Smooth Muscle Antibody
The Test
How is it used?
The anti-actin test may sometimes be ordered as an initial screening test for autoimmune hepatitis instead of the SMA test. If it is positive, it may be followed by the SMA to confirm the finding. The anti-actin test is relatively new and, in some cases, is taking the place of the SMA test. The ultimate clinical utility of the anti-actin test has yet to be established.
When is it ordered?
SMA and ANA are usually ordered to help diagnose and/or rule out other causes of liver injury. These causes can include viral infections, such as viral hepatitis, drugs, alcohol abuse, toxins, genetic conditions, metabolic conditions, and primary biliary cirrhosis.
What does the test result mean?
When F-actin antibodies are present in significant quantities in a patient with clinical signs of autoimmune hepatitis, then it is likely that the patient has this condition. In most cases, if the anti-actin is positive, the SMA will also be positive. Since actin is only one of several cytoskeleton proteins, it is possible for a person to have smooth muscle antibodies even when the anti-actin test is negative.
If the SMA test is negative, then symptoms may be due to causes other than autoimmune hepatitis. However, up to 20% of patients with autoimmune hepatitis will not be positive for SMA, ANA, or LKM1 antibodies.
Is there anything else I should know?
Small amounts of SMA may be present, along with AMA, in up to 50% of patients with primary biliary cirrhosis and may be found in other conditions such as infectious mononucleosis and some cancers. F-actin antibodies may be present in about 22% of patients with primary biliary cirrhosis
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 doctor suspects autoimmune hepatitis, then a liver biopsy may be performed to look for characteristic signs of damage and scarring in the liver tissue.
