Smooth Muscle Antibody
When a patient has hepatitis or a liver disorder that the doctor suspects may be due to an autoimmune-related process
A blood sample taken from a vein in your arm
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.
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?
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?
On This Site
Tests: Antinuclear antibody (ANA); Aspartate aminotransferase (AST); Bilirubin; Antimitochondrial antibody (AMA); F-Actin Antibody; Anti-cytoskeleton antibodies (ACTA); Liver Kidney Microsomal (LKM) antibodies
Conditions: Autoimmune disorders, Liver disease, Hepatitis, Autoimmune disorders, Liver disease, Hepatitis