Alanine aminotransferase, commonly called ALT, is an enzyme found primarily in liver cells. When your doctor orders a blood test that includes ALT, they’re looking for signs of liver damage. Healthy ALT levels generally fall between 7 and 55 U/L for males and 7 and 45 U/L for females, though labs vary slightly in their reference ranges. When liver cells are injured or inflamed, they release ALT into the bloodstream, causing levels to rise.
What ALT Does in Your Body
ALT plays a key role in how your body processes amino acids (the building blocks of protein) and generates glucose. Specifically, it transfers a chemical group from the amino acid alanine to another molecule, producing pyruvate and glutamate. Pyruvate feeds into your body’s energy production and glucose-making pathways. This is part of a process called the glucose-alanine cycle, which shuttles energy between your muscles and liver throughout the day.
While ALT exists in small amounts in other tissues like the kidneys and heart, it’s concentrated most heavily in the liver. That concentration is what makes it such a useful marker. A significant rise in blood ALT levels points toward the liver far more reliably than many other enzymes.
Why Doctors Order an ALT Test
An ALT test is typically part of a liver function panel, a group of blood tests ordered together. Your doctor might request one if you’re experiencing symptoms like fatigue, nausea, abdominal pain, or jaundice (yellowing of the skin or eyes). It’s also used as a routine screening tool during annual checkups, especially if you have risk factors for liver disease such as heavy alcohol use, obesity, diabetes, or a family history of liver problems.
ALT results help doctors detect liver damage early, sometimes before symptoms appear. Mild elevations can show up years before a person notices anything wrong, making this a valuable early-warning signal.
Common Causes of Elevated ALT
The most common reason for a high ALT level today is nonalcoholic fatty liver disease (NAFLD), driven largely by excess weight and insulin resistance. Other frequent causes include viral hepatitis (types A, B, and C), excessive alcohol intake, and cirrhosis (scarring of the liver from long-term damage).
Dozens of common medications can also raise ALT. Statins (cholesterol-lowering drugs) frequently cause mild, asymptomatic elevations. Over-the-counter pain relievers like acetaminophen and NSAIDs can do the same, particularly at higher doses or with prolonged use. Antidepressants including fluoxetine, sertraline, and trazodone, as well as blood pressure medications like lisinopril and losartan, are also on the list. Even some herbal supplements, notably kava and green tea extract, have been linked to elevated liver enzymes. If your ALT comes back high, your provider will want to know every medication and supplement you’re taking.
Not every ALT elevation comes from the liver. Intense exercise can temporarily spike your levels because ALT is also present in muscle tissue. Muscle injury or trauma can have the same effect. This is why your doctor may ask about your recent activity before interpreting results.
How ALT Levels Are Interpreted
A single elevated ALT reading doesn’t necessarily mean serious liver disease. Mildly elevated levels (just above the reference range) are common and often resolve on their own, especially if the cause is a temporary factor like a medication or a hard workout. Doctors typically recheck levels after a few weeks before pursuing further testing.
When ALT is significantly elevated, or stays elevated over time, doctors look at the pattern alongside other liver enzymes, particularly AST (aspartate aminotransferase). The ratio between AST and ALT helps narrow down the cause. In most forms of acute liver injury, ALT rises higher than AST, producing an AST-to-ALT ratio at or below 1. In alcohol-related liver damage, the pattern flips: AST tends to be at least twice as high as ALT, producing a ratio greater than 2. This distinction gives doctors an important clue about what’s driving the damage.
Very high ALT levels, sometimes 10 to 20 times the upper limit of normal, point toward acute liver injury from causes like viral hepatitis, drug toxicity (particularly acetaminophen overdose), or sudden loss of blood flow to the liver.
Preparing for an ALT Test
Because ALT is usually ordered alongside other blood tests, you may need to fast for several hours beforehand. Your provider will give you specific instructions. Let them know about all medications and supplements you take, as some may need to be paused before the test. Avoid intense exercise in the day or two leading up to your blood draw, since it can artificially inflate your results.
The test itself is a simple blood draw from a vein in your arm, with results typically available within a day or two. If your ALT comes back outside the normal range, your provider will interpret it in the context of your other lab results, symptoms, medical history, and medications before deciding on next steps.