Alanine aminotransferase (ALT) is an enzyme primarily located within liver cells (hepatocytes), where it helps convert amino acids into energy. Under normal circumstances, ALT levels in the bloodstream are low because the enzyme remains contained inside these cells. When the liver is damaged or inflamed due to disease, injury, or toxins, the cell walls become compromised, causing ALT to leak into the circulating blood. A blood test measuring serum ALT concentration functions as a sensitive marker for liver cell injury, with higher levels indicating a greater degree of damage.
The Standard Reference Range
The concept of a “normal” ALT level is not a single, universal number, as the reference range varies slightly between clinical laboratories. A commonly accepted standard range for adults is approximately 7 to 55 units per liter (U/L). This range is not fixed and is influenced by demographic factors.
ALT levels tend to be slightly higher in men than in women, with some guidance suggesting an upper limit of 50 U/L for men and 38 U/L for women. Body mass index (BMI) and age also affect typical levels, as obesity and metabolic factors can slightly elevate baseline results. The variability in published reference ranges often stems from the different populations used by laboratories to establish their limits.
Many medical professionals suggest that traditional upper limits may be too high because they often include individuals with undiagnosed liver conditions. Some guidelines propose an optimal upper limit closer to 35 U/L for men and 25 U/L for women to improve the early detection of subtle liver stress. Therefore, a result within the standard laboratory range does not guarantee the absence of liver stress, and a physician must consider individual factors when interpreting the result.
Common Reasons for Elevated Levels
An elevated ALT level signals liver cell injury, and the most frequent causes relate to metabolic health and lifestyle factors. Non-Alcoholic Fatty Liver Disease (NAFLD), now often called metabolic dysfunction-associated steatotic liver disease (MASLD), is the leading cause of chronic mild-to-moderate ALT elevation. This condition is linked to obesity, insulin resistance, type 2 diabetes, and high cholesterol, where fat accumulation triggers inflammation and damage.
Excessive alcohol consumption is a major cause, leading to alcohol-related liver disease (ARLD) through toxic effects on liver cells. Acute viral infections, such as Hepatitis A, B, or C, cause significant liver inflammation, often resulting in very high ALT levels (sometimes more than ten times the upper limit). While many acute viral hepatitis cases resolve, Hepatitis B and C can become chronic and continue to cause damage.
Certain medications and dietary supplements are well-known for causing drug-induced liver injury, ranging from mild to severe elevation. Commonly implicated drugs include antibiotics, non-steroidal anti-inflammatory drugs (NSAIDs), and statin medications. Over-the-counter pain relievers, particularly acetaminophen taken in high doses, can severely damage the liver. Various herbal and dietary supplements have also been linked to liver toxicity.
Clinical Significance and Follow-Up
The interpretation of an elevated ALT result requires a complete clinical picture, including the patient’s medical history and the levels of other liver enzymes. Physicians often compare the ALT result with the level of Aspartate Aminotransferase (AST) to determine the likely cause of the damage. Since ALT is more concentrated in the liver than AST, an ALT level higher than AST (an AST/ALT ratio less than 1) commonly suggests fatty liver disease or acute viral hepatitis.
Conversely, an AST/ALT ratio greater than 1 (where AST is higher) can point toward more advanced liver disease, such as cirrhosis. A ratio exceeding 2:1 is highly suggestive of alcohol-related liver disease, as alcohol metabolism disproportionately affects AST release. The magnitude of the elevation also provides clues; mild elevation (up to twice the upper limit) may be transient, while severe elevation (more than ten times the upper limit) often signals acute liver failure or severe viral hepatitis.
If an initial blood test shows elevated ALT, the typical follow-up involves repeating the liver function tests within a few weeks to determine if the elevation is persistent. If the levels remain high, a comprehensive evaluation is initiated, which includes a broader panel of blood tests, such as bilirubin and alkaline phosphatase, to assess overall liver function and bile duct health. Imaging tests, particularly an abdominal ultrasound, may also be ordered to visualize the liver structure and check for fat deposits or scarring.