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What Is the Sm Antibody Test?

It is one of the diagnostic tests used for determining an autoimmune condition called Systemic Lupus Erythematosus (SLE). It is a highly specific test for SLE because it is not found in normal individuals and patients with other autoimmune systemic rheumatic diseases. 

Smith(sm) is a protein antigen found inside the cell's nucleus. In SLE, antibodies are synthesized against these proteins, which attack different body cells. These antibodies are called anti-smith antibodies, and their increased levels in blood establish the diagnosis of SLE. Besides SLE, these antibodies may also be present in mixed connective tissue disorder (MCTD). It is a rare condition that presents with the overlapping signs and symptoms of a combination of disorders like polymyositis, scleroderma, systemic lupus, etc.

Though this test is exclusive for lupus, these antibodies are not always positive in all patients with SLE. But if present is considered the marker antibodies. They are only present in 20-30% of cases of SLE, and the incidence is higher in black and Asian females with SLE than white patients with SLE.

Anti-smith antibodies are associated with the milder form of SLE involving the Raynaud phenomenon (discoloration of the skin of fingers due to decrease blood flow towards fingers) and sclerodactyly (hardening of the skin of hands and fingers leading towards claw hand) and less associated with involvement of CNS and kidney. This test does not help monitor disease progression, lupus flares, and prognosis of the disease.

What Is Sm Antibody Test Used For?

This test is an important diagnostic tool for systemic lupus erythematosus, an autoimmune disease in which your immune system produces autoantibodies against your body cells which attack your body, causing widespread inflammation, damage, and even destruction of cells. SLE is a chronic autoimmune condition that may involve several systems of your body, with the severity ranging from mild to life-threatening conditions. You may undergo a variety of symptoms, including:

  • Fatigue (most common symptom)
  • Skin rashes (butterfly rash or discoid rash on face)
  • Photosensitivity (sun sensitivity)
  • Muscle ache and joint pain
  • Mouth ulcers
  • Fever
  • Recurrent miscarriages
  • Kidney, lung, and heart involvement
  • Seizures and psychosis due to CNS involvement
  • Blood cells (decreases in all blood cell types)
  • Immunological problems (impaired immunity)
  • Raynaud phenomenon (decrease blood flow in fingers leading to avascular necrosis)

It is a chronic disease of unknown etiology; it is supposed that its cause may be multifactorial, linked to environmental, hormonal, and genetic factors. The disease courses with flare (symptomatic phase of SLE) followed by a period of remission (a symptom-free period). Multiple tests may be required to reach the accurate diagnosis; hence this test is usually run with other tests like antinuclear ribonuclease protein test (anti-RNP test), Antinuclear antibody test (ANA), etc.

Blood tests alone are usually non-conclusive; your healthcare provider may evaluate your physical findings and correlate them with test results to make a definitive diagnosis.

Why And When Do You Need To Get Tested?

SLE is a difficult disease to diagnose as it presents with multiple symptoms. It requires numerous investigations and their correlation with your sign and symptoms. You may need to be evaluated for SLE or MCTD:

  • If you have a long history of signs and symptoms of this disease.
  • If you have a family history of SLE or any other autoimmune disease.
  • If you develop symptoms after taking a certain type of medication.
  • If you experience a recurrent miscarriage without any obvious cause.

Anti-smith antibody test is only carried out in patients whose antinuclear antibody test (ANA) is positive. It should not be ordered if ANA tests are negative or weakly positive because patients with negative ANA will rarely come positive with an anti-sm antibody test.
SLE has no definitive treatment available; therefore, it is essential to diagnose it early to prevent the disease's progression and its disastrous complications.

What Type Of Sample Is Required?

A blood (serum) sample is required for this test to be carried out. Your healthcare provider will use a small needle to draw a small amount of blood from the veins of your arm. The sample is obtained in a test tube or vial and transported to the laboratory.

Do You Need To Prepare For This Test?

This sm Antibody test does not require any specific preparation. Anti-smith antibodies may disappear after treatment; hence you must inform about your medicinal history to your healthcare provider before running this test. This test is usually ordered for simplifying the diagnosis of SLE in those patients who already have tested positive for ANA.

Is There Any Risk To Sm Antibody Test?

This sm Antibody test is not associated with any remarkable risks. Since a needle is used to draw blood, you may feel pain at the prick site. You may often develop bruises or bleeding from the prick site or very rarely experience infection.

What Does The Result Mean?

The sm Antibody test result is usually obtained between 5-7 days and is reported as positive or negative.

  • A positive result suggests that a significant number of anti-smith antibodies (> or = 1.0U) are detected in your blood, which concludes the diagnosis of SLE.
  • A negative result defines that not enough anti-smith antibodies (<1.0U) are detected in your blood. A negative result does not exclude SLE diagnosis, as this antibody is only found in 20-30% of patients with SLE. Your healthcare provider may order some more tests to bring about the diagnosis.

If you have any questions regarding your test report, you must consult your healthcare provider.

Related Tests:

  • antinuclear antibody test (ANA)
  • anti-RNP test
  • anti-dsDNA antibody test
  • serum complement levels
  • anti-histone antibody test
  • anti-Ro/SSA and anti-La/SSB antibodies test
  • SCL-70 antibody test

Our clinical experts continually monitor the health and medical content posted on CURA4U, and we update our blogs and articles when new information becomes available. Last reviewed by Dr.Saad Zia on June 03, 2023. 


Anti-Smith antibody is associated with disease activity in patients with new-onset systemic lupus erythematosus - PubMed (

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