Group A Streptococcal Rapid Test: Antigen Detection After 18-24 Hours of Penicillin Therapy

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 concentrated 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.