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Comparison of an immunochromatographic method for rapid identification of group A streptococcal antigen with culture
T P Ehrlich1, R H Schwartz, R Wientzen
1Department of Family Practice, Medical College of Virginia, Fairfax Family Practice Center Inc.
Objectives:
To compare the sensitivity and specificity of Concise Strep A (Hybritech, San Diego, Calif), an immunochromatographic group A streptococcal rapid antigen detection system, with a two-plate culture method for the diagnosis of streptococcal pharyngitis, and to evaluate the need for routine back-up culture when this rapid test is used.
Design:
Throat cultures were obtained from 351 children with acute pharyngitis by duplicate rayon-tipped swabs held in parallel and vigorously rubbed against both tonsils and the posterior pharyngeal wall. One swab was tested for group A streptococcal antigen by a registered licensed laboratory technologist in the pediatrician's office. The other swab was streaked over each of two sheep blood agar plates, one of which was enhanced with trimethoprim in combination with sulfamethoxazole. The plain sheep blood agar plate was then incubated in a candle-extinguish jar. The enhanced agar plate was placed in a gas-pack anaerobic jar. Both plates were incubated for up to 48 hours at 35 degrees C.
Setting:
A six-person group pediatric practice.
Participants:
Three hundred fifty-one children.
Results:
The Concise Strep A antigen detection test produced 129 positive results. Only six of the 129 were not confirmed by culture method. There were four false-negative rapid streptococcal antigen detection test results, all of which were found after a single overnight incubation. The sensitivity for the Concise Strep A test was 96.9% and the specificity was 97.4%. The plain 5% sheep blood agar plate (without trimethoprim and sulfamethoxazole), which was incubated in a candle-extinguish jar, identified 123 (97%) of the 127 positive throat cultures. The second 24-hour incubation and use of trimethoprim and sulfamethoxazole agar were not rewarding for this study.
Conclusions:
Concise Strep A, a polyclonal antibody test, in conjunction with a color immunochromatographic assay for soluble streptococcal carbohydrate antigen A appears to be accurate, sensitive, and specific when throat swabs are carefully obtained and when qualified, licensed laboratory technologists perform the procedure. Further studies should be done to confirm our findings, especially when nurses or office staff perform the rapid test procedure in the office setting. If our findings are confirmed, the use of back-up cultures for negative rapid test results obtained using Concise Strep A would be unnecessary.