Smooth Muscle Antibody (SMA) Test

Submitted on March 27, 2012

The smooth muscle antibody test is primarily done to diagnose autoimmune hepatitis. It is always ordered along with antinuclear antibodies (ANA). Autoimmune hepatitis is an inflammation of the liver that is associated with high concentration of auto antibodies (antibodies which instead of acting against foreign invaders, attack the body's own cells), pronounced hypergammaglobulinemia, and the absence of cholestasis. Smooth Muscle Antibody (SMA) is protein which is produced by the body's immune system and is directed against its own cytoskeletal proteins of the smooth muscle. Autoimmune hepatitis can be found in anyone at any age, but mostly women are affected.

The SMA testing involves the detection of anti smooth muscle antibody against smooth muscle antibody. Sometimes SMA testing is done after the anti-actin test to confirm the finding. The anti-actin test is an initial screening test for autoimmune hepatitis.

The common symptoms of autoimmune hepatitis to be noticed for ordering SMA blood test are:

  • Fatigue
  • Jaundice
  • Abnormal elevated findings (aspirate aminotransferase and bilirubin) on routine liver test.

Among the three types of muscles in the human body, smooth muscles are the ones found in the walls of several hollow organs of the digestive tract in the body, such as the stomach, the liver, and the intestines. It is the waves of contraction and relaxation created by the action of these muscles that pushes the ingested food down slowly from the throat to be digested in the gut and the intestines until what remains of it is eliminated by the rectum.

Anti Smooth Muscle Antibody Test

Auto antibodies are antibodies made by the body's own immune system that attack the body's own cells rather than attacking cells of any foreign body like a virus or a bacterial infection. This results as these antibodies are unable to distinguish between the two. The presence of these auto antibodies can be detected by performing a blood test. One such auto antibody originating from the body's own immune system is the anti smooth muscle antibody, a protein that targets the smooth muscles in the liver.

The medical anti smooth muscle antibody test is usually ordered when a doctor suspects that the patient suffers from autoimmune hepatitis. Also called the SMA test, it is ordered along with the antinuclear antibodies test to eliminate other reasons that may be the cause of liver malfunction.


SMA test along with ANA is usually ordered to rule out other causes of liver injury. When the test shows significantamounts of SMA and ANAin the blood, the most likely cause is autoimmune hepatitis .The test requires a blood sample drawn from a vein in your arm to measure the amount of smooth muscle antibodies (SMA) in the blood. If the SMA blood test is negative, then symptoms may be due to causes other than autoimmune hepatitis, which may include viral infections such as viral hepatitis, drugs, alcohol abuse, toxins, genetic conditions, and primary biliary cirrhosis.


The blood sample is drawn from the patient and sent for a SMA lab test. In the lab, serum from the patient's blood specimen is added to microscope slides which already have commercially prepared cells. If the patient's serum contains antinuclear antibodies, they bind to the cells on the slide. A second antibody, commercially tagged with a fluorescent dye, attaches to the serum antibodies and cells which have bound together. This is then viewed under an ultraviolet microscope to test the results.

SMA blood test can at times give positive results in people without any known autoimmune disease. While this is not common, the frequency of a false positive SMA result increases as people get older.

Titer Ranges To Detect the SMA

A titer is determined by repeating the positive test with serial dilutions until the test yields a negative result. Below the titer ranges which detect the SMA, testing is either positive or negative:

  • Less than 1:20 = Normal-No antibody detected.
  • 1:20-1:80 = Weak Positive-Suggest repeat testing in 2-3weeks with fresh specimen.
  • 1:160 or greater = Positive-Suggestive of autoimmune hepatitis.

What Do The Test Results Indicate?

The doctor may order an anti smooth muscle antibody test if the patient complains of fatigue, loss of appetite and nausea, and presents the clinical symptoms of someone suffering from jaundice. Abnormal levels of bilirubin in liver tests also indicate the need for a smooth muscle antibody exam. An enlarged liver, abdominal pain, unexplained weight loss, dark urine, and pale stools, and a yellowish pallor of the skin are other symptoms that may prompt the medical practitioner to call for a smooth muscle antibody blood test. At the outset an actin smooth muscle antibody test may be performed as the presence of anti-actin antibodies is more frequent in autoimmune diseases and may help in their diagnosis. Very often when the anti-actin antibodies test is positive, the SMA test is also positive. The reverse, however, is not true. A person whose anti-actin test comes out negative may still have smooth muscle antibodies as actin is only one of several proteins affected by the disease.

When smooth muscle antibody test results indicate the presence of significantly high smooth muscle antibody levels and when the anti-actin antibodies test is also positive, it is likely that the patient is suffering from autoimmune hepatitis.However, it has been found that patients with biliary cirrhosis and certain types of cancers may also show positive smooth muscle antibody test results.

The medical professionals may decide to do a liver biopsy to check for damaged liver tissue and scarring of the tissue to substantiate their diagnosis for autoimmune hepatitis. Absence of smooth muscle antibodies in the test does not negate autoimmune hepatitis because a fifth of patients with the disease do not test positive for SMA. Though it is not conclusive, it is probable that a negative SMA test suggests that the clinical symptoms exhibited by the patient may be due to liver problems other than autoimmune hepatitis.

Prognosis for patients with autoimmune hepatitis is varied, depending on their age, sex and lifestyle. Women and young girls are more likely to suffer from the disease than men. Corticosteroids and immunosuppressants have been used effectively in arresting the progress of autoimmune hepatitis in some patients. There are cases where the disease has eventually resulted in cirrhosis of the liver, leading to irreversible liver failure. Such patients can survive only with a liver transplant. Anti smooth muscle antibodies test can help in the early diagnosis and treatment of autoimmune hepatitis so that patients can live longer with a healthier lifestyle.

The SMA lab test results are just one factor in diagnosing, and must be considered together with the various clinical symptoms and other supportive diagnostic tests. The severity of autoimmune hepatitis ranges from acute to chronic. It can lead to liver cirrhosis and, in some cases, to liver failure. If the person has SMA due to autoimmune hepatitis, it will be present throughout the patient's life. But the levels may vary over a period of time and could also drop below detectable levels.