Selection criteria for an antimicrobial susceptibility testing system

The clinical microbiology laboratory plays an important role in antibiotic selection and use through performance of routine antimicrobial susceptibility testing on patients' clinical isolates. There are now a number of bacterial species which may exhibit acquired resistance to one or more classes of antimicrobial agents. For that reason, susceptibility testing is most important with members of the family Enterobacteriaceae, Pseudomonas spp., Staphylococcus spp., Enterococcus spp., Haemophilus influenzae, and Streptococcus pneumoniae, since clinical isolates may not be susceptible to empiric drug choices. Indeed, the most important function of a susceptibility test is the accurate detection of clinically relevant antimicrobial resistance in individual isolates. Failure of a test to predict antimicrobial resistance could place a patient in jeopardy of increased morbidity or mortality. Clinical microbiology laboratories can choose from among several manual or instrument-based methods for performance of their routine susceptibility tests. These include the disk diffusion (or Bauer-Kirby) test, antibiotic gradient methods, agar dilution with mechanized inoculator, the broth microdilution method (with or without use of an instrument or a growth indicator), and short-incubation automated instrument methods. The broth microdilution susceptibility test with commercially prepared antibiotic panels has become the most popular of the current methods available to U.S. clinical microbiology laboratories (4). MICs can be determined with the aid of a simple viewing box, with a more elaborate viewer incorporating a computerassisted data recording system, or with an automated photometer or fluorometer, which reads the growth endpoints in the trays. The reader devices may also be connected to a personal computer, which generates printed reports and can facilitate data storage and retrieval for periodic generation of cumulative susceptibility profiles for organisms which have been tested. Automated instruments for susceptibility testing may provide test results following a conventional overnight incubation period, or they may allow results to be determined in a shortened analysis period of 3.5 to 10 h (11) by taking advantage of the greater sensitivity of the instruments' optical systems to detect subtle increases in bacterial growth. Most of the instruments may also be used to perform identifications of gram-negative or gram-positive bacteria and may be able to merge and print identification and antimicrobial susceptibility results into a single report (22). This guest commentary focuses on criteria which may be used for evaluation and selection of antimicrobial susceptibility testing systems. ADVANTAGES AND DISADVANTAGES OF CURRENT SYSTEMS