratio below 1. As the disease progresses to fibrosis or cirrhosis, the ratio may shift upward toward 1 or above.

In alcoholic liver disease, AST is characteristically elevated more than ALT, often in a 2:1 or 3:1 ratio. This pattern occurs because alcohol specifically depletes a cofactor (pyridoxal 5'-phosphate, the active form of vitamin B6) that is needed for ALT production, suppressing ALT levels relative to AST. The AST/ALT ratio is one clue, but it is always interpreted alongside the clinical history and other lab findings.

When Both Are Elevated vs. Only One

Both ALT and AST elevated: This pattern most commonly indicates liver inflammation or damage. Possible causes include:

• Viral hepatitis (hepatitis A, B, or C)
• Non-alcoholic fatty liver disease (NAFLD) or non-alcoholic steatohepatitis (NASH)
• Alcoholic liver disease
• Medication or supplement toxicity (notably acetaminophen)
• Autoimmune hepatitis
• Bile duct obstruction

Only AST elevated (ALT normal): When AST is elevated without a corresponding rise in ALT, non-liver causes should be considered:

• Muscle damage — rhabdomyolysis, intense exercise, muscle trauma
• Heart conditions — myocardial infarction (heart attack), myocarditis
• Hemolysis — breakdown of red blood cells
• Thyroid disorders

Only ALT elevated (AST normal): This is less common but may occur in mild or early liver inflammation, particularly in NAFLD, where ALT rises before AST. Certain medications may also selectively elevate ALT.

Muscle Damage, Exercise, and AST

Because AST is found in skeletal muscle, intense physical activity can temporarily raise AST levels without any liver involvement. Marathon runners, weightlifters, and people who have recently started a new exercise program may see elevated AST on blood work drawn within 24 – 48 hours of strenuous activity.

If a healthcare provider suspects that elevated AST is due to muscle rather than liver damage, they may order a creatine kinase (CK) test. CK is highly specific to muscle tissue. When CK is elevated alongside AST but ALT is normal, muscle damage is the likely explanation.

This is one reason why some labs recommend avoiding vigorous exercise for 24 – 48 hours before blood work that includes liver enzymes, particularly if the purpose is to evaluate liver health.

Should I Be Concerned About Elevated Liver Enzymes?

Mildly elevated ALT or AST levels are common and do not always indicate serious disease. Many temporary and benign factors can cause a slight increase, including recent alcohol consumption, certain medications (such as statins or NSAIDs), herbal supplements, intense exercise, or even a recent high-fat meal.

However, persistent elevation — meaning levels remain high on repeat testing over weeks or months — warrants further investigation. Significantly elevated levels (more than 5 – 10 times the upper limit of normal) may suggest acute liver injury and require prompt medical evaluation.

Your healthcare provider will interpret ALT and AST results in the context of your symptoms, medical history, medications, alcohol use, and other lab values. A single abnormal result is rarely diagnostic on its own.

Common Next Steps

If ALT or AST levels are abnormal, a healthcare provider may recommend:

• Repeat testing in 2 – 4 weeks to confirm whether the elevation is persistent
• A complete liver function panel including alkaline phosphatase (ALP), bilirubin, and albumin
• Hepatitis B and C screening if not previously tested
• Imaging such as an abdominal ultrasound to evaluate liver structure
• Creatine kinase (CK) if muscle damage is suspected
• Review of current medications, supplements, and alcohol intake
• Lifestyle modifications such as reducing alcohol, managing weight, and reviewing medications

Frequently Asked Questions

Sources

• Mayo Clinic. Liver Function Tests. mayoclinic.org
• American College of Gastroenterology (ACG). ACG Clinical Guideline: Evaluation of Abnormal Liver Chemistries. Am J Gastroenterol. 2017;112(1):18-35.
• MedlinePlus. ALT Blood Test; AST Test. U.S. National Library of Medicine.
• American Association for the Study of Liver Diseases (AASLD). Practice Guidelines for Evaluation of Elevated Liver Enzymes.

Related Tests and Biomarkers