What Does a Low ALT and AST Signify?

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The alanine transaminase (ALT) test and aspartate aminotransferase (AST) test are two of the most common liver function tests that are ordered. Your doctor typically orders these types of blood tests because he or she suspects some type of liver disease or other abnormality. AST and ALT are both enzymes that the liver releases, usually in response to infection, damage, or disease. While these two tests measure different enzymes, they are typically ordered concurrently, often along with other liver function tests.

Why Does a Doctor Order an ALT and AST Test?

Your healthcare provider orders ALT and AST testing to check on liver function. The underlying cause could be due to several reasons. You may be experiencing symptoms of liver dysfunction, or you may have other conditions that affect liver function, such as anemia, diabetes, or high blood pressure. If you are a heavy or alcoholic drinker or have hepatitis B or C, your physician may want to check liver function. If you already suffer from liver disease or have in the past, your doctor may want a closer look to ensure your health. 

What Is the ALT Test?

ALT is an enzyme whose function is to metabolize protein. If your liver is not functioning properly and not metabolizing protein correctly, ALT enzymes are released into the blood. This shows up on an ALT screening test.

What Are Normal, Low, and High ALT Ranges?

Normal ALT ranges are below 25 IU/L (international units per liter) in females, and below 33 IU/L in males, according to the American College of Gastroenterology. A low ALT result is not an indicator of poor health. A low level means that your liver is functioning well and normally. It is only high results (above 25 IU/L in females and 33 IU/L in males) that would concern a physician. 

What Is the AST Test?

Like ALT, AST is another enzyme, but AST is not only found in the liver. It is also found in the heart and muscles. It is not as comprehensive of an indicator for liver disease alone, which is the reason that ALT and AST are routinely tested together. A high level of both enzymes can be indicative of liver malfunctioning. Like ALT, when the liver is damaged, the level of AST is released by the liver into the bloodstream, creating a higher level.

What Are Normal, Low, and High AST Ranges? 

A normal result for AST is below 40 IU/L in both males and females. If an infant or young child is being tested, the values may be slightly higher due to their age. Like ALT, a low AST value means that your liver is functioning properly and there is likely no cause for alarm. a low AST range does not indicate disease. It is a high range (above 40 IU/L) that is concerning. 

What Are Other Liver Function Tests?

If you receive ALT and AST values that are elevated, your doctor may want to perform other tests to check your liver function more thoroughly. These include:

  • Alkaline phosphatase (ALP) test, which is another liver enzyme blood test
  • Albumin test, which checks the main type of protein secreted by your liver
  • Bilirubin test, which checks for waste byproduct from red blood cells (RBCs)
  • Gamma-glutamyl transferase test (GGT), which is a liver enzyme test that checks for liver disease related to alcoholism 

Your doctor may also order other testing or procedures if liver function tests prove to be abnormal, such as a liver biopsy, CT scan, or ultrasound.