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 a patient has hepatitis that the doctor suspects may be due to an autoimmune-related process
Sample Required?
A blood sample taken from a vein in your arm
Test Preparation Needed?
None
The Test Sample
What is being tested?
This test looks for the presence (and if present the level) of smooth muscle antibodies (SMA) in the blood. Antibodies usually protect the body against infecting organisms such as bacteria and viruses. However, antibodies which are produced against parts of the body rather than outside organisms are known as autoantibodies and can cause disease rather the protect against it. SMA are autoantibodies produced by the body’s immune system that are directed against proteins of the smooth muscle. These ‘cytoskeletal’ proteins form a framework within a cell and support its structure. Smooth muscle differs from skeletal muscle we use for walking, lifting, and other movements e.g. biceps, triceps muscles. Smooth muscle is present in the walls of blood vessels, the bowel and bladder, amongst other locations. F-actin is a specific cytoskeleton protein which SMA may target. Testing for autoantibodies to F-actin is performed in some laboratories. It is thought that its performance is similar, or possibly better, than SMA itself. The clinical utility of the anti-actin test has yet to be established, and it is not widely used in the UK.
The presence of SMA is strongly associated with autoimmune hepatitis (more specifically type 1 autoimmune hepatistis). It may also be seen much less commonly in other forms of liver disease such as primary biliary cirrhosis, and in viral infections. Usually higher levels of SMA are seen in autoimmune hepatitis compared with other causes.
Autoimmune hepatitis is an acute or chronic inflammation of the liver caused by the immune system mistakenly attacking the cells of the liver. It is not the direct result of other causes 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.
How is the sample collected for testing?
A blood sample is taken by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
The Test
How is it used?
The smooth muscle antibody (SMA) test is usually requested 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 used to help diagnose autoimmune liver disease and distinguish it from other causes of liver disease or injury. LKM1 antibodies are found in autoimmune hepatitis type 2, and AMA in primary biliary cirrhosis.
When is it requested?
The SMA test and the ANA test are used when a doctor suspects that the patient has autoimmune hepatitis. They are usually requested when a patient shows symptoms and/or signs of liver disease such as tiredness and jaundice. They may also be requested if abnormal findings are found on routine liver tests, such as elevated aspartate aminotransferase (AST) and/or bilirubin.
SMA and ANA are usually used to help diagnose autoimmune hepatitis and/or help exclude other causes of liver injury. Other causes of liver disease can include viral infections, such as viral hepatitis, drugs, alcohol abuse, toxins, genetic conditions, metabolic conditions, and other autoimmune disease e.g. primary biliary cirrhosis.
What does the test result mean?
When high levels of SMA, or SMA and ANA, or SMA and F-actin antibodies are present in the blood, the most likely cause is autoimmune hepatitis.
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 the liver disease 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. The best means to diagnose autoimmune hepatitis is a liver biopsy.
Low levels of SMA may be seen in viral infection, sometimes transiently. Examples include Epstein-Barr virus (infectious mononucleosis), adenovirus and hepatitis B.
Is there anything else I should know?
Levels of SMA may be lower in children and in those with compromised immune systems. The levels may vary over the course of the disease, but may not be related to the severity of autoimmune symptoms or to a patient’s prognosis. Hence SMA are not used for monitoring in autoimmune hepatitis.
The presence of SMA, and F-actin antibodies 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. Even if these autoantibodies are negative, a liver biopsy may still be performed if the doctor suspects autoimmune hepatitis, or to needs to establish the cause of the liver disease more precisely.
Common Questions
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Article Sources
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.
