A comparison of the BioStar Strep A OIA™ rapid antigen assay, Group A Selective Strep Agar (ssA™), and Todd-Hewitt broth cultures for the detection of group A Streptococcus in an outpatient Family Practice setting

Abstract

In some studies the BioStar™ Strep A OIA (optical immunoassay) has yielded inconsistent results, although originally it was reported to be more sensitive than conventional culture for the detection of group A Streptococcus (GAS). The Group A Selective Strep Agar with 5% sheep blood (ssA™) incubated anaerobically has been reported to be more sensitive than conventional culture in the detection of GAS. We compared the BioStar™ Strep A OIA GAS rapid antigen detection kit to anaerobic culture on ssA™ with and without preincubation in Todd-Hewitt broth (THB) for the detection of GAS. From September 1995 through January 1996, throat swabs were collected in duplicate from 75 children (≤18 years) and 188 adults (>18 years) who presented with pharyngitis in the outpatient University of New Mexico Family Practice Clinic. Thirty-one (12%) of the 263 cases were positive for GAS by culture and/or broth. Compared with anaerobic culture on the ssA™, with and without preincubation in THB, the sensitivity, specificity, positive predictive value, and negative predictive value of the BioStar™ Strep A OIA were 77, 62, 22, and 95%, respectively. Compared with enrichment in THB followed by subculture on ssA™ and anaerobic incubation, the sensitivity, specificity, positive predictive value, and negative predictive value of direct culture on ssA™ and anaerobic incubation were 79, 99, 98, and 96%, respectively. All isolates were serologically grouped. The BioStar™ Strep A OIA is as sensitive as direct culture on ssA™ incubated anaerobically, but the low specificity and low positive predictive value when the OIA is used in low prevalence populations could lead to unnecessary antibiotic treatment.