Smith Antibodies

Other names: SM ANTIBODY

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By Healthmatters Editorial Team

Peer-reviewed sources

Anti-Smith antibodies are present in some cases of systemic lupus erythematosus (SLE) and mixed connective-tissue disease (MCTD).

- Anti-Smith antibody is more common in blacks and Asians with SLE than in whites with SLE.
- Elevated anti-Smith levels persist even after anti-DNA levels have returned to the normal range. This is useful when testing with decreased signs or symptoms of SLE (ie, a waning phase).

Smith antigens, along with RNP antigens, are part of small nuclear RNAs. Levels of antibodies to these two antigens are often elevated in SLE. The Smith antigen is composed of the B1, D, and E proteins.

Anti-Smith antibody has low sensitivity but high specificity for systemic lupus erythematosus (SLE). It is occasional present in mixed connective-tissue disease (MCTD).

Anti-Smith antibody levels are elevated in up to 30% of SLE cases and 8% of MCTD cases.

An antibody to Sm, a ribonucleoprotein found in the nucleus of a cell, is found almost exclusively in people with lupus. It is present in 20-30% of people with the disease, but it is rarely found in people with other rheumatic diseases and its incidence in healthy individuals is less than 1%. Therefore, it can also be helpful in confirming a diagnosis of systemic lupus. Unlike anti-dsDNA, anti-Sm does not correlate with the presence of kidney lupus.

Anti-Smith antibody testing should be considered in people with signs or symptoms of SLE or MCTD, such as the following:

- Malar rash
- Discoid rash
- Photosensitivity
- Mucosal ulcers
- Avascular necrosis
- Anemia
- Arthritis/arthralgia
- Serositis
- Glomerulonephritis

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Written by Healthmatters Editorial Team

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