Special topics in antimicrobial susceptibility testing: test accuracy against methicillin-resistant Staphylococcus aureus, pneumococci, and the sensitivity of beta-lactamase methods.

Three contemporary problems in antimicrobial susceptibility testing were assessed by the CAP-Microbiology Surveys Program in 1982. A penicillin-resistant Streptococcus pneumoniae was categorized correctly as resistant by nearly 78% of Infectious Disease Survey subscribers. This rate compares with a 15% accuracy in the 1981 Surveys challenge, and all results reported from the NCCLS recommended 1 microgram oxacillin screening test correctly found penicillin resistance. Further improvement in the microbiology laboratories' ability to recognize pneumococcal penicillin resistance is critical to good patient care. A methicillin-resistant Staphylococcus aureus (MRSA) strain was assessed accurately by 96.8% of NCCLS disk diffusion test users (methicillin, nafcillin, and oxacillin disks). The microdilution broth method using methicillin as the representative of the penicillinase-stable penicillins performed well. Only nafcillin and oxacillin broth microdilution tests and the automated MIC methods had reduced accuracy. Automated systems also failed to recognize an associated erythromycin resistance in the MRSA strain. Suggestions for improved microdilution test accuracy are made. Three survey challenges have evaluated the sensitivity and specificity of commercial and other beta-lactamase test methods. The false-positive rates for staphylococci range from 3.9 to 4.5%. The false-negative results on a Haemophilus paraphrophilus (beta-lactamase producer) were highest for the iodometric technic (8.7%) and lowest for the chromogenic cephalosporins (1.9%) such as nitrocefin and PADAC. Recommendations for their more limited use in generally emergent clinical settings are offered.