Anti-Smith Antibody 

Updated: Apr 25, 2013
  • Author: Niral M Patel; Chief Editor: Eric B Staros, MD  more...
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Reference Range

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

The reference range of anti-Smith antibodies is negative.

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Interpretation

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 30% of SLE cases and 8% of MCTD cases [1]

Anti-Smith antibody is more common in blacks and Asians with SLE than in whites with SLE. [2] Elevated anti-Smith levels persist even after anti-DNA levels have returned to the normal range. This is useful when testing a patient with decreased signs or symptoms of SLE (ie, a waning phase).

Sensitivities and specificities vary by laboratory technique. Immunoblotting and immunoprecipitation are very sensitive. Immunodiffusion, passive hemagglutination, and counterimmunoelectrophoresis are not as sensitive. [3]

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Collection/Panels

See the list below:

  • There are no special collection conditions or timings.
  • A standard red-top tube is used
  • Hemolysis and lipemia should not affect the results but should be avoided, if possible.
  • There are no special storage conditions.
  • Anti-Smith antibody testing is usually not part of a laboratory panel.
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Background

Description

Anti-Smith antibodies are present in some cases of systemic lupus erythematosus (SLE) and mixed connective-tissue disease (MCTD). Smith antigens are part of the extractable nuclear antigens (ENAs); specifically, they are the proteins that are resistant to ribonuclease. Ribonucleoproteins, the other part of ENAs, are ribonuclease susceptible. [4]

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. [3]

Indications/Applications

Anti-Smith antibody testing should be considered in patients 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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