PCR Replaces Group A Strep Culture |
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Group A beta-hemolytic streptococcus (S. pyogenes) is responsible for up to 30% of cases of acute pharyngitis. The clinical features of pharyngitis caused by group A strep include purulent exudate, fever, and anterior cervical adenopathy. Beta-hemolytic streptococci groups C & G can cause pharyngitis with symptoms and signs that are indistinguishable from group A strep infections, but are more commonly found in adolescents and young adults.
The mainstay of diagnosis for group A strep pharyngitis is rapid antigen testing of throat swabs. The obvious advantage of rapid antigen testing is the immediate availability of results and the ability to perform testing in the clinic setting. However, a major disadvantage of these tests is less than optimal sensitivity, so that culture back-up of negative tests is essential. Culture is also necessary to identify the presence of group C or G streptococci, which are not detected by rapid antigen tests. Unfortunately, final culture results are generally not available until 24-48 hours after specimen collection.
Roche Lightcycler real-time PCR has been evaluated for the detection of group A strep from throat swabs. Samples included those previously tested by rapid antigen testing and those tested by culture alone. The greatest advantage of real-time PCR was seen in comparison to culture. Real-time PCR increased the yield of positive group A strep (GAS) results by nearly 20%.
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GAS PCR pos |
GAS PCR neg |
GAS Culture pos |
13 |
2 |
GAS Culture neg |
6 |
168 |
Although some literature reports have shown group A strep PCR to yield significantly more positive results than rapid antigen tests with direct testing of throat swabs, this was not apparent during our evaluation.
As a result of this data, real-time PCR should replace back-up culture of throat swabs PCR. Beta-strep groups C & G are also detected by the same assay & are reported when present. No change in sample submission is required.
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