To help diagnose autoimmune hepatitis and distinguish it from other causes of liver injury
Smooth Muscle Antibody (SMA) and F-actin Antibody
When you have hepatitis that your healthcare practitioner suspects may be due to an autoimmune-related process
A blood sample drawn from a vein in your arm
None
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How is it used?
The smooth muscle (SMA) or actin antibody test is primarily ordered along with antinuclear antibodies (ANA) and liver kidney microsomal type 1 (LKM-1) antibodies to help diagnose autoimmune hepatitis and to differentiate between the two major types of autoimmune hepatitis, type 1 and type 2.
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When is it ordered?
The SMA (or actin antibody), ANA, and rarely LKM1 tests are ordered when a healthcare practitioner suspects that someone has autoimmune hepatitis. They are usually ordered when a person presents with symptoms such as fatigue and jaundice along with abnormal findings on routine liver tests such as aspartate aminotransferase (AST) and/or bilirubin.
These autoantibody tests may be ordered along with other testing when a healthcare practitioner is investigating liver disease and wants to distinguish between different causes of liver injury, including viral infections, drugs, alcohol abuse, toxins, genetic conditions, metabolic conditions, and autoimmune disorders.
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What does the test result mean?
A significant increase in the blood titer of SMA or actin antibody and/or ANA is usually due to autoimmune hepatitis type 1. A small increase in SMA or actin antibody may be present in up to 50% of patients with primary biliary cholangitis (PBC). The SMA autoantibody may also be found in other conditions, such as infectious mononucleosis, hepatitis C, and some cancers.
A negative SMA or actin antibody test and an increased titer of LKM1 may indicate autoimmune hepatitis type 2. Type 2 is uncommon in the United States.
If the SMA or actin antibody test and other antibody tests are negative, then symptoms and liver injury may be due to causes other than autoimmune hepatitis.
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Is there anything else I should know?
Titers of SMA may be lower in children and in those with compromised immune systems. The levels may vary over the course of the disease and are not closely related to the severity of autoimmune symptoms or to a person's prognosis.
The presence of SMA, F-actin antibodies, and ANA are highly suggestive of autoimmune hepatitis but not diagnostic. When significant concentrations of both are present and the healthcare practitioner suspects autoimmune hepatitis, then a liver biopsy may be performed to look for characteristic signs of damage and scarring in the liver tissue.
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Will smooth muscle antibody (SMA) ever go away?
If it is due to a temporary condition, such as infectious mononucleosis, SMA may drop below detectable levels once the condition has resolved. If SMA is produced because of autoimmune hepatitis, then it will be present throughout a person's life, although levels may vary over time.
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Can I have more than one cause of hepatitis?
Yes. For instance, autoimmune hepatitis can co-exist with a viral hepatitis, such as hepatitis B or hepatitis C, and can be exacerbated by liver damage caused by alcohol abuse. Since the treatment of hepatitis depends on the cause, it is very important that your healthcare practitioner understand the underlying cause(s) of your condition.
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How fast does autoimmune hepatitis progress?
The course and severity of autoimmune hepatitis is hard to predict. It may be acute or chronic. Some people will have no or few symptoms for many years and are diagnosed when routine liver tests are abnormal. For more information, consult with your healthcare practitioner and see the links to other resources in the Related Pages section.