Changes in throat swabs for Rapid Group A Strep testing

New PCR-based test means quicker, more reliable negative results

Throat swabs used for detection of Group A Strep changed from rapid Group A Strep antigen assay to a rapid PCR-based assay on May 9. This impacts what type of throat swabs are acceptable for Rapid Group A Strep testing.

Which swabs to use

The manufacturer lists the following FDA-approved swab types for use with the Xpert® Xpress Strep A assay:

• ESwab Collection and Transport System
  • Copan 480C (white top)
  • Copan 480CE (pink top)
  • Copan 480CFA (purple top)

Epic Beaker ordering information

·         GROUP A STREP BY NAA [LAB5101]

·         Test Code: 1230170001

·         Synonym: STRAL.

Benefits of PCR testing

The diagnosis of Group A Streptococcal pharyngitis traditionally relies on the use of rapid antigen detection testing, with a reflex to throat culture with negative results.

However, the overall sensitivity of these rapid antigen tests compared to throat culture is low (86 to 91% versus 90 to 95%). Throat culture remains the gold standard for detection of Group A strep but requires additional laboratory time (18 to 24 hours) to isolate any potential pathogens.

Compared to rapid antigen detection testing, PCR has a higher sensitivity and specificity for the detection of Group A Streptococcus in patients with bacterial pharyngitis. PCR testing also has the advantage of providing a shorter turnaround time to results versus throat culture (less than 30 minutes versus 18 to 24 hours from time of collection).

The PCR test (Xpert® Xpress Strep A test) reports a sensitivity of 100%, specificity of 94.1%, positive predictive value (PPV) of 84.1%, and a negative predictive value (NPV) of 100% from 577 samples in the U.S. Based on this data, the need for confirmation of negative PCR results by throat culture is not recommended unless clinical symptoms persist, or if the patient has an outbreak of acute rheumatic fever (ARF).

Recommended treatment for penicillin allergy

Penicillin or amoxicillin is the antibiotic of choice to treat Group A Streptococcal pharyngitis. In patients with an allergy to penicillin, recommended treatment regimens include narrow spectrum cephalosporins (cephalexin, cefadroxil), clindamycin, azithromycin and clarithromycin. A throat culture is recommended in this case to allow for antimicrobial susceptibility testing to be performed, if clinically indicated.

Important

·         Negative PCR results for Group A Strep will not receive an automatic reflex to throat culture.

·         This test is not intended to monitor treatment for Group A Streptococcus infections.

Questions?

Contact Jeff Luong, senior technical specialist in the molecular/microbiology department.