What Does a Low ALT and AST Signify?

Lower levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are good. These indicate that the liver is functioning properly, states WebMD. Liver problems may be suspected if ALT and AST levels are high.

A blood test for ALT and AST checks for functional abnormalities in a person's liver, says WebMD. These tests are usually performed together, and they are part of a comprehensive metabolic panel, or CMP, which is used by doctors to check a patient for a liver disease and begin treatment if a liver problem exists. While both tests are used to evaluate liver function, the ALT test is more precise than the AST test in evaluating liver function, says labtestsonline.org. Physicians might sometimes compare the results of an ALT test and an AST test in a side-by-side comparison. They do so to check for different causes of liver damage and determine whether a person's problem stems from a liver condition or from injury or damage to the heart or muscles.

The Liver
The liver is an important organ that performs many valuable functions. It plays a key role in digestion, and it also helps the body fight off illnesses. One of the liver's main tasks is to produce a substance called bile, which facilitates digestion. The liver also eliminates toxins and other harmful products from the bloodstream. The liver produces proteins and clotting agents that help blood clot properly. The liver's function can be impaired in several ways, such as through an injury, illness or drug and alcohol abuse.

Liver Function Tests
Physicians order a liver function test for several reasons, according to Lab Tests Online. They will most likely recommend a test if a patient presents with symptoms of a malfunctioning liver. Symptoms associated with liver problems include jaundice, nausea and vomiting, loss of appetite, itching, swelling in the legs and ankles, easy bruising, dark urine, light-colored stools, fatigue and general weakness. Physicians might also recommend that their patients get an AST test, which is sometimes ordered in conjunction with other liver function tests if they meet certain conditions. Liver function tests are commonly prescribed for people who have been exposed to the hepatitis virus, people who have a family history of liver disease, people who drink excessive amounts of alcohol or abuse drugs and individuals who are overweight or have diabetes. Doctors may also recommend that patients have their liver function regularly monitored if they are taking medications that are known to cause liver problems.

What the Results Mean
While low ALT and AST levels are good, high levels can indicate a problem, states WebMD. Sometimes, high levels of AST are associated with certain types of infections, such as viral infections. Exposure to drugs and other substances considered toxic to the liver will also raise AST levels. Sometimes, AST levels are elevated in patients with problems stemming from other parts of the body. If a person recently had a heart attack, for instance, AST levels might be higher than normal. Muscle injury can also cause higher levels of AST and ALT, but AST levels are usually significantly higher than ALT levels, says WebMD. Some factors, such as pregnancy, strenuous exercise and the use of certain medications can affect the results of liver function tests. Therefore, people are encouraged to tell their doctors about all medications they are taking before having a liver function test.