Extrapharyngeal Group A Streptococcus Infection: Diagnostic Accuracy and Utility of Rapid Antigen Testing

Herbert W. Clegg, MD, Steven D. Dallas, PhD, Oliver F. Roddey, MD, Edward S. Martin, MD, Raymond L. Swetenburg, MD, Elizabeth W. Koonce, MD, Mary B. Felkner, MD, Amy G. Ryan, MD, The Presbyterian Pediatric Research Group

Disclosures

Pediatr Infect Dis J. 2003;22(8) 

In This Article

Discussion

In previous studies of ADTs applied to extrapharyngeal sites, only two have included a substantial number of patients. Kokx et al.[2] identified perianal GAS infection in 31 children, 27 of whom had an ADT (Culturette Group A Rapid Strep ID, Marion Laboratories, Kansas City, MO) performed. Of these children 89% had a positive ADT, whereas none in a group of 119 well children was positive by ADT. Kaplan et al.[3] evaluated an ADT (Culturette Group A Rapid Strep ID) in 129 children with impetigo (84 culture-positive) and found a sensitivity of 94% and a specificity of 96%. Coffey[4] found positive ADTs in 6 children with impetigo, and Bourgeois and Bourgeois[5] used an ADT to diagnose GAS skin and soft tissue infection in 8 children. In case reports others have reported use of an ADT to make a rapid diagnosis of extrapharyngeal GAS infection.[6,7,8]

In our evaluation the Test Pack Plus ADT performed slightly better than its successor, Signify Strep A, for extrapharyngeal specimens, but the Test Pack Plus test is no longer commercially available. The waived ADT, Signify Strep A, had better sensitivity than the moderately complex Directigen test (88% vs. 78%), but the Directigen test was slightly more specific (100 vs. 97%). It is possible that the delay in performance of the Directigen test (because of transportation to the hospital laboratory) resulted in its poorer sensitivity, but we did not evaluate this possibility by direct parallel comparison in the office or hospital laboratory. We sought to compare the tests under the conditions in which they would usually be done (the waived test in an office and the moderately complex test in a referral or hospital laboratory).

Our findings are consistent with those in published studies of Test Pack Plus, Signify Strep A and Directigen ADTs for throat specimens. Sensitivities in these studies ranged from 82 to 91% for Test Pack Plus,[19,20,21,22] 80 to 89% for Signify Strep A[23,24,25] and 39 to 95% for Directigen.[26,27,28,29,30] In these same studies specificities are reported as between 85 and 100%.[19,20,21,22,23,24,25,26,27,28,29,30]

As with throat swabs for the recovery of GAS,[31] it appears that a moistened swab from perineal and other skin sites is not necessary for the culture recovery of GAS as the dry swab and swab placed in modified Stuart's transport medium showed 100% concordance for the 86 positive cultures. Use of a dry swab from these sites may also make interpretation easier as there was little overgrowth of other organisms with the dry swab compared with results from the swab placed in transport medium.

This study has several limitations. We enrolled only 42 patients with specimens from nonperineal cutaneous infections, and only 13 of these patients had positive cultures from impetiginous lesions. However, our findings are in accordance with those of the one other study that addressed ADT use for impetigo.[3] We also encountered no patients with invasive soft tissue or toxin-associated streptococcal infections. Nonetheless the spectrum of patients who were included represents the usual variety of suspected cutaneous GAS infections seen in pediatric outpatients.

We conclude that the three ADTs evaluated have acceptable performance characteristics for the detection of GAS infections in extrapharyngeal sites. Their use can aid in the rapid differentiation of cutaneous lesions, especially in the perianal area, and may limit inappropriate antibiotic therapy for non-GAS lesions. The Signify Strep A test is particularly attractive for office laboratory use given its excellent performance characteristics and its waived status. For hospitalized patients with suspected necrotizing fasciitis, a GAS ADT might be very useful in making a presumptive, rapid etiologic diagnosis.

Although the current generation of ADTs for GAS may eventually be supplanted by newer technology,[32] ADTs for GAS can be used for extrapharyngeal specimens as with throat specimens. The controlled conditions in this study may not reflect those in an office or other setting where an occasional rapid test for GAS may be done and, as with throat specimens, a negative result of a rapid test for an extrapharyngeal site should be confirmed with a conventional culture. ADTs for GAS should also be validated in the individual laboratory setting in which they will be used, especially when used for sites of infection for which there is no formal Food and Drug Administration approval.

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