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Interpretation of Test Results and Limitations of OTC Rapid Strep Test

Interpretation of test results

1. Negative results
A purple-red control line will appear only in the upper half of the window, indicating that the result is negative. The control line means that the test was performed correctly, but group A streptococcal antigen was not detected.

2. Positive results
Two purple-red lines will appear, indicating the detection of group A streptococcal antigen. Low-level antigen specimens can give a dim sample line. Any visible purple-red sample line is positive.

3. Invalid results
If there are no lines or only sample lines, the experiment is invalid. Invalid samples should be re-tested with new samples.

Limitations of test methods

OTC rapid strep test could not distinguish between live and dead bacteria. Because of the presence of group A streptococcal antigen, patients who have recently received treatment for group A streptococcal or similar infections will still show positive results for a period of time after effective treatment. Pharyngitis can also be caused by bacteria other than group A streptococcus. When laboratory diagnosis does not match clinical manifestations, further diagnosis including bacterial culture should be made.
OTC rapid strep test could not distinguish between asymptomatic carriers and infected persons of group A streptococcus. If the number of antigens extracted is below the sensitivity, a negative result may be obtained. It is suggested that the negative results of OTC rapid strep test should be confirmed by culture. Using a swab in OTC rapid strep test for both vaccination and rapid diagnosis may reduce sensitivity.
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