What Is anAnti-Smooth Muscle Antibody Test?
An anti-smooth muscle antibody (ASMA) test detects antibodies that attack smooth muscle. This test is performed on a blood sample.
The immune system detects substances that may be harmful in the body. These are known as antigens. Examples of antigens include microorganisms such as viruses or bacteria. When the immune system recognizes an antigen, it produces a protein known as an antibody to attack it.
Every antibody produced is unique, and each one defends against only one type of antigen. Sometimes antibodies are mistakenly produced against the body’s own healthy cells. These are called autoantibodies. If the body starts attacking itself, it causes an autoimmune disorder.
ASMA testing looks for one type of autoantibody—the type that attacks smooth muscle. Autoimmune diseases that attack smooth muscle include the liver diseases cirrhosis and hepatitis.
An ASMA test is often performed in patients who have chronic liver disease. It can confirm a diagnosis of chronic, active autoimmune hepatitis.
The ASMA test can also distinguish between autoimmune hepatitis and systemic lupus erythematosus. ASMAs are present in patients with autoimmune hepatitis. They are not found in patients with lupus.
How the Anti-Smooth Muscle Antibody Test Is Performed
The ASMA test is performed on a blood sample. It can be done at a hospital, clinic, or laboratory.
To perform the test, a health professional will take a blood sample.
First an elastic band is wrapped around your upper arm. This stops blood flow and makes your veins more visible so that it is easier to insert the needle. After a vein is located, the skin is cleaned with alcohol and the needle is inserted. A tube is attached to collect the blood.
After enough blood is collected, the elastic band is removed from your arm. Gauze or cotton is placed onto the site of the injection as the needle is removed. Pressure is applied and the gauze or cotton is secured with a bandage.
When the needle is inserted, you may feel a brief pinching or stinging sensation. There may also be some minor discomfort when the needle is positioned in the vein. Finally, you may feel some throbbing at the site after the needle is removed. Many people do not feel anything at all. Serious discomfort is rare.
You do not need to prepare for an ASMA test.
What Are the Risks?
The ASMA test carries minimal risk. There may be a small amount of bruising at the needle site. This can be minimized by applying pressure on the puncture site for several minutes after the needle is removed.
Continued bleeding after the needle is removed is a potential risk for some people. Tell the health professional if you are taking blood thinners or have problems with bleeding or clotting.
Inflammation of the vein may occur in rare cases after the blood sample is taken. This condition is known as phlebitis. It is treated by applying a warm compress several times a day.
In very rare cases, having blood drawn may result in:
- excessive bleeding
- lightheadedness or fainting
- hematoma (blood accumulating under the skin)
- infection at the needle site
What do the Test Results Mean?
Normal results mean that no ASMAs are detected in the blood.
A test that comes back positive for anti-smooth muscle antibodies may be due to:
- chronic active autoimmune hepatitis
- infectious mononucleosis
Diagnosis of autoimmune hepatitis means that the immune system is mistakenly producing antibodies that attack healthy cells in the liver. This attack can eventually result in:
- destruction of the liver
- liver cancer
- the need for liver transplantation