Clostridioides difficile: Speeding Up Rigor and Reproducibility of Susceptibility Testing

Testing for C difficile

Testing for Clostridioides difficile in a medical setting can be time-consuming and should be stringent. However, with the ever-increasing demands of the medical are setting, clinicians need a test to diagnose a patient more quickly with C difficile, so investigators keep attempting to create a test that can be reproduced in and stand up to the demands of many laboratories.

In a study presented as a poster at the IDWeek Conference in Washington, DC, on October 19-23, 2022, investigators from Texas describe a process they developed to verify the rigor and reproducibility (R&R) of Clostridioides difficile susceptibility testing. They used 2 laboratories, decreased the time needed to perform the tests, and decreased costs for the facilities, while reducing the possibility of human error.

The study, “Rigor and Reproducibility of Clostridioides difficile susceptibility testing,” was presented by Chris Lancaster, MS, senior research manager University of Houston, Houston, Texas, and demonstrated that a little over 10 minutes were saved by using automation versus a technicians’ work.

“The Clinical Laboratory Standards Institute (CLSI) recommends 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. R&R of these assays can lead to difficulty in comparing results between studies,” Lancaster, et al, noted on the poster. The goal for the investigators was “to assess the intra-and inter-laboratory reproducibility of MIC testing for C difficile within our lab and colleagues.”

Using AD and BMD techniques, the investigators used Vancomycin MIC testing on 30 C difficile isolates. According to the poster, “To test intralab reproducibility, proficiency testing to develop a reproducible MIC testing process using 18 isolates was implemented across two multidisciplinary labs, which was then validated in a prospective cohort of 116 isolates.”

The investigators compared the MICs obtained by each method and lab and calculated the essential agreement (EA) and major and minor error rates. Because the AD testing is labor- and time-intensive, the investigators tested the automation with Integra Assist Plus. Then twice they measured the times for plate setup using a technician against the Integra Assist Plus and compared the average times.

The investigators noted that they used discarded stool samples transported to the centralized lab. The stool samples were plated on selective cefoxitin-cycloserinefructose agar (CCFA) plates and incubated anaerobically for 48 – 72 hours. The investigators defined reduced vancomycin susceptibility by MIC greater than 2 mg/L. A total of 10’4” minutes were saved using automation (12’44” mean) versus a technician (24’40” mean).

Future research on this topic will include authentication with a larger sample and more academic partners, the investigators noted.

Lancaster, et al, reviewed the results of the study, “The 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.” One increase was EA which improved in the larger cohort to 93.9% (109/116). In that, minor and major disagreements occurred in 17% (21/116) and 25% (29/116), respectively. The study showed that using automation (12’44” mean) versus a technician (24’40” mean), a total of 10’4” minutes were saved.

Co-authors of the study include Taryn A. Eubank, PharmD, BCIDP, postdoctoral ID research fellow, University of Houston College of Pharmacy, Houston, Texas; Anne J. Gonzales-Luna, PharmD, BCIDP, assistant professor, University of Houston; Chetna Dureja, PhD, Texas A&M Health Science Center Institute of Biosciences and Technology, Houston, Texas; Julian Hurdle, PhD, associate professor, Texas A&M Health Science Center, Houston, Texas; Kevin W. Garey, PharmD, MS, professor and chair, University of Houston, Houston, Texas.