Evaluation of the Strep A OIA (R) assay versus culture methods: Ability todetect different quantities of Group A Streptococcus

The Strep A OIA(R) assay by Biostar (Boulder, Co., USA) is a unique optical immunoassay system for the rapid detection of Group A streptococcal carboh ydrate. As part of a community-based pediatric cohort study of Group A Stre ptococcus (GAS) persistence following antibiotic therapy of pharyngitis, th e performance of the Str ep A OIA(R) assay was compared with the amount of growth from standard throat swab culture methods. A total of 363 throat swa bs taken over the course of the study was evaluated from 248 children betwe en 2 and 18 years of age. Two culture methods were performed: an agar plate with the throat swab using Columbia agar base with 5% sheep blood incubate d under an anaerobic environment for 48 it and Todd-Hewitt broth (THB) enha ncement. The Strep A OIA(R) runs then performed. A total of 144 of 363 (39. 7%) samples was positive for GAS by one or more of the laboratory tests acr oss study visits: agar culture detected 132 of 144 (91.7%), THE culture det ected 128 of 144 (88.9%), and the Strep A OIA(R) assay detected 129 of 144 (89.6%). Complete agreement among all three laboratory tests was found for 333 of 363 (91.7%) of the samples. Agar culture results were comparable to THE cultures with a sensitivity of 96.9%, specificity of 96.6%, a positive predictive value of 93.9%, and a negative predictive value of 98.3%. Althou gh the performance of the Strep A OIA(R) assay had similar specificity (96. 5%) and positive predictive value (93.8%) compared with the combined result s of the two culture methods, the sensitivity (89.0%) and negative predicti ve value (93.6%) were lower. A significant difference (p < 0.001) was found in the ability of the Strep A OIA(R) assay to detect agar culture-positive swabs that had a light growth (1+ or 2+) (63.0%) versus a moderate (3+) or heavy (4+) growth (98.1%) of GAS. Although the Strep A OIA(R) assay allows GAS throat swab results to be reported art average of 24 h sooner than eit her of the cultures, the rapid assay was not as sensitive in defecting ligh t growth GAS-positive cultures. (C) 1999 Elsevier Science Inc.