ALT, or alanine transaminase, is a liver enzyme included in the comprehensive metabolic panel (CMP), a routine blood test that measures 14 different substances in your blood. When liver cells are damaged or inflamed, ALT leaks into the bloodstream, making it one of the most useful markers for detecting liver problems early.
What ALT Actually Measures
ALT is an enzyme, a protein that speeds up chemical reactions inside cells. It’s found mainly in the liver, though smaller amounts exist in muscle, kidney, heart, and brain tissue. Under normal conditions, only a small amount of ALT circulates in your blood. When liver cells are injured, they release more ALT into the bloodstream, and that spike shows up on your blood test.
This is why ALT is often called a “liver function test,” though that label is somewhat misleading. ALT doesn’t measure how well your liver is functioning. It measures whether liver cells are being damaged. You can have significant liver damage with normal function, or mildly elevated ALT from something that has nothing to do with your liver at all.
Where ALT Fits in the Metabolic Panel
There are two versions of the metabolic panel: basic (BMP) and comprehensive (CMP). ALT is only included in the comprehensive version. The BMP covers electrolytes, blood sugar, and kidney markers. The CMP adds liver-related tests on top of those, including ALT, AST (a related enzyme), and alkaline phosphatase (ALP), along with protein levels and bilirubin. Together, these give your provider a broad snapshot of how your liver, kidneys, and metabolism are performing.
If your doctor ordered a basic metabolic panel, ALT won’t appear in your results. A CMP is typically ordered during annual physicals, before starting certain medications, or when symptoms suggest a possible liver or metabolic issue.
Normal ALT Ranges
Normal ALT levels differ by sex, and the exact cutoff depends on which lab processes your blood. The American College of Gastroenterology defines a healthy ALT as 29 to 33 U/L for males and 19 to 25 U/L for females. Many labs use slightly higher upper limits, around 42 U/L for men and 30 U/L for women, based on large population studies of healthy individuals.
This inconsistency can be confusing. Your result might be flagged as normal at one lab but abnormal at another. If your ALT is in the borderline zone (say, the mid-30s for a woman or the low 40s for a man), the number alone doesn’t tell the full story. Your provider will consider your other CMP results, your medical history, and any medications or supplements you’re taking.
Common Reasons ALT Goes Up
The most common cause of a mildly elevated ALT is fatty liver disease, which affects roughly one in four adults. Obesity, insulin resistance, and high triglycerides all contribute. Alcohol use is another frequent driver: in alcohol-related liver disease, about 90% of patients show a characteristic pattern where AST rises higher than ALT, often at a ratio of 2 to 1 or greater. That ratio helps providers distinguish alcohol-related damage from other causes.
Viral hepatitis (types A, B, and C), autoimmune liver conditions, and bile duct blockages can all push ALT higher. In acute situations like a sudden viral infection or loss of blood flow to the liver, ALT can spike into the thousands, which signals a medical emergency.
More than 1,000 types of drugs have been linked to liver injury and ALT elevations. Common culprits include certain antibiotics, antiseizure medications, antifungals, anabolic steroids, and even niacin supplements. Herbal and dietary supplements are an underappreciated source of liver stress. Green tea extract, turmeric supplements, kava kava, kratom, and aloe vera have all been associated with liver enzyme elevations. If your ALT is unexpectedly high, your provider will likely ask about everything you’re taking, including over-the-counter products.
Non-Liver Causes of Elevated ALT
Because ALT also exists in skeletal muscle, intense exercise can raise your levels without any liver involvement. In one study, 15 healthy men who didn’t regularly lift weights completed a single intense workout. Their ALT and AST rose significantly and stayed elevated for at least seven days. Markers of muscle injury confirmed the source was skeletal muscle, not the liver.
There are two main explanations for this. Resistance training causes microscopic tears in muscle fibers, and ALT leaks into the bloodstream as part of the normal repair process. Intense exercise can also temporarily reduce blood flow to the liver, stressing liver cells enough to release small amounts of enzymes. If you had a hard workout in the days before your blood draw, mention it to your provider before assuming the worst about a mildly elevated result.
What Happens if Your ALT Is Abnormal
A single elevated ALT reading isn’t a diagnosis. It’s a signal to look deeper. Your provider will typically start by reviewing the rest of your CMP. If AST, ALP, and bilirubin are also elevated, that points more strongly toward a liver issue. The pattern matters: ALT that’s higher than AST suggests metabolic or viral causes, while AST higher than ALT leans toward alcohol-related damage or advanced scarring (cirrhosis) from any cause.
Next steps usually include repeating the test in a few weeks to see if the elevation persists, checking for hepatitis B and C, and sometimes ordering an ultrasound of the liver. If the repeat test comes back normal, it was likely a temporary bump from exercise, a medication, or a supplement. Persistent elevations warrant further investigation.
Mildly elevated ALT often produces no symptoms at all, which is exactly why it’s included in routine bloodwork. By the time liver damage causes noticeable symptoms like fatigue, yellowing of the skin, or abdominal pain, the problem is usually more advanced. Catching a rising ALT early gives you the chance to address the underlying cause, whether that’s modifying a medication, cutting back on alcohol, or managing weight and metabolic health, before real damage sets in.