Smooth Muscle Antibody
Other names: ASMA; Anti-Smooth Muscle Antibody; SMA
SMA is a type of autoantibody, which attack healthy tissue; autoantibodies are produced when the body mistakes healthy tissue for something ’foreign’. Antibodies are usually produced by the body to fight off illnesses and infections. SMA is produced against proteins of the smooth muscle; smooth muscle is present inside the walls of the bladder, bowel and blood vessels.
When is the test used?
The test is used to detect SMA, which is commonly associated with autoimmune hepatitis. A test for ANA (antinuclear antibodies) and AMA (antimitochondrial antibodies) may also be ordered; the results of the test can be used to distinguish between autoimmune liver disease and other types of liver disease.
The test is usually ordered when a patient shows symptoms of liver conditions, including autoimmune hepatitis; these include jaundice, weakness and fatigue. The test may also be ordered if the patient’s liver function tests show abnormal results.
How is the test performed?
The test is performed by collecting and analysing a sample of the patient’s blood; a needle is inserted into the vein and the blood is drawn out and collected in a syringe. Once the doctor or nurse has a sufficient sample, the blood will be bottled, labelled with the patient’s name and sent away to the laboratory for testing.
What do the test results mean?
If levels of SMA, SMA and ANA or SMA and F-actin antibodies are high, this usually indicates that the patient has autoimmune hepatitis; however, the test result is not sufficient evidence to reach a diagnosis and further tests will be ordered.
If the test result is negative and there are no SMA antibodies, this usually indicates that liver disease is due to a different cause; however, in some cases, patients with autoimmune hepatitis may not test positive for SMA antibodies and a liver biopsy test will usually be ordered.
Low levels of SMA may be caused by viral infections, such as hepatitis B and infections caused by the Epstein-Barr virus.
Levels of SMA are lower in children and those who have a compromised immune system.
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Blood Test Guide
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