AST and ALT are two liver enzymes measured in a standard blood test to check how well your liver is working. When liver cells are damaged, these enzymes leak into the bloodstream, causing levels to rise. Doctors order these tests as part of a liver function panel, and elevated results can point to conditions ranging from fatty liver disease to hepatitis.
What AST and ALT Actually Do
Both AST and ALT are enzymes, proteins that speed up chemical reactions your body needs to process amino acids (the building blocks of protein). ALT helps convert proteins into energy for liver cells. AST helps break down amino acids throughout the body.
The key difference between them is where they live. ALT is found almost exclusively in the liver, which makes it a more specific marker of liver health. AST exists in the liver too, but also in your heart, brain, kidneys, pancreas, and skeletal muscles. This means an elevated AST level doesn’t automatically point to a liver problem. It could reflect muscle damage from intense exercise, a heart issue, or injury to another organ. When both enzymes are elevated together, the liver is the most likely source.
Normal Ranges
Normal AST and ALT levels typically fall between 7 and 56 units per liter (U/L), though the exact reference range varies slightly between labs. Men tend to have somewhat higher baseline levels than women. Your results will appear on your lab report alongside the specific lab’s reference range, making it easy to see whether your numbers fall outside the expected window. A result slightly above the upper limit isn’t always cause for concern, since factors like a recent intense workout, a large meal, or even the time of day can temporarily nudge levels higher.
What Elevated Levels Can Mean
Higher-than-normal ALT levels can signal liver damage from a variety of causes:
- Fatty liver disease: the most common reason for mildly elevated liver enzymes, often linked to excess weight or metabolic syndrome
- Hepatitis: viral infections (hepatitis A, B, or C) that inflame the liver
- Cirrhosis: long-term scarring of liver tissue, often from chronic alcohol use or untreated hepatitis
- Liver cancer
- Reduced blood flow to the liver
- Medications and toxins
The degree of elevation matters. Mildly elevated levels (less than three times the upper limit of normal) often show up in fatty liver disease or as a side effect of medication. Moderate elevations may warrant testing for autoimmune liver conditions. Very high levels, sometimes ten or more times the normal range, suggest acute injury such as a drug overdose, severe viral hepatitis, or a sudden loss of blood flow to the liver.
Why the AST-to-ALT Ratio Matters
Doctors don’t just look at each number in isolation. The ratio of AST to ALT provides additional diagnostic clues. In most forms of liver disease, ALT rises higher than AST, giving a ratio below 1. When the ratio flips and AST is higher than ALT (a ratio above 1), it can suggest alcoholic liver disease or advanced cirrhosis. A ratio significantly above 1.67 has been associated with greater cardiovascular risk, including higher rates of heart attack and heart failure, because elevated AST can reflect damage to heart muscle or other tissues beyond the liver.
Medications and Supplements That Raise Levels
More than 1,000 drugs have been linked to liver injury, so medication is one of the most common explanations for unexpected enzyme elevations. Over-the-counter pain relievers are frequent culprits. Ibuprofen (Advil, Motrin) and naproxen (Aleve) can stress the liver, especially with regular use or when combined with alcohol. Acetaminophen (Tylenol) is safe at recommended doses but is the most common cause of acute liver failure in overdose.
Several prescription medications are also known to raise liver enzymes, including certain antibiotics, antiseizure drugs, antifungals, anabolic steroids, and some cancer immunotherapy drugs. If your levels come back high and you’re on any of these, your doctor may adjust your dose or switch medications and recheck your labs.
Herbal supplements deserve special attention because many people don’t think of them as medications. In the United States, roughly 20% of drug-induced liver injury cases are caused by herbal products. Green tea extract, turmeric supplements, kava kava, kratom, black cohosh, and valerian are among the supplements most commonly implicated. If you take any herbal or dietary supplements and your liver enzymes are elevated, mention them to your doctor.
What Happens After Abnormal Results
A single set of elevated AST or ALT results rarely leads to a diagnosis on its own. The workup usually starts with repeat blood tests to confirm the elevation wasn’t temporary, along with additional blood work to check for viral hepatitis, autoimmune conditions, and other markers of liver and bile duct health. One of those markers, called ALP, rises in conditions affecting the bile ducts rather than the liver cells themselves. Another enzyme called GGT helps confirm whether an ALP elevation is coming from the liver or from bone disease, which can also raise ALP.
Imaging is the next step for many people. An abdominal ultrasound can reveal fatty deposits, tumors, or blocked bile ducts. A newer technique called transient elastography (often known by the brand name FibroScan) measures liver stiffness to estimate how much scarring or fat accumulation is present, without requiring a needle. Liver biopsy, where a small tissue sample is taken for examination under a microscope, is reserved for situations where the diagnosis remains unclear or doctors need to determine the stage of a known liver disease.
Lifestyle Factors That Affect Your Levels
Not every elevation signals disease. Intense exercise, particularly heavy weightlifting or endurance training, can temporarily raise AST because the enzyme is abundant in skeletal muscle. Alcohol consumption, even moderate amounts in some individuals, elevates both enzymes. Obesity and a diet high in sugar and processed carbohydrates are the primary drivers of nonalcoholic fatty liver disease, the leading cause of mildly elevated ALT in otherwise healthy adults.
If your results are mildly elevated and your doctor suspects lifestyle factors, you’ll typically be asked to cut back on alcohol, lose weight if needed, review your medications and supplements, and retest in a few weeks to a few months. For many people, these changes alone bring enzyme levels back to normal.