Extrapharyngeal group A Streptococcus infection: diagnostic accuracy and utility of rapid antigen testing
Herbert W Clegg1, Steven D Dallas, Oliver F Roddey
1Department of Pediatrics, Eastover Pediatrics, Presbyterian Hospital, Charlotte, NC, USA.
Insights
Rapid antigen detection tests (ADTs) for group A Streptococcus (GAS) are effective for diagnosing skin infections in children. These tests accurately detect GAS in cutaneous lesions, offering a reliable alternative to traditional methods.
Area of Science:
- Pediatric infectious diseases
- Microbiology
- Diagnostic test evaluation
Background:
- Antigen detection tests (ADTs) are established for diagnosing group A Streptococcus (GAS) pharyngitis in children.
- Limited data exists on the efficacy of ADTs for detecting GAS in pediatric skin infections, such as perineal or impetiginous lesions.
Purpose of the Study:
- To evaluate the performance and accuracy of antigen detection tests (ADTs) for diagnosing group A Streptococcus (GAS) skin infections in pediatric patients.
- To compare the effectiveness of different ADTs and sample collection methods for GAS detection in cutaneous sites.
Main Methods:
- A 3-year study (1999-2002) involving 239 pediatric patients with suspected GAS skin infections across 5 practices.
- Three Dacron swabs were collected per site: one for office-based ADT (Abbott Test Pack Plus or Signify Strep A), one for hospital laboratory ADT (BD Directigen 1-2-3), and one for culture (reference standard).
- Test performance metrics including sensitivity, specificity, positive predictive value, and negative predictive value were calculated for each ADT.
Main Results:
- Of 239 patients, 86 (35%) had positive GAS cultures, primarily from perineal sites (73) and impetiginous lesions (13).
- Abbott Test Pack Plus and Signify Strep A ADTs demonstrated high performance for skin infections (sensitivity 88-92%, specificity 97-99%).
- BD Directigen 1-2-3 ADT showed lower sensitivity (78%) but high specificity (100%). Test accuracy ranged from 92% to 97%.
Conclusions:
- Antigen detection tests (ADTs) designed for pharyngitis are accurate and rapid tools for diagnosing group A Streptococcus (GAS) in pediatric skin infections.
- These ADTs provide reliable results for cutaneous GAS detection, supporting their use in clinical practice for suspected skin infections.
Background:
Antigen tests have been well-studied and are widely used in pediatric practice for rapid detection of group A Streptococcus (GAS) infections in the throat, but they have not been examined sufficiently for the detection of infection of skin sites, such as the perineal region or impetiginous lesions.
Methods:
During the 3-year period 1999 to 2002, we evaluated 239 patients with suspected GAS skin infection, in 5 pediatric practices, using 3 Dacron swabs for each site. The first swab was tested in the pediatric office laboratory with an antigen detection kit. For the first 91 patients, the Abbott Test Pack Plus antigen detection test (ADT) was used. The Abbott Signify Strep A ADT was used to test subsequent patients. The second swab was tested with BD Directigen 1-2-3 ADT in the hospital laboratory. The third swab was placed in modified Stuart's transport medium for comparison of recovery of GAS from culture in broth or on agar. A positive culture served as the reference standard. Test performance and test accuracy were determined for each ADT.
Results:
Of the 247 ADTs and cultures performed on 239 patients, 91 with suspected skin infection were tested with the Test Pack Plus test, 149 with the Signify Strep A test and 247 with the Directigen test. Eighty-six (35%) cultures were positive, 73 from perineal sites (54 rectal, 13 vaginal, 6 penile) and 13 from impetiginous lesions. There was 100% concordance for the 86 cultures positive for GAS in a comparison between dry Dacron swabs and swabs that had been placed in modified Stuart's transport medium. Test Pack Plus and Signify Strep A ADTs had similar performance characteristics for skin infections: sensitivity, 92 and 88%; specificity, 99 and 97%; positive predictive value, 96 and 94%; and negative predictive value, 97 and 93%. Directigen ADT had sensitivity 78%, specificity 100%, positive predictive value 100% and negative predictive value 89%. Accuracy for the tests varied from 92 to 97%.
Conclusion:
Tests designed to detect GAS carbohydrate antigen in patients with pharyngitis can be used rapidly and accurately to detect GAS antigen in patients with cutaneous lesions suspected of GAS infection.
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01:30Acute Pharyngitis
Acute pharyngitis is the inflammation of the back of the throat (pharynx), commonly resulting in a sore throat. It is a frequently encountered condition that prompts individuals to seek medical advice.
Classification
Acute pharyngitis can be categorized based on its underlying cause: