Smooth Muscle Antibody (SMA) Test

Explanation

A smooth muscle antibody, or SMA, test is a blood test for antibodies that react with some of the body's own proteins. It can be one part of an assessment for autoimmune hepatitis, a condition in which the immune system attacks the liver.

Does this test measure muscles?

No. The test does not measure muscle strength or muscle damage. It looks for autoantibodies in blood. Autoantibodies are antibodies that react with the body's own proteins.

The name comes from the type of cell material these antibodies react with in laboratory testing. The result is mainly used as one clue when doctors assess possible autoimmune liver disease.

Does a positive result diagnose autoimmune hepatitis?

No. Smooth muscle antibodies can support the diagnosis, but they can also occur in other conditions. The result alone does not establish autoimmune hepatitis or show how severe a liver condition is.

Doctors consider the result with symptoms, liver enzymes and other antibodies. Liver enzymes are substances measured in blood that can rise when liver cells are injured. They may also measure immunoglobulin G, or IgG, a type of antibody.

Can a negative result rule it out?

No. A negative SMA result does not exclude autoimmune hepatitis. Doctors may use other blood tests, assess other possible causes of liver disease and sometimes examine a small liver tissue sample with a biopsy.

Laboratories may offer related antibody tests, but names such as SMA and F-actin, a particular protein target, do not always describe identical tests. Interpret the exact test name and result with the laboratory's information.

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