COMPARISON OF 3 COMMERCIAL MIC SYSTEMS, E-TEST, FASTIDIOUS ANTIMICROBIAL SUSCEPTIBILITY PANEL, AND FOX FASTIDIOUS PANEL, FOR CONFIRMATION OF PENICILLIN AND CEPHALOSPORIN RESISTANCE IN STREPTOCOCCUS-PNEUMONIAE

The performances of three commercial broth microdilution MIC assays adapted for use with fastidious organisms-the E test (ET), Fastidious Antimicrobial Susceptibility panel (FAS), and FOX Fastidious panel (FOX)-were compared with a MIC using Mueller-Hinton broth with 5% lysed horse blood (MHLHB) to confirm penicillin and cephalosporin resistance in clinical isolates of Streptococcus pneumoniae. Of the isolates screened for penicillin resistance, 5 (12.8%) were categorized as susceptible, 16 (41.0%) were categorized as intermediate, and 18 (46.2%) were categorized as resistant by MHLHB. Only the isolates exhibiting intermediate-to-resistant MICs were included in the comparison. Agreement within +/- log(2) dilution was found in 91, 21, and 76% of the ET, FAS, and FOX MICs, respectively, compared with the MHLHB MIC. No very major or major discrepancies occurred with the ET or FOX; however, two very major interpretive errors occurred with the FAS. Agreement between the ET and MHLHB for cefotaxime, ceftriaxone, and cefuroxime was 88, 85, and 100%, respectively. Less than 50% of cephalosporin MICs categorized as >0.5 mu g/ml by MHLHB were detected by FAS or FOX. Of the methods compared, the ET was the most reliable alternative for susceptibility testing of pneumococci.