Smooth muscle antibodies (SMA)

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Also known as: SMA; ASMA; F-actin antibody; SMA-T; SMA-VGT' SMA-V; anti-smooth muscle antibody
Formal name: Smooth muscle antibody; anti-actin antibody

At a Glance

Why Get Tested?

To help diagnose autoimmune hepatitis and distinguish it from other causes of liver injury

When to Get Tested?

When a patient has hepatitis that the doctor suspects may be due to an autoimmune-related process

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?


The Test Sample

What is being tested?

This test measures the amount of smooth muscle antibodies (SMA) in the blood. SMAs are proteins produced by the body’s immune system to work against its own cytoskeletal proteins. The production of SMA is strongly associated with chronic autoimmune hepatitis but may also be seen in other forms of liver disease and with other autoimmune disorders such as primary biliary cirrhosis. Autoimmune hepatitis presents as an acute or chronic inflammation of the liver that is not caused by another discernable cause (such as a viral infection, drug, toxin, hereditary disorder or alcohol abuse). It can lead to cirrhosis (liver damage and scarring) and, in some cases, to liver failure.

There are 3 subtypes of SMA antibodies, which reflect their staining pattern when measured by indirect immunofluorescence: SMA-V, SMA-VG and SMA-VGT, the latter often called just SMA-T.

In adults, the SMA-T pattern is strongly correlated with auto-immune hepatitis, though when treated any pattern may be present (the antibodies may even disappear).

In children, the antibodies may be present at lower level (titre) than adults and the SMA-V pattern may also indicate autoimmune hepatitis. In adults, SMA-V or VG often indicates a viral infection (hepatitis C), or other auto-immune condition e.g. SLE.

Autoimmune hepatitis can be found in anyone at any age, but about 80% of those affected are women. More than 80% of patients with this disorder will have SMA, either alone or along with antinuclear antibodies (ANA).

Anti-F-actin is an antibody targeted at actin, a specific cytoskeletal protein. Some recent studies suggest that it is a more specific test than SMA for diagnosing autoimmune liver disease, with about 52% to 85% of those affected having the anti-F-actin antibody. Anti-F-actin antibody measurements are only available in a few laboratories and these assays lack sensitivity at present.

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.

The Test

Common Questions

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.