Smooth Muscle Antibody

This test measures the amount of smooth muscle antibodies (SMA) in the blood. SMA are proteins produced by the body’s immune system that are directed against its own cytoskeletal proteins of the smooth muscle. Cytoskeletal proteins form the framework within a cell that serve to support or reinforce the cell structure. The production of SMA is strongly associated with chronic active autoimmune hepatitis but may also be seen in other forms of liver disease and with other autoimmune disorders such as primary biliary cirrhosis, but usually at lower titers. Autoimmune hepatitis presents as an acute or chronic inflammation of the liver that is not a result of another discernable cause – such as a viral infection, drug, toxin, hereditary disorder, or alcohol abuse. It can lead to liver cirrhosis and, in some cases, to liver failure.

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.

A blood sample is obtained by inserting a needle into a vein in the arm.

No test preparation is needed.