What is ALT?

Alanine aminotransferase (ALT) is one of the tests that is often used to check for liver health. ALT is an enzyme found primarily in the liver. Although ALT is also present in smaller amounts in the kidneys, heart, muscles, and pancreas, its concentration in the liver and its release into the bloodstream in response to liver cell damage makes it a significant marker for liver health.

Why is it important?

The importance of ALT lies in its role as an indicator of liver health. When liver cells are damaged due to causes such as viral infections (like hepatitis), excessive alcohol consumption, or drug toxicity, ALT is released into the bloodstream. Therefore, an elevated ALT level often suggests liver inflammation or damage. But what's the difference between the two liver enzymes ALT and AST (aspartate aminotransferase)? "Both aminotransferases are highly concentrated in the liver. AST is also diffusely represented in the heart, skeletal muscle, kidneys, brain and red blood cells, and ALT has low concentrations in skeletal muscle and kidney; an increase in ALT serum levels is, therefore, more specific for liver damage." (Giannini, 2005)

Frequently checking ALT levels can help diagnose liver diseases early and monitor the progression of a liver condition or the effectiveness of a certain treatment regimen. If ALT is chronically elevated, it is important to try and understand the potential causes. While ALT is more specific to the liver than AST, your muscles, kidneys and other organs can also make ALT. So to better understand what an abnormal ALT means, it's best to consult with your physician if you are concerned about your liver enzyme results.

How can I better understand my results?

ALT typically be presented in units per liter (U/L). The "normal reference" ranges can vary slightly between laboratories but generally fall within the range of:

  • Adults: ~10 to ~40 units per liter (U/L). For “normal” reference range for women is often lower then men.

It's important to understand that these "normal" ranges are based on average values from a specific population, and it is hard to know what the general health of this population that was used to come up with the “normal” range. Factors such as age, gender, race, and even muscle mass can influence ALT levels. Therefore, the population used to develop these ranges might not align with your specific health profile or goals. In general, lower is typically better.

For instance, slightly elevated ALT levels could be considered normal for some individuals, depending on various health and lifestyle factors. Conversely, a result within the "normal" range doesn't always guarantee a perfectly healthy liver. It's also important to consider other liver enzyme tests, like AST, and the overall clinical context.

Where can I learn more?

Cleveland Clinic: Alanine Transaminase (ALT)

Medscape: Alanine Aminotransferase

Kalas, MA, et al. 2021. Abnormal liver enzymes: A review for clinicians