Alanine aminotransferase (ALT) is an enzyme routinely measured in blood tests, often included as part of a comprehensive metabolic panel or a dedicated liver function panel. This measurement serves as a sensitive indicator for assessing liver health. When a healthcare provider orders this test, they are looking for evidence of liver cell injury or inflammation. Understanding what a normal ALT level signifies is the first step in interpreting the results.
What Alanine Aminotransferase (ALT) Does
ALT is a specialized protein found predominantly inside liver cells, known as hepatocytes. Like all enzymes, its purpose is to speed up chemical reactions within the body. Specifically, ALT plays a role in protein metabolism by helping to convert the amino acid alanine into pyruvate, which the cell uses to generate energy.
In a healthy individual, ALT levels circulating in the bloodstream are relatively low. When liver cells are damaged due to inflammation or injury, their membranes rupture and release their contents into the blood. This leakage causes a measurable increase in the serum ALT level, signaling a liver problem. Because of its high concentration within the liver, ALT is considered a specific marker for liver injury.
Interpreting the Normal Reference Range
The standard reference range for ALT in adults typically falls between 7 and 55 Units per Liter (U/L). This range is not an absolute cutoff, as it can vary slightly depending on the specific laboratory equipment and methods used. The specific “normal” value for a patient is always provided on the test report.
A person’s biological sex influences the normal range, with men generally having slightly higher ALT values than women. Several non-disease factors can temporarily skew the result, making pre-test preparation important. Intense physical exercise within 24 hours of the blood draw can cause a transient elevation because small amounts of the enzyme are also found in muscle tissue. Certain medications or a recent high-fat meal can also influence the measured level.
Common Causes of Elevated ALT Levels
An elevated ALT result indicates that liver cell damage has occurred, causing the enzyme to spill into the blood. The most frequent cause of mild to moderate elevation is Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD), previously known as Non-Alcoholic Fatty Liver Disease (NAFLD). This condition is associated with obesity, type 2 diabetes, and high cholesterol, leading to fat accumulation within liver cells.
Excessive consumption of alcohol is another common culprit, which directly harms liver cells over time. Drug-induced liver injury is also a frequent cause, stemming from various over-the-counter and prescription medications. Common examples include high doses of acetaminophen, statins, and certain antibiotics.
Viral infections, specifically Hepatitis A, B, and C, can cause inflammation leading to elevated ALT levels. The degree of elevation can offer a clue to the cause; acute viral hepatitis often results in extremely high ALT levels, while MASLD or chronic drug use typically causes more modest increases. Other causes include iron overload (hemochromatosis) or autoimmune hepatitis.
Steps Following an Abnormal Result
If a blood test reveals an ALT level outside the normal reference range, the first step is to consult a healthcare provider for a thorough review. An elevated ALT level does not automatically confirm a diagnosis of severe liver disease, as less than 5% of people with high levels have a serious liver condition. The provider will consider the result alongside the patient’s medical history and other accompanying blood markers.
Further diagnostic steps often involve ordering secondary tests to pinpoint the underlying cause. These may include additional liver function tests, screens for viral hepatitis, or blood tests to check for conditions like iron overload. Imaging studies, such as an abdominal ultrasound, might also be requested to visualize the liver structure. Treatment often focuses on lifestyle adjustments, such as dietary changes, regular exercise, and limiting alcohol intake, which can effectively lower ALT levels.