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Group A streptococcal rapid test. Antigen detection after 18-24 hours of penicillin therapy
P S Beach1, L C Balfour, H L Lucia
1University of Texas Medical Branch, Galveston 77550-2776.
Insights
Commercial kits for detecting group A beta-hemolytic streptococci (GABHS) antigen in children showed limited accuracy after partial treatment. GABHS antigen was not detected in urine samples using these kits.
Area of Science:
- Pediatrics
- Microbiology
- Infectious Diseases
Background:
- Pharyngitis is a common childhood illness, often caused by group A beta-hemolytic streptococci (GABHS).
- Accurate and rapid diagnosis of GABHS pharyngitis is crucial for appropriate treatment and prevention of complications.
- Commercially available diagnostic kits aim to detect GABHS antigen, but their accuracy, especially after partial treatment, requires evaluation.
Purpose of the Study:
- To assess the rapid disappearance of detectable pharyngeal GABHS antigen after oral penicillin therapy in children.
- To determine if GABHS antigen is excreted into the urine of infected children.
- To evaluate the diagnostic accuracy of latex agglutination (LA) and enzyme-linked immunoassay (EIA) kits for GABHS detection.
Main Methods:
- Study included 29 children (19 months to 16 years) with pharyngitis and positive GABHS tests, no recent antibiotic use.
- Pharyngeal swabs and urine samples were collected before and after 18-24 hours of oral penicillin therapy.
- GABHS detection was performed using blood agar plate (BAP) culture, latex agglutination (LA), and enzyme-linked immunoassay (EIA).
Main Results:
- After antibiotic treatment, throat swab positivity decreased significantly across all methods (BAP culture: 17%, LA: 29%, EIA: 17%).
- GABHS antigen was consistently undetectable in urine samples by LA or EIA, even with concentrated specimens.
- The study highlights potential limitations of current rapid antigen detection kits in diagnosing GABHS pharyngitis post-treatment.
Conclusions:
- Rapid antigen detection kits may not be reliable for diagnosing GABHS pharyngitis after partial antibiotic treatment.
- Urine testing for GABHS antigen using current kits is not a viable diagnostic approach.
- Clinicians should consider a history of prior antibiotic use when assessing pharyngitis in children.
Abstract:
We studied 29 children, aged 19 months to 16 years, prior to and after 18-24 hours of oral penicillin therapy to confirm the rapid disappearance of detectable pharyngeal antigen and to determine whether the antigen detectable by commercially available kits was excreted into the urine. Patients were recruited based on the presence of pharyngitis, no antibiotic therapy in the preceding 2 weeks, and a positive latex agglutination (LA) for group A beta hemolytic streptococci (GABHS) antigen on pharyngeal swab. Diagnosis was confirmed by positive GABHS culture on blood agar plates. Twenty-five of these children were also tested for GABHS antigen by enzyme-linked immunoassay (EIA). After 18-24 hours of oral antibiotic therapy, only 10 patients had a positive test for GABHS on throat swab. Five of 29 subjects (17%) remained positive by blood agar plate (BAP) culture, eight of 29 (29%) by LA, and four of 23 (17%) by EIA. GABHS antigen was undetectable by LA or EIA in the urines of any of these patients, either prior to or after initiation of treatment, even in specimens concentration as high as 100 fold. Clinicians should routinely seek a history of prior antibiotic therapy in assessing pharyngitis. Neither of the kits tested are reasonably accurate for GABHS disease by detection of antigen in the pharynx after partial treatment or in the urine at any time.