Also Known As
Anti-Smooth Muscle Antibody
Formal Name
Smooth Muscle Antibody
This article was last reviewed on
This article waslast modified on 13 July 2023.
At a Glance
Why Get Tested?

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

When To Get Tested?

When a patient has hepatitis or a liver disorder 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?


On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

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 than 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 that 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 one 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 hepatitis). It may also be seen much less commonly in other forms of liver disease such as primary biliary cirrhosis, and in viral infections (e.g. hepatitis B). 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.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Accordion Title
Common Questions
  • 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 an SMA test is requested, it automatically includes screening for the presence of AMA or LKM1 antibodies, as well as gastric parietal antibodies (GPA) using mouse kidney, stomach, liver tissues, to help aid diagnosis.

  • 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 blood tests, such as an 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 diseases 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 with a liver biopsy. A low titre positive SMA result may also occur in those with Non-Alcoholic Fatty Liver Disease (NAFLD).

  • 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 purposes 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.

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

    If it is due to a temporary condition, such as a viral infection, SMA may become negative (not be detected in the blood) once the condition has resolved. If SMA is produced because of autoimmune hepatitis, then it will be present in the long-term, though levels may vary over time.

  • Can I have more than one cause of hepatitis?

    Yes. For instance, autoimmune hepatitis and a viral hepatitis, such as hepatitis C, and can be made worse by liver damage caused by alcohol abuse. Since the treatment of hepatitis depends on the cause, it is very important that your doctor understands 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. Many patients will have no or few symptoms for many years and are diagnosed when routine liver tests are abnormal. For more information, consult with your doctor and see the related links.