# The Evaluation of Two Colistin Minimum Inhibitory Concentration (MIC) Methods: A Comparative Study of MicroScan Walkaway 96 Plus ID/AST and Colistin Broth Disc Elution Test at a Tertiary Care Hospital in Punjab

## TL;DR

This study compares two methods for testing colistin resistance in bacteria and finds discrepancies that could affect treatment decisions.

## Contribution

The study evaluates the reliability of two colistin MIC methods in detecting resistance in MBL-producing Gram-negative bacteria.

## Key findings

- MicroScan showed 86.67% intermediate susceptibility, while CBDE showed 95.24%.
- 10 major errors and 1 very major error were identified between the two methods.
- Discrepancies were observed in resistance rates for Acinetobacter baumannii and Klebsiella pneumoniae.

## Abstract

Background and objective

The rise of multidrug-resistant (MDR) Gram-negative bacteria poses significant challenges in clinical settings, especially concerning colistin, a last-resort antibiotic. This study sought to compare two colistin minimum inhibitory concentration (MIC) testing methods: the automated MicroScan Walkaway 96 Plus AST system and the colistin broth disc elution (CBDE) test, particularly in (MBL)-producing Gram-negative organisms. The study aimed to evaluate the incidence of major errors (MEs) and very major errors (VMEs) in colistin susceptibility results between the two methods and to assess the comparative reliability and accuracy of these techniques for detecting colistin resistance.

Materials and methods

A study was conducted from October 2022 to September 2023 at a tertiary care hospital in Ludhiana, Punjab, involving 105 Gram-negative bacterial isolates from patient samples, including pus, blood, body fluids, and urine. Colistin susceptibility was assessed using the MicroScan Walkaway 96 Plus system and the CBDE method. Errors in susceptibility results were categorized as MEs and VMEs. Descriptive statistics were used for categorical and quantitative variables. Kappa agreement was calculated to evaluate the agreement between the two methods. Statistical analysis was performed using IBM SPSS Statistics software (IBM Corp., Armonk, NY), with a significance level set at p < 0.05.

Results

The study found that 86.67% of isolates showed intermediate susceptibility by MicroScan, compared to 95.24% using the CBDE test. A total of 10 MEs (9.52%) and one VME (0.95%) were identified. The study also observed discrepancies in colistin resistance rates, particularly for Acinetobacter baumannii (A. baumannii) and Klebsiella pneumoniae (K. pneumoniae).

Conclusions

MicroScan demonstrated a moderate agreement with the CBDE test in colistin susceptibility testing, with notable MEs and VMEs. These discrepancies highlight the need for further validation and the complementary use of both methods to ensure accurate susceptibility testing and effective clinical management of MDR infections.