Anti-smooth muscle antibody

Definition

Anti-smooth muscle antibody is a blood test that detects the presence of antibodies against smooth muscle. The antibody is useful in making a diagnosis of autoimmune hepatitis.

How the Test is Performed

A blood sample is needed. This may be taken through a vein. The procedure is called a venipuncture.

How to Prepare for the Test

No special steps are needed to prepare for this test.

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others may feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the Test is Performed

You may need this test if you have signs of certain liver diseases, such as hepatitis and cirrhosis. These conditions can trigger the body to form antibodies against smooth muscle.

Anti-smooth muscle antibodies are not often seen in diseases other than autoimmune hepatitis. Therefore, it is helpful to make the diagnosis. Autoimmune hepatitis is treated with immunosuppressive medicines. People with autoimmune hepatitis often have other autoantibodies. These include:

• Antinuclear antibodies.
• Anti-actin antibodies.
• Anti-soluble liver antigen/liver pancreas (anti-SLA/LP) antibodies.
• Other antibodies may be present, even when the anti-smooth muscle antibodies are absent.

The diagnosis and management of autoimmune hepatitis may require a liver biopsy.

Normal Results

Normally, there are no antibodies present.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your health care provider about the meaning of your specific test results.

What Abnormal Results Mean

A positive test may be due to:

• Chronic active autoimmune hepatitis
• Cirrhosis
• Infectious mononucleosis

The test also helps distinguish autoimmune hepatitis from systemic lupus erythematosus.

Risks

Risks associated with having blood drawn are slight, but may include:

• Excessive bleeding
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

References

American College of Rheumatology website. Antinuclear antibodies (ANA). rheumatology.org/patients/antinuclear-antibodies-ana. Updated February 2023. Accessed March 6, 2025.

Czaja AJ. Autoimmune hepatitis. In: Feldman M, Friedman LS, Brandt LJ, eds. Sleisenger and Fordtran's Gastrointestinal and Liver Disease. 11th ed. Philadelphia, PA: Elsevier; 2021:chap 90.

Peng SL. Anti-nuclear antibodies. In: Firestein GS, McInnes IB, Koretzky GA, Mikuls TR, Neogi T, O'Dell JR, eds. Firestein & Kelley's Textbook of Rheumatology. 12th ed. Philadelphia, PA: Elsevier; 2025:chap 55.

von Mühlen CA, Chan EKL, Ceribelli A, Fritzler MJ. Clinical and laboratory evaluation of systemic autoimmune rheumatic diseases. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 53.