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.

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 (< or = 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.