Anti-smooth muscle antibody (ASMA)
An autoantibody directed against components of the body's own smooth muscle (particularly the protein actin). Its main use is in diagnosing type 1 autoimmune hepatitis, where it is a hallmark finding, read together with the antinuclear antibody, a raised IgG and the liver enzymes.
What it measures
The anti-smooth muscle antibody is an autoantibody — an antibody the immune system makes, mistakenly, against the body's own tissues, in this case components of smooth muscle, most importantly the structural protein actin. Its main clinical value is in the diagnosis of autoimmune hepatitis, a chronic condition in which the immune system attacks the liver. In the commonest form, type 1 autoimmune hepatitis, ASMA is one of the characteristic autoantibodies, usually appearing alongside a positive antinuclear antibody (ANA) and a raised immunoglobulin G (IgG). The test is reported as a titre — how far the blood can be diluted and still show the antibody — and a higher titre, especially the anti-actin pattern, is more meaningful. It is requested when someone has unexplained raised liver enzymes or features suggesting autoimmune liver disease, as part of an autoantibody panel. A positive ASMA supports the diagnosis but does not make it alone: autoimmune hepatitis is diagnosed from the combination of autoantibodies, raised IgG, the liver enzyme pattern and, usually, a liver biopsy. Low-level positives can also occur in other liver diseases and infections.
What a high value can mean
- Type 1 autoimmune hepatitis — the key use; a hallmark autoantibody, especially at higher titres and with the anti-actin pattern.
- Read with ANA and a raised IgG — the classic type 1 autoimmune hepatitis picture.
- Other liver diseases and viral infections — can cause low-level positives, so context matters.
- Occasionally in healthy people — usually at low titre and without disease.
What a low value can mean
- Negative — the expected result; it makes type 1 autoimmune hepatitis less likely but does not exclude it (some cases are seronegative or carry other antibodies).
- A falling titre on treatment — is not routinely used to monitor; response is followed by liver enzymes and IgG.
When to discuss with a doctor
The anti-smooth muscle antibody is a specialist test, used when autoimmune liver disease is being considered — typically for unexplained raised liver enzymes or suspected autoimmune hepatitis — rather than as a general screen. A positive result, especially at a higher titre and alongside a positive ANA, a raised IgG and a matching liver enzyme pattern, supports type 1 autoimmune hepatitis, which is important to identify because it is treatable and, untreated, can progress to cirrhosis. The diagnosis is made by a specialist from the whole picture, usually with a liver biopsy, not from this antibody alone. Mediora.AI shows the anti-smooth muscle antibody alongside ANA, IgG and the liver enzymes so the autoimmune-hepatitis pattern is visible in context; the diagnosis and treatment belong with your doctor.