Quality control of agar diffusion susceptibility tests.

Area of Science:

• Clinical Microbiology
• Pharmaceutical Quality Control

Background:

• The College of American Pathologists (CAP) Microbiology Program has provided quality assurance since 1974.
• Over 180 laboratories have participated, generating extensive data on antibiotic susceptibility testing.
• This study analyzes a significant dataset of antibiotic determinations on reference strains.

Purpose of the Study:

• To evaluate the performance of antibiotic susceptibility testing methods within a large-scale quality assurance program.
• To compare current quality assurance data against established precision and accuracy control limits.
• To identify trends and potential areas for improvement in antibiotic testing methodologies.

Main Methods:

• Analysis of 2,372,000 antibiotic determinations submitted to the CAP Quality Assurance Service (QAS).
• Primary method analyzed: standard Bauer-Kirby method (89.5% of data).
• Secondary method analyzed: agar overlay modification by Barry et al. (8.4% of data).

Main Results:

• Standard statistical analysis was performed for each antimicrobial/reference strain combination using the Bauer-Kirby method.
• Quality Assurance Service (QAS) data were compared to historical precision and accuracy control limits from collaborative studies.
• A significant proportion of QAS data exceeded existing control limits, indicating potential variability or shifts in performance.

Conclusions:

• The extensive data from the CAP QAS highlights the performance of antibiotic susceptibility testing on a national scale.
• Exceedance of established control limits suggests a need to review and potentially update current performance benchmarks.
• Continuous quality assessment is crucial for maintaining accuracy and reliability in clinical microbiology antibiotic testing.