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
- Mucosal ulcers
- Avascular necrosis
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).
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