Clinical trial comparing bacitracin with Strep-A-Chek for accuracy and turnaround time in the presumptive identification of Streptococcus pyogenes

Abstract

In a clinical trial, Strep-A-Chek (a 10-min chromogenic test) was compared with the bacitracin disk susceptibility test for accuracy and turnaround time in the presumptive identification of Streptococcus pyogenes. Among 461 isolates of beta-hemolytic streptococci (344 throat isolates and 117 isolates from other sites), 303 group A S. pyogenes isolates were found. The sensitivities of the Strep-A-Chek and bacitracin tests were high (96.4 and 100%, respectively), but the bacitracin test had a lower specificity (84.2%) than the Strep-A-Chek test (98.7%). The predictive values for positive and negative test results were 99.3 and 93.4%, respectively, for Strep-A-Chek and 92.4 and 100%, respectively, for bacitracin. Strep-A-Chek correctly identified all isolates upon repeat testing. All bacitracin tests were performed on subcultures of isolates from the primary plate. Strep-A-Chek testing was performed on colonies from the primary plate when isolated colonies were available. This shortened the turnaround time for Strep-A-Chek compared with bacitracin by at least 24 h on nearly one-half (45%) of the isolates. A peripheral finding of this study was that sulfamethoxazoletrimethoprim blood agar offered no advantage over conventional blood agar with regard to the number of false-positive bacitracin tests obtained from each medium.

Rapid, cost-effective identification of group A streptococci and enterococci by pyrrolidonyl-beta-naphthylamide hydrolysis

“…The Strep-A-Chek results were comparable to those previously reported (5). Although this method offers the advantage of rapid positive results (<10 min), the color developer provided in the kit is unstable, must be reconstituted for each test or batch of five tests, and used within 30 min of reconstitution.…”

“…and others (3,4) have demonstrated that L-pyrrolidonyl-pnaphthylamide (PYR) hydrolysis is a reliable test for the identification of GAS and ENT. Whereas earlier tests required incubation for 4 h to overnight to obtain results, recent modifications allow detection of PYR hydrolysis in 10 to 15 min (1,4,5). This study describes two PYR tests.…”

“…However, colonies must be carefully picked from throat cultures or other specimens with a heavy growth of normal flora to avoid picking up nonstreptococci on the swab. Organisms other than streptococci may produce falsepositive PYR results (5), and mixed cultures may result in false-negative tests.…”

Comparison of Meritec-Strep with Streptex for direct colony grouping of beta-hemolytic streptococci from primary isolation and subculture plates

“…For cultures with no isolated colonies, subculture and subsequent direct plate testing offer an easier method than testing from broth culture. In the present study, 59.8% of all primary isolation cultures yielded colonies that could be tested as others have reported (1,9). Because of the design of the study, only six cultures had too few isolated colonies to be tested from primary culture.…”

Evaluation of three disk tests for identification of enterococci, leuconostocs, and pediococci

“…In most cases, the test can be interpreted after overnight incubation. A disk test for determining the presence of pyrrolidonylarylamidase (PYRase) has been previously described for the identification of group A streptococci as well as enterococci (6,8,17). We have previously described a vancomycin susceptibility (Van s ) screening test to aid in the identification of viridans group streptococci (4).…”