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Smooth Muscle 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 you have hepatitis that your healthcare practitioner suspects may be due to an autoimmune-related process

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

The Test Sample

What is being tested?

Smooth muscle antibodies (SMA) are autoantibodies, proteins produced by the body's immune system that recognize and attack its own actin, a protein found in smooth muscle and other tissues, especially the liver. This test detects and measures the amount (titer) of SMA (or antibody against actin) in the blood.

The production of smooth muscle or actin antibodies is strongly associated with autoimmune hepatitis. It may also sometimes be seen in other forms of liver disease, such as primary biliary cholangitis (PBC), but usually at lower antibody titers.

Autoimmune hepatitis occurs when the immune system attacks the body's liver cells. It presents as an acute or chronic inflammation of the liver that is not due to another cause, such as a viral infection, exposure to a drug or toxin, a hereditary disorder, or alcohol abuse. It can lead to liver cirrhosis and, in some cases, to liver failure. Autoimmune hepatitis can affect anyone at any age, but about 80% of patients are women. In the United States, more than 70% of people with this disorder will have SMA or actin antibodies, either alone or along with antinuclear antibodies (ANA).

The majority of smooth muscle antibodies produced with autoimmune hepatitis is specifically directed against a protein called actin or F-actin. Testing is available for specific actin autoantibodies, but it is not available in every laboratory. Tests for actin antibodies detect more cases of autoimmune hepatitis but, in some studies, give more false-positive results than tests for smooth muscle antibodies.

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

How is it used?

The smooth muscle (SMA) or actin antibody test is primarily ordered along with antinuclear antibodies (ANA) and liver kidney microsomal type 1 (LKM-1) antibodies to help diagnose autoimmune hepatitis and to differentiate between the two major types of autoimmune hepatitis, type 1 and type 2.

When is it ordered?

The SMA (or actin antibody), ANA, and rarely LKM1 tests are ordered when a healthcare practitioner suspects that someone has autoimmune hepatitis. They are usually ordered when a person presents with symptoms such as fatigue and jaundice along with abnormal findings on routine liver tests such as aspartate aminotransferase (AST) and/or bilirubin.

These autoantibody tests may be ordered along with other testing when a healthcare practitioner is investigating liver disease and wants to distinguish between different causes of liver injury, including viral infections, drugs, alcohol abuse, toxins, genetic conditions, metabolic conditions, and autoimmune disorders.

What does the test result mean?

A significant increase in the blood titer of SMA or actin antibody and/or ANA is usually due to autoimmune hepatitis type 1. A small increase in SMA or actin antibody may be present in up to 50% of patients with primary biliary cholangitis (PBC). The SMA autoantibody may also be found in other conditions, such as infectious mononucleosis, hepatitis C, and some cancers.

A negative SMA or actin antibody test and an increased titer of LKM1 may indicate autoimmune hepatitis type 2. Type 2 is uncommon in the United States.

If the SMA or actin antibody test and other antibody tests are negative, then symptoms and liver injury may be due to causes other than autoimmune hepatitis.

Is there anything else I should know?

Titers 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 are not closely related to the severity of autoimmune symptoms or to a person's prognosis.

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 healthcare practitioner suspects autoimmune hepatitis, then a liver biopsy may be performed to look for characteristic signs of damage and scarring in the liver tissue.



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