Antimicrobial susceptibility testing (AST): a review of changing trends, quality control guidelines, test accuracy, and recommendation for the testing of beta-lactam drugs.

The review that follows presents the changing trends in antimicrobial susceptibility testing observed from the author's clinical laboratory experience and the proficiency testing surveys of the College of American Pathologists (CAP). The CAP Microbiology Surveys show a clear trend toward standardized test methods of the National Committee for Clinical Laboratory Standard (NCCLS) and greater compliance with specified methods' technical steps. This has favorably influenced the laboratory performance on proficiency challenges where a 3-5% improvement has been noted over the last 5 years for the disk tests (overall acceptable rate of 95.2% in 1981). A concurrent increase in dilution test use, mainly broth microdilution methods, has resulted in greater than 25% of larger hospital laboratories reporting results as MICs (overall acceptable or good performance = 98%). Automated systems use also continues to increase, with user performance being monitored at an acceptable level. Quality control frequency may be reduced to once weekly without compromising test accuracy or patient care, but only after adequate daily or concurrent QC performance has been documented. Most methods continue to have problems in testing enterococci, methicillin-resistant staphylococci, and the class-disk concept appears to be less applicable. Recommendations are made for the testing of the newer semisynthetic penicillins and cephalosporins based on their spectrum comparability and cross-resistance studies with bacteria possessing known susceptibility or resistance mechanisms. The concept of "spectrum-class" is introduced with peer drugs within classes. The general trends and quality of antimicrobial susceptibility tests seem outstanding and point toward continued excellent intra- and interlaboratory reproducibility at the national level, primarily due to the efforts of the inspection and accreditation agencies, CAP, CDC, and other concerned professional groups.