What are the diagnostic lab studies for pediatric mixed connective-tissue disorder (MCTD)?

Updated: Oct 24, 2018
  • Author: Marisa S Klein-Gitelman, MD, MPH; Chief Editor: Lawrence K Jung, MD  more...
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Diagnostic laboratory studies include the following:

  • Antinuclear antibody: This test result is usually positive in high titers. [12]

  • Anti–double-stranded DNA: This test result is usually negative but is occasionally positive in individuals with mixed connective tissue disease.

  • Autoantibody panel, including antibodies against ribonucleoprotein (RNP), Smith, Ro(SSA), La(SSB), Scl-70, phospholipids, cardiolipin and histone, total hemolytic complement, C3, C4, quantitative immunoglobulins, and thyroid studies

    • Other than anti-RNP and anti-Sm, these laboratory test findings may be positive or negative depending on the characteristics of the individual patient's disease.

    • By definition, anti-RNP should be positive, and anti-Sm should be negative. Anti-Sm and anti-RNP antibodies can be measured by double diffusion, counterimmunoelectrophoresis, passive hemagglutination, and enzyme immunoassay. The double diffusion assay uses crude antigens, and positivity is based on the identity of precipitation lines for test standard and test sera. This test is specific but not sensitive.

    • Purified Sm proteins are available for testing; however, RNP antigen has not been separated from Sm and is detected as a complex.

    • Counterimmunoelectrophoresis and passive hemagglutination improve the ability to distinguish anti-Sm from anti-RNP by modifying the antigen extract.

    • The antigen for anti-Sm antibodies is treated with RNase, removing RNP from the preparation.

    • A decrease in titer of approximately 5 tubes (or more) dilution from before and after RNase digestion is characteristic of mixed connective tissue disease sera.

    • The antigens in the enzyme immunoassay are prepared by immunoaffinity chromatography using human autoantibodies and murine monoclonal antibodies to separate Sm from the RNP/Sm antigen complex and are sensitive to detecting anti-RNP antibodies.

  • Thyroid studies (thyroid stimulating hormone [TSH], free T4, thyroid autoantibody screen). In one study of 1517 adult patients with rheumatic diseases, 21% of the patients with mixed connective tissue disease had Hashimoto thyroiditis, and 2.5% had Graves disease, with prevalence rates 556- and 76-fold higher than the general population, respectively.

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