Interpretation of Pneumococcal Susceptibility Tests with Cefaclor

Abstract

Chiou et al. (1) have reported an alarming incidence of b-lactam resistance among Streptococcus pneumoniae isolates in Taiwan, Republic of China. There are two points relating to data interpretation in the article that should be discussed and clarified. The first point regards application of nonvalidated interpretive criteria to cefaclor MIC data for pneumococci. In Materials and Methods, the authors indicate that they interpreted the cefaclor MICs using breakpoints recommended for “other cephalosporins” (#0.5 5 susceptible [S]) because the National Committee for Clinical Laboratory Standards (NCCLS) has not established pneumococcal breakpoints for this cephalosporin (3). Such a practice is not justified. Susceptibility breakpoints are established and verified through careful consideration of the native in vitro activity of the compound, its in vivo distribution and pharmacokinetics following the recommended doses, and its clinical efficacy against indicated infections due to target pathogens having known in vitro responses to the antibiotics (2). Thus, it is not acceptable to apply interpretive criteria developed for one agent to the test results of another without first verifying that the breakpoints should be the same for both agents. It should be noted that recent editions of the NCCLS approved standards for dilution susceptibility tests contain a table of criteria specific for pneumococci. After the appearance of significant numbers of non-penicillin-susceptible pneumococci (NPSP), criteria for cefaclor and several other b-lactam antibiotics were removed from the table because the committee had not seen clinical data to validate interpretations for these agents when tested against NPSP (3). A footnote to that table instructs the user to inform the physician that penicillin-susceptible pneumococci can be safely assumed to be susceptible to those deleted agents but that susceptibility to those agents among NPSP is unknown. The second point pertains to apparent use of two different