Smooth Muscle Antibody
Autoimmune hepatitis can be found in anyone at any age, but about 80% of those affected are women. In the United States, more than 70% of patients with this disorder will have SMA, either alone or along with ANA (antinuclear antibodies). F-actin is a specific cytoskeletal protein whereby autoantibodies directed against it may be increased. Some recent studies suggest that it is a more specific test than SMA in diagnosing autoimmune liver disease, with about 52% to 85% of those affected having the F-actin autoantibody.
How is the sample collected for testing?A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?No test preparation is needed.
How is it used?The smooth muscle antibody (SMA) test is primarily ordered along with antinuclear antibodies (ANA) to help diagnose autoimmune hepatitis. Other autoantibodies, such as liver-kidney microsomal type 1 (LKM1) antibodies and antimitochondrial antibodies (AMA), may also be ordered to help diagnose autoimmune hepatitis and distinguish it from other causes of liver disease or injury.
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?This test and the ANA test are ordered when a doctor suspects that the patient has autoimmune hepatitis. They are usually ordered when a patient presents with symptoms such as fatigue and jaundice along with abnormal elevated findings on routine liver tests, such as aspartate aminotransferase (AST) and/or bilirubin.
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 significant amounts of SMA and ANA are present in the blood, the most likely cause is autoimmune hepatitis. When both are present, then systemic lupus erythematosus, another autoimmune disorder, can be essentially ruled out as ANA will be positive with lupus but SMA is not.
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?Concentrations 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 may not be related to the severity of autoimmune symptoms or to a patient’s prognosis.
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.
Will smooth muscle antibody (SMA) ever go away?If it is due to a temporary condition, such as infectious mononucleosis, SMA may drop below detectible levels once the condition has resolved. If SMA is produced because of autoimmune hepatitis, then it will be present throughout the patient's life, although levels may vary over time.
Can I have more than one cause of hepatitis?Yes. For instance, autoimmune hepatitis can co-exist with a viral hepatitis, such as 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 doctor understand the underlying cause(s) of your condition.
How fast does autoimmune hepatitis progress?