More about cefuroxime screening test for pneumococci

Jorgensen et al. published an extensive evaluation of disk diffusion tests for Streptococcus pneumoniae (3). Several antimicrobial agents were tested by a rigorous method. However, the authors did not show the results they said could be obtained using cefuroxime. They said that cefuroxime zone diameter data generated during their study were not useful for accurate recognition of cephalosporin-resistant (MICs of .2 mg/ml) strains. Similar conclusions were obtained by Tenover et al. (6). Neither of these groups has tried to use the cefuroxime disk diffusion test as an screening method to detect both cefotaxime-intermediate and cefotaxime-resistant strains (MIC, .0.25 mg/ml) (5). One hundred and seventeen strains of S. pneumoniae isolated from clinically significant samples obtained from the same number of pediatric patients were selected to be tested by both a broth macrodilution method with ceftriaxone and a disk diffusion method with cefuroxime. The dilution test was performed with Mueller-Hinton broth plus 2% lysed horse blood, and the tubes were incubated for 18 to 24 h at 358C in room air. Ceftriaxone was obtained from Productos Roche S.A. Q. e I. The diffusion method was performed by following the National Committee for Clinical Laboratory Standards (NCCLS) guidelines for pneumococci (4) using 30-mg cefuroxime disks (Laboratorios Britania S.R.L., Buenos Aires, Argentina) and 5% sheep blood Mueller-Hinton agar plates. They were incubated for 18 to 24 h at 358C in a candle jar. S. pneumoniae ATCC 49619 was included as the reference strain. No ‘‘very major’’ errors were obtained, and only 7.7% of the susceptible strains would be classified as resistant (or intermediate) by the screening method (Fig. 1). From this single experience, the cefuroxime disk diffusion test seemed to be a good screening method for categorizing strains of S. pneumoniae nonsusceptible to cefotaxime. Zone sizes of ,35 mm may be indicative of either cefotaxime-resistant or cefotaximeintermediate pneumococci by NCCLS breakpoints (5). It could be a matter of discussion which of the cutoffs is appropriate, #0.25 mg/ml, as the NCCLS recommended (5), or #0.5 mg/ml, as Jorgensen et al. proposed (3). At least one clinical failure was reported when a child with meningitis caused by S. pneumoniae for which the MIC of cefotaxime was 0.5 mg/ml was treated with that antibiotic (1). We have no personal data to support one or the other criterion, but we preferred to be conservative in this matter and used the lower value. Considering diameters of $35 mm as indicative of susceptibility by the cefuroxime screening test and adopting NCCLS breakpoints for the dilution method, we found that even in the scattergram published by Tenover et al. (6) neither resistant (MIC, $2 mg/ml) nor intermediate (MIC, between 1 and 0.5 mg/ml) strains of S. pneumoniae would be considered susceptible to cefotaxime. Twenty-seven percent of the minor errors would be introduced in this case (6). Moreover, in another study, all the strains for which MICs were $1 mg/ml produced inhibition zones of ,35 mm when challenged with 30-mg cefuroxime disks (1). Later, our results and some previous information tempted us to use the cefuroxime screening test for predicting not resistance but susceptibility to broad-spectrum cephalosporins, in the same way that the oxacillin test is being used for detecting penicillin susceptibility in pneumococci (4). In cases of meningitis, we believe that it will be more useful to accurately predict susceptibility, rather than resistance, to broad-spectrum cephalosporins.