Smooth Muscle Antibody (SMA) and F-actin Antibody

Also Known As

SMA

Anti-Smooth Muscle Antibody

ASMA

Actin Antibody

F-Actin Antibody

ACTA

Formal Name

Smooth Muscle Antibody

This article was last reviewed on

March 17, 2017.

This article waslast modified on February 1, 2018.

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

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Lab Tests Online is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.

The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called "normal" values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are "within normal limits."

For more information, please read the article Reference Ranges and What They Mean.

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.

Common Questions

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.
Will smooth muscle antibody (SMA) ever go away?

If it is due to a temporary condition, such as infectious mononucleosis, SMA may drop below detectable levels once the condition has resolved. If SMA is produced because of autoimmune hepatitis, then it will be present throughout a person'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 B or 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 healthcare practitioner understand the underlying cause(s) of your condition.
How fast does autoimmune hepatitis progress?

The course and severity of autoimmune hepatitis is hard to predict. It may be acute or chronic. Some people will have no or few symptoms for many years and are diagnosed when routine liver tests are abnormal. For more information, consult with your healthcare practitioner and see the links to other resources in the Related Pages section.

See More Common Questions See Less Common Questions

On This Site

Tests: ANA; Autoantibodies; AST; Bilirubin; Antimitochondrial Antibody and AMA M2; Liver Kidney Microsomal Type 1 (LKM-1) Antibodies

Conditions: Autoimmune Diseases, Liver Disease, Autoimmune Hepatitis

Elsewhere On The Web

American Liver Foundation: Autoimmune Hepatitis
National Digestive Diseases Clearinghouse: Autoimmune Hepatitis
Mayo Clinic: Autoimmune hepatitis

View Sources

Sources Used in Current Review

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Sloan, J. et. al. (2014 March 4, Updated). Anti-Smooth-Muscle Antibody. Medscape Drugs and Diseases. Available online at http://emedicine.medscape.com/article/2086774-overview. Accessed on 02/18/17.

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Slev, P. and Tebo, A. (2016 December Updated). Autoimmune Hepatitis - Hepatitis, Autoimmune. ARUP Consult. Available online at https://arupconsult.com/content/autoimmune-hepatitis. Accessed on 02/18/17.

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(© 2005). Smooth Muscle Antibody, IgG with Reflex to Titer. ARUP's Guide to Clinical Laboratory Testing [On-line information]. Available online at http://www.arup-lab.com/guides/clt/tests/clt_203b.jsp#3219875.

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