Alanine aminotransferase (ALT) is an enzyme found mostly in the cells of the liver and kidney. Much smaller amounts of it are also found in the heart and muscles. In healthy individuals, ALT levels in the blood are low. When the liver is damaged, ALT is released into the bloodstream, usually before more obvious signs of liver damage occur, such as jaundice. This makes ALT a useful test for detecting liver damage.
The liver is a vital organ located in the upper right-hand side of the abdominal area. It is involved in many important functions in the body. The liver helps to process the body's nutrients, manufactures bile to help digest fats, produces many important proteins such as blood clotting factors, and breaks down potentially toxic substances into harmless ones that the body can use or excrete.
A number of conditions can cause damage to liver cells, resulting in an increase in ALT. The test is most useful in detecting damage due to hepatitis or as a result of drugs or other substances that are toxic to the liver.
ALT is commonly tested in conjunction with aspartate aminotransferase (AST), another liver enzyme, as part of a liver panel. Both ALT and AST levels usually rise whenever the liver is being damaged, although ALT is more specific for the liver and, in some cases, may be the only one of the two to be increased. An AST/ALT ratio may be calculated to aid in distinguishing between causes and severity of liver injury and to help distinguish liver injury from damage to heart or muscles.
Why it is done?
A health practitioner usually orders an ALT test to evaluate a person who has signs and symptoms of a liver disorder. Some of these may include:
ALT may also be ordered, either by itself or with other tests, for people who are at an increased risk for liver disease since many people with mild liver damage will have no signs or symptoms. Some examples include:
- Persons who have a history of known or possible exposure to hepatitis viruses
- Those who are heavy drinkers
- Individuals whose families have a history of liver disease
- Persons who take drugs that might damage the liver
- Persons who are overweight and/or have diabetes
In persons with mild symptoms, such as fatigue or loss of energy, ALT may be tested to make sure they do not have chronic liver disease.
What does the test result mean?
Normally, levels of ALT in the blood are low.
Very high levels of ALT (more than 10 times normal) are usually due to acute hepatitis, sometimes due to a viral infection. In acute hepatitis, ALT levels usually stay high for about 1-2 months but can take as long as 3-6 months to return to normal. Levels of ALT may also be markedly elevated (sometimes over 100 times normal) as a result of exposure to drugs or other substances that are toxic to the liver as well as in conditions that cause decreased blood flow (ischemia) to the liver.
ALT levels are usually not as high in chronic hepatitis, often less than 4 times normal. In this case, ALT levels often vary between normal and slightly increased, so the test may be ordered frequently to see if there is a pattern.
Other causes of moderate increases in ALT include obstruction of bile ducts, cirrhosis (usually the result of chronic hepatitis or bile duct obstruction), heart damage, alcohol abuse, and with tumors in the liver.