16.2.3: Serological Typing of Streptococci
- Page ID
- 122731
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The Clearview® Strep A Exact II Dipstick is an example of a lateral flow immunologic assay. It is is a qualitative serologic test for detecting Group A Streptococcal antigen (the unknown antigen) directly from throat swabs and is used as an aid in diagnosing streptococcal pharyngitis caused by Streptococcus pyogenes (Group A Beta Streptococci).
The test consists of a membrane strip that is precoated with rabbit anti-Strep A monoclonal antibody-red latex conjugate (known antibodies made in rabbits against strep A antigen with red latex particles attached) located in a pad at the beginning of the strip. It is also precoated with rabbit anti-Strep A monoclonal antibody (known antibodies made in rabbits against the strep A antigen but without attached red latex particles) that is immobilized at the test line where the test results are read (see Fig. \(\PageIndex{1A}\)). The red latex particles attached to the rabbit anti-Strep A antibodies are what ultimately causes the “positive” red band.
The throat swab is placed in an extraction solution that lyses the Streptococcus pyogenes, if present, and exposes the strep A antigen in the bacterial cell wall. When the test strip is immersed in the extracted sample, the Group A Streptococcal antigen extracted from the Streptococcus pyogenes on the throat swab of a person with strep throat begins to move chromatographically up the membrane and binds to the known antibody-red latex conjugate in the pad located at the beginning of the strip, forming a Strep A antigen-antibody complex (see Fig. \(\PageIndex{1B}\)). This Strep A antigen-antibody complex continues to moves up the membrane to the test line region where the immobilized rabbit anti-Strep A antibodies are located.
If Group A Streptococcal antigen is present in the throat swab, a red-colored sandwich of known antibody/Strep A antigen/red latex conjugate forms in the test line region of the strip (see Fig. 6C). The control region of the strip has immobilized anti-rabbit antibodies, that is, antibodies made in a different animal against rabbit antibodies. The red color at the control line region appears when molecules of the rabbit anti-Strep A antibody-red latex conjugate not trapped at the test line reach the control area and are stopped by binding to the anti-rabbit antibodies. This indicates that the test is finished. As a result, a positive test for Group A Strep antigen appears as a red band in the test result area and a red band in the control area (see Fig. \(\PageIndex{1C}\)).
If there is no Group A Streptococcal antigen present in the throat swab no red band appears in the test result region of the strip and a single red band appears in the control line region, indicating a negative test for Group A Strep antigen (see Fig. \(\PageIndex{1D}\)).
Contributors and Attributions
Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)