Alanine Aminotransferase (ALT) is an enzyme found predominantly inside liver cells, where its primary function is to help convert food into usable energy. This process is a fundamental part of the body’s metabolism. When liver cells are damaged, they release ALT into the bloodstream. A blood test measures the concentration of this leaked enzyme. This test is a sensitive indicator of hepatocellular injury and is frequently included in routine health checkups or comprehensive liver function panels.
Defining the Normal Range
Determining a “normal” ALT level involves a reference range, typically measured in Units per Liter (U/L). Historically, the conventional upper limit was around 55 U/L, but this range is being re-evaluated to improve detection of subtle liver issues. Current recommendations often suggest a tighter upper limit, such as 35 U/L for adult men and 25 U/L for adult women, to exclude individuals with hidden liver disease. These lower thresholds reflect a better understanding that even previously normal levels might indicate underlying risk factors like excess body weight or metabolic changes.
The acceptable range for ALT is not a universal number and varies based on the specific testing laboratory and the patient’s characteristics. Men generally have slightly higher ALT levels than women due to differences in muscle mass and hormonal profiles. Age and body mass index also influence the reference interval, which is why the test report includes the specific range used by that lab. While falling within the established normal range is desirable, a lower ALT value within that range suggests better liver health.
Causes of Elevated Levels
An elevated ALT level signifies that liver cells have been damaged, leading to the release of the enzyme into the blood. The most frequent cause of chronic, mild to moderate elevation is Non-Alcoholic Fatty Liver Disease (NAFLD), now often referred to as Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD). This condition is strongly linked to obesity, type 2 diabetes, and high cholesterol, representing a buildup of fat in the liver that causes inflammation and cell damage. Chronic, heavy alcohol consumption is also a common cause, leading to alcoholic liver disease and enzyme leakage.
Acute and often more severe elevations can be caused by viral infections, such as Hepatitis A, B, or C, which directly inflame and destroy liver cells. Drug-induced liver injury is a significant factor, caused by numerous prescription and over-the-counter medications. High doses of acetaminophen, certain antibiotics, and cholesterol-lowering statins are among the most recognized pharmaceutical culprits. Even some herbal supplements and dietary products can cause toxicity that results in high ALT readings.
Less common, but serious, conditions also cause ALT release. These include hemochromatosis, a genetic disorder causing excessive iron accumulation, and autoimmune hepatitis, where the body’s immune system mistakenly attacks liver cells. Extremely high, sudden spikes in ALT can be a sign of acute liver failure caused by severe toxicity or a sudden lack of blood flow. Because a small amount of ALT is found in other tissues, elevations can sometimes be related to severe muscle injury or heart failure.
Interpreting Results and Next Steps
The magnitude of the ALT elevation provides a clue about the underlying cause. A mild elevation, defined as an ALT level less than twice the upper limit of normal, is often seen in chronic conditions like fatty liver disease or with certain medications. Levels that are ten times the normal range or higher suggest an acute and more aggressive process, such as acute viral hepatitis or severe drug toxicity. However, the level alone does not indicate the extent of long-term liver scarring or function.
A more complete picture emerges when ALT is assessed alongside Aspartate Aminotransferase (AST). In most forms of acute liver injury, ALT levels are higher than AST levels, resulting in an AST-to-ALT ratio less than 1. Conversely, a ratio where AST is significantly higher than ALT, particularly a ratio of 2:1 or greater, is a strong indicator of alcoholic liver disease or cirrhosis. This ratio is a valuable tool for healthcare providers to narrow down the potential diagnosis.
If an elevated ALT is detected, consultation with a healthcare provider is necessary for personalized interpretation. They will consider your medical history, symptoms, and other blood markers to determine the cause. Follow-up actions may involve repeating the test after lifestyle changes, such as weight loss or reducing alcohol intake, or ordering further diagnostic tests. These additional tests might include an ultrasound of the liver to check for fat or scarring, or specific blood work to screen for viral hepatitis or genetic conditions. Low ALT levels are generally not considered clinically significant, though very low readings can sometimes be linked to vitamin B6 deficiency.