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Streptococcus, Group A, Direct Detection (Antigen, Nucleic Acid)

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Subject: Streptococcus, Group A, Direct Detection (Antigen, Nucleic Acid)

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Definition

  • The results of direct tests for group A streptococci may guide early therapy. In antigen tests, the group A cell wall polysaccharide is extracted from a throat swab.

Use

  • Direct detection tests for group A beta-hemolytic streptococci (Streptococcus pyogenes) are used for direct diagnosis of streptococcal pharyngitis. Patients present with symptoms including sore throat, fever, headache, and abdominal pain.

  • Method:

    • Extracted antigen is detected by specific antibodies using standard immunologic techniques, such as LA or EIA. The sensitivity of antigen tests varies by technique and specific kit used, ranging from 60% to 95%; the specificity of most tests exceeds 95%. Therefore, throat culture has been recommended to confirm negative antigen tests, but is not needed to confirm positive tests.

    • The Group A Streptococcus Direct Test (Gen-Probe, San Diego, CA) is an FDA-approved molecular diagnostic assay for the detection of S. pyogenes in pharyngeal specimens. Group A streptococci are detected using a specific DNA probe directed against specific S. pyogenes rRNA sequences. Sensitivity of the assay is 88–95% with specificity of 98–99.7%. The high sensitivity and specificity for this test allow test results to stand without the need for confirmation of positive or negative tests.

  • Special collection and transport instructions: Throat swab specimens are collected as recommended for throat cultures.

  • Turnaround time: <4 hours for antigen tests; <24 hours for molecular tests.

Interpretation

  • Expected results: Negative.

  • Positive results: Positive results are diagnostic of group A streptococcal pharyngitis in patients with consistent clinical findings.

  • Negative results: Negative antigen tests decrease the likelihood of group A streptococcal pharyngitis but must be confirmed by a more sensitive technique, such as throat culture or molecular detection.

Limitations

  • Only swabs specified by the manufacturer may be used for the Gen-Probe assay.

 
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