Colistin Agar; Evaluation of a Novel Diagnostic Approach to Detection of Colistin Resistance

Abstract

Objective: To determine the diagnostic accuracy of Colistin agar for detection of Colistin resistance in clinical isolates of MultiDrug Resistant Gram-Negative Bacilli.
 Study Design: Cross-sectional validation study.
 Place and Duration of Study: Department of Microbiology, Armed Forces Institute of Pathology, Rawalpindi Pakistan, from Feb to Aug 2019.
 Methodology: A total of 100 Multi-Drug Resistant Gram-Negative Bacilli in clinical isolates were included. Isolates were identified using Gram stain, Catalase, Oxidase, API 20E, and API 20NE. After approval from the institutional ethical review committee, Colistin susceptibility was determined simultaneously by Colistin agar and Broth Micro Dilution Minimum Inhibitory Concentration method as per CLSI. For susceptibility criteria, EUCAST guidelines were followed. Results were validated with the gold standard test, i.e., Broth Micro Dilution.
 Results: Out of 100 Multi-Drug Resistant clinical isolates, the distribution was K. pneumoniae n=60, E.coli n=16, A. baumannii n=11, C. freundii n=8, and E. cloacae n=5. The sensitivity, specificity, positive predictive value, negative predictive value, and diagnostic accuracy of Colistin agar for detection of Colistin resistance, keeping Broth Micro Dilution Minimum Inhibitory Concentration method as the gold standard was 96.67%, 97.14%, 93.55%, 98.55%, and 97%, respectively.
 Conclusion: Colistin agar has excellent diagnostic accuracy for the detection of colistin resistance with standardized inoculum density. Due to its ease of use, cost-effectiveness, and accurate results, it can be used in lab setups deficient in manpower and advanced equipment for Broth Micro Dilution or genetic sequencing.

Evaluation of Six Commercial and Non-Commercial Colistin Resistance Diagnostics

“…Although the specificity recorded in this study and that of Osei Sekyere et al (at 66,67%) agree, two other studies recorded a significantly higher specificity of ≥ 97% [11,22,23].…”

“…These discrepancies could be attributed to the different bacterial inoculum concentrations used. Ali et al demonstrated that when a 0.5 McFarland standard inoculum was used, colistinsusceptible isolates were able to grow on the media [23], whereas the same isolates were inhibited when the inoculum was diluted to a density of 1×10 ହ CFU/mL. Therefore, the high rate of MEs in this present study could be due to the inoculum used being a 0.5 McFarland standard (1.5×10 ଼ CFU).…”

Evaluation of Broth Disk Elution Test and Agar Test to Determine Colistin In Vitro Activity in Enterobacteriaceae.

“…In a study conducted by Ali and his team in Pakistan, they recommended that Colistin agar can be employed routinely as a credible method for the identification of Colistin resistance for early medication and infection control in small laboratories where BMD and genetic sequencing are not available, as well as those with a high workload (24) . Additionally, CAT is relatively easier to execute than CBDE because up to ten isolates can be inoculated per dilution plate.…”