Interlaboratory performance of disk agar diffusion and dilution antimicrobial susceptibility tests, 1979-1981. A summary of the microbiology portion of the College of American Pathologists (CAP) surveys.

The antimicrobial susceptibility test results of the 1979-1981 College of American Pathologists (CAP) Microbiology Surveys are summarized for the disk diffusion and dilution test methods. Improved interpretive accuracy (97.5%) was detected for the Special Bacteriology participants compared with those reported for 1972 and 1977-1978. The performance accuracy rates for the Comprehensive and Basic Surveys subscribers was 95.8% and 93.7%, respectively (all surveys accuracy rates was 95.2% from 196,775 responses). The dilution methods were graded since 1980 and showed an acceptable or good performance rate of 98.0%. Most reported MICs were from commercial systems, especially those performed in frozen broth microdilution systems. Eighteen per cent of all susceptibility tests are now reported as MICs. Quality control data shows a trend away from costly daily control practices without compromise of test accuracy. Several problem areas continue to plague these methods: (1) poor predictive value of disk results for enterococci; (2) availability of low potency disks with suboptimal accuracy rates, e.g., the 50 micrograms carbenicillin disk; (3) the release of more new drugs requiring individual testing beyond the capacity of the test systems; (4) vague recommendations for reporting and testing certain bacterial species, e.g., methicillin-resistant staphylococci and penicillin-resistant pneumococci; and (5) the lack of contemporary modification in FDA package insert quality control and interpretive criteria guidelines that might reflect new test or therapeutic information. Overall, the 1979-1981 surveys results show improved performance to a high level of predictive and interpretive accuracy among all subscriber groups coupled with a shift toward the more informative quantitative dilution methodologies.