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Blood Test Tracker

Anti-Smith (Sm) Antibody

Looks for antibodies to the Smith (Sm) protein. A positive result is uncommon and is most closely linked with lupus.

AI-verified against cited sources. Awaiting expert reviewAlso called Anti-Sm, Sm AbLOINC 33564-6

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What it is

Sm is a protein inside the nucleus of your cells. Anti-Sm antibodies are autoantibodies, which means your immune system has made antibodies that react with your own proteins. Clinicians usually order this test after an antinuclear antibody (ANA) test comes back positive. Many labs report a number plus a call of negative, equivocal (in between) or positive. The number and the cutoffs depend on the lab's test kit.

Why it matters

Anti-Sm antibodies are found in a minority of people with lupus (systemic lupus erythematosus). ARUP Laboratories cites about 30% to 35%, and the share varies with ancestry. They are uncommon in people without lupus, so a positive result is a useful clue. Lupus classification criteria give anti-Sm the same weight as anti-dsDNA antibodies. A negative result does not rule lupus out, because most people with lupus do not have anti-Sm. One result cannot settle the question. Your clinician reads it together with your symptoms, your ANA and your other tests.

If it’s high

A positive result means your blood has antibodies that react with Sm. This is most often seen in lupus. A positive result by itself does not mean you have lupus. Your clinician looks at your symptoms and your other test results. If your result is called equivocal, it fell between your lab's negative and positive cutoffs, and it may be repeated.

Common causes

  • Lupus (systemic lupus erythematosus), the most common reason
  • Other autoimmune conditions, less often
  • No known condition: a positive result is uncommon in people without lupus, because ARUP Laboratories reports the test is more than 90% specific for lupus

What can help

  • Ask your clinician how this result fits with your symptoms, your ANA and your other lupus-related tests
  • Keep a short list of symptoms and when they happen, such as joint pain, rashes, mouth sores or sun sensitivity, to share at your visit

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 0–40 AU/mL. Source: ARUP Laboratories.

ARUP Laboratories interpretation (multiplex bead assay; lab-specific)

  • Negativebelow 30 AU/mL
  • Equivocal30 to under 41 AU/mL
  • Positive41 and above AU/mL

What can affect your result

  • Lab and test kit: units and cutoffs differ between labs, so compare results only from the same lab
  • Ancestry: in one New York City lupus registry, anti-Sm was found in a larger share of Black and Asian patients
  • Sex: in the same registry, men with lupus were more likely to be anti-Sm positive
  • Hemolyzed, severely lipemic or contaminated samples can be rejected or give unreliable results (ARUP specimen requirements).
  • Labs use different test kits, scales and cutoffs (for example AU/mL, antibody index or chemiluminescent units), so a number from one lab can't be compared with a number from another. Compare results from the same lab and test.

Not a short-term marker. These antibodies are usually checked to help sort out a condition, not to follow day-to-day changes.

When to talk to a clinician

A positive anti-Sm result is not an emergency by itself. Contact your clinician soon if you have new symptoms such as swollen or painful joints, a rash that gets worse in the sun, mouth sores, chest pain when you breathe in, foamy or dark urine, or swelling in your legs. If you have severe chest pain, trouble breathing, confusion or fainting, call 911 or your local emergency number.

Questions you could ask

  • Why was the anti-Sm test ordered for me?
  • Which other antibodies, such as anti-dsDNA or anti-RNP, were checked?
  • How does this result fit with my symptoms and my other lab results?
  • Would it help to see a rheumatologist, a specialist in autoimmune conditions?
  • Is a repeat test useful, or is this result enough?

Questions people ask

Does a positive anti-Sm result mean I have lupus?

Not by itself. Anti-Sm is quite specific for lupus, but a result is always read with your symptoms, your ANA and your other tests. A positive result in someone without lupus is uncommon, and most people with lupus test negative for anti-Sm. Your clinician decides what it means for you.

What does equivocal mean?

Some labs sort results into negative, equivocal and positive. Equivocal means the number fell between the negative and positive cutoffs. At ARUP Laboratories, for example, 29 AU/mL or less is negative, 30 to 40 is equivocal and 41 or more is positive. Other labs use different numbers, so use the cutoffs on your own report. Your clinician may repeat the test.

Why does my number look so different from a friend's report?

Labs use different test kits and scales, such as AU/mL or antibody index (AI), each with its own cutoffs. A number from one lab can't be compared with a number from another. Look at the negative or positive call and the range printed on your own report.

Show the science and sources

For clinicians and researchers

Sm antigens are core proteins (B/B', D1 to D3, E, F and G) shared by several small nuclear ribonucleoprotein particles (snRNPs) that help process RNA in the nucleus. Anti-Sm is one of the extractable nuclear antigen (ENA) antibodies. ARUP Laboratories states it is more than 90% specific for systemic lupus erythematosus but occurs in only about 30 to 35% of cases, with variable links to clinical features. It scores 6 points in the immunologic domain of the 2019 EULAR/ACR SLE classification criteria, the same as anti-dsDNA; those criteria require a positive ANA as an entry criterion. Anti-Sm and anti-U1-RNP can occur together (the U1 snRNP particle contains Sm proteins), so a Sm/RNP complex result should be read alongside the Sm result. Scales differ by platform: ARUP reports a semi-quantitative multiplex bead assay in AU/mL (negative 29 or less, equivocal 30 to 40, positive 41 or more), while other labs use other kits and units such as antibody index, so numbers are not interchangeable. In a Manhattan SLE registry, anti-Sm was most frequent in non-Latino Black and Asian patients.

LOINC 33564-6 (primary), 11090-8, 18323-6, 43182-5. Specimen: serum.

Sources

  1. Smith (ENA) Antibody, IgG (test 0050085), ARUP Laboratories
  2. 2019 European League Against Rheumatism/American College of Rheumatology Classification Criteria for Systemic Lupus Erythematosus, Arthritis & Rheumatology (2019)
  3. Systemic Lupus Erythematosus (Lupus): Diagnosis, Treatment, and Steps to Take, National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIH) (2022)
  4. Prevalence of concomitant rheumatologic diseases and autoantibody specificities among racial and ethnic groups in SLE patients, Frontiers in Epidemiology (2024)
  5. Autoantibody Testing: MedlinePlus Medical Test, MedlinePlus (National Library of Medicine) (2025)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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