Alanine Aminotransferase (ALT)
When a healthcare practitioner thinks that you have symptoms of a liver disorder, such as abdominal pain, nausea and vomiting, jaundice (yellowish discoloration of the skin or white of the sclerae), or potential exposure to hepatitis viruses; it can also be ordered as part of a comprehensive metabolic panel (CMP) when you have a routine health exam.
A blood sample drawn from a vein in your arm
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. Normally, ALT levels in blood are low, but when the liver is damaged, ALT is released into the blood and the level increases. This test measures the level of ALT in the blood and is useful for early detection of liver disease.
The function of ALT is to convert alanine, an amino acid found in proteins, into pyruvate, an important intermediate in cellular energy production. In healthy individuals, ALT levels in the blood are low. When the liver is damaged, ALT is released into the blood, usually before more obvious signs of liver damage occur, such as jaundice.
The liver is a vital organ located in the upper right side of the abdomen, just beneath the rib cage. 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 albumin, and breaks down potentially toxic substances into harmless ones that the body can use or eliminate.
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 (inflammation of the liver) 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 different causes and severity of liver injury and to help distinguish liver injury from damage to heart or muscles.
How is the test used?
The alanine aminotransferase (ALT) test is typically used to detect liver injury. It is often ordered in conjunction with aspartate aminotransferase (AST) as part of a liver panel or comprehensive metabolic panel (CMP) to screen for and/or help diagnose liver disease.
AST and ALT are considered to be two of the most important tests to detect liver injury, although ALT is more specific to the liver than is AST. Sometimes AST is compared directly to ALT and an AST/ALT ratio is calculated. This ratio may be used to distinguish between different causes of liver damage and to help recognize heart or muscle injury.
ALT values are often compared to the results of other tests such as alkaline phosphatase (ALP), gamma glutamyl transferase (GGT), total protein, albumin-to-globulin ratio (A/G ratio), and bilirubin to help determine which form of liver disease is present.
ALT is often used to monitor the treatment of liver disease, to see if the treatment is working, and may be ordered either by itself or along with other tests for this purpose.
When is it ordered?
ALT may be ordered as part of a comprehensive metabolic panel when you have a routine health examination.
- Weakness, fatigue
- Loss of appetite
- Nausea, vomiting
- Abdominal distension/fullness and/or pain
- Jaundice (icterus)
- Dark urine or tea-colored urine
- Light-colored stool or clay-colored stool
- Skin itchiness (pruritus)
ALT may also be ordered, either by itself or with other tests, when you have an increased risk for liver disease. This is because many people with mild liver damage will have no signs or symptoms. Even without symptoms, ALT can be increased with mild liver damage. You may be at risk of liver disease when you:
- Have a history of known or possible exposure to hepatitis viruses
- Are a heavy alcohol drinker
- Have a family history of liver disease
- Take drugs that might damage the liver, such as acetaminophen
- Are overweight, obese and/or have diabetes or metabolic syndrome
ALT may be ordered on a regular basis when it is used to monitor the treatment of liver disease.
What does the test result mean?
A low level of ALT in the blood is expected and is normal. Liver disease is the most common reason for higher than normal levels of ALT.
Very high levels of ALT (more than 10 times normal) are usually due to acute hepatitis, sometimes due to a viral hepatitis 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 or in conditions that cause decreased blood flow to the liver.
ALT levels are usually not much elevated 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.
In certain types of liver diseases, the ALT level is higher than AST and the AST/ALT ratio will be low (less than 1). There are a few exceptions; the AST/ALT ratio is usually greater than 1 in alcoholic hepatitis, cirrhosis, and with heart or muscle injury and may be greater than 1 for a day or two after onset of acute hepatitis.
Is there anything else I should know?
A shot or injection of medicine into the muscle tissue, or strenuous exercise, may increase ALT levels.
Many drugs may raise ALT levels by causing liver damage in a very small percentage of patients taking the drug. This is true of both prescription drugs and some "natural" health products. Be sure to tell your healthcare provider about all of the drugs and/or health supplements you are taking.
What conditions other than liver problems can cause increased ALT?
ALT is more specific for the liver than AST and so is much less affected by conditions affecting other parts of the body. Nevertheless, injury to organs other than the liver, such as the heart and skeletal muscle, can cause elevations of ALT. For example, small increases may be seen with skeletal muscle damage or heart attacks.
What other tests may be performed to help determine the cause of liver damage?
After a thorough physical exam and evaluation of a person's medical history, there are several other tests that may be performed as follow up depending on what is suspected to be the cause of liver damage. Some of these include:
- Tests for hepatitis A, hepatitis B, and hepatitis C
- Testing for exposure to drugs and other substances toxic to the liver (see Drug Abuse Testing and Emergency and Overdose Drug Testing)
- Ethanol level
- Serum copper level and ceruloplasmin for Wilson disease
- Iron tests and genetic tests for hereditary hemochromatosis
A liver biopsy may be performed to help determine the cause of liver injury and to evaluate the extent of liver damage.
For more information, see the article on Liver Disease.