The growth and susceptibility testing of anaerobic bacteria presents many unique challenges. The Clinical Laboratory Standards Institute (CLSI) recommends performing minimum inhibitory concentration (MIC) testing for C. difficile through agar dilution (AD) assay, which carries logistical and time burdens compared to broth microdilution (BMD) methods. In this project we aim to assess the intra-and inter-laboratory reproducibility of MIC testing for C. difficile.
A total of 30 C. difficile isolates underwent MIC testing using AD and BMD techniques. To test intra-lab reproducibility, proficiency testing to develop a reproducible MIC testing process using 18 isolates was implemented across two multi-disciplinary labs, which was then validated in a prospective cohort of 116 isolates. MICs obtained by each method and lab were compared and essential agreement (EA) and major and minor error rates were calculated. To combat the labor and time-intensive demands of AD testing, automation with Integra Assist Plus was tested. Times for plate set up using a technician versus the Integra Assist Plus were measured twice and the average times were compared.
AD and BMD yielded discordant resistant/susceptible results in 16.7% (5/30) of isolates tested. During proficiency testing, intra-lab comparison of AD MICs yielded 88.9% (16/18) EA and no disagreements occurred with more than 1 dilution difference. EA improved in the larger cohort to 93.9% (109/116), of which minor and major disagreements occurred in 17% (21/116) and 25% (29/116), respectively. A total of 104 minutes were saved using automation (1244 mean) versus a technician (2440 mean).
Here we present the process undertaken to ensure the rigor and reproducibility of C. difficile susceptibility testing. We developed a procedure that yields accurate results from two different labs, minimizes cost, and lowers the overall time required. Our future research will include validation with a larger sample and more academic partners.
Kevin W. Garey, PharmD, MS, Acurx: Grant/Research Support|cidara: Advisor/Consultant|cidara: Grant/Research Support|Paratek: Grant/Research Support|Seres Health: Grant/Research Support|Summit: Grant/Research Support.