A comparative evaluation of mikrolatest kit broth microdilution and VITEK2 in assessing colistin susceptibility to Gram-negative bacterial isolates

Introduction and Aim: The increasing resistance in colistin is a major concern. The aim of the study was to compare the methods to identify the prevalence of colistin resistance and antibiotic susceptibility patterns of colistin resistant strains isolates from blood, urine and sputum samples at all ICUs including NICU and PICU and wards.   Materials and Methods: A total of 1458 consecutive Gram-negative isolates were tested for colistin susceptibility by standard broth microdilution method, Mikrolatest method and VITEK 2 automation system. Intrinsically colistin resistant organisms including Proteus sps, Providencia sps, Serratia sps., and Morganella morganii were excluded. Enterobacteriaceae (e.g., Escherichia coli, Klebsiella), Pseudomonas   aeruginosa and Acinetobacter baumanii were included. The sensitivity, specificity, positive predictive value, negative predictive value of Vitek-2, BMD and Mikrolatest methods were compared.    Results: Sixteen (1.09%) colistin?resistant isolates were reported over 24 months. K. pneumoniae constituted 8(50%), E. coli 6 (37.5%) and Enterobacter cloacae 2 (12.5%) of the 16 resistant isolates. The sensitivity, specificity, positive predictive value, negative predictive value of Mikrolatest compared with that of VITEK2 were 87.5% vs 56.25%, 90.84% vs 82.65%, 9.58% vs 3.47%and 99.8% vs 99.3%, respectively for resistant isolates.  Mikrolatest shared good Category agreement of 1.24% with BMD Essential agreement was 1.5%. Comparing MICs of BMD with other tests, essential agreement was the lowest for the VITEK2, while the Mikrolatest MIC showed essential agreement greater than 1.5%. No errors and 100% categorical agreement (for E. coli, K. pneumoniae) were observed while comparing colistin susceptibility test results of the Mikrolatest MIC and BMD tests. The difference between the two methods in assessing colistin resistance were not statistically significant (P=0.89). Blood[37.5%] and pus[37.5%] samples recorded as the common sources of the isolate, followed by Urine [12.5%], and respiratory [12.5%) samples.   Conclusion: Automated VITEK, Mikrolatest MIC methods give variable susceptibility results and colistin should be prescribed only after Mikrolatest and BMD. K. pneumoniae and E. coli, the Mikrolatest showed better performance for isolates with ? 0.5 or ? 16 µg/mL MICs. For P. aeruginosa isolates, colistin resistant isolates must be confirmed Colistin resistance among Gram-negative bacteria, especially K. pneumoniae, is emerging in Indian hospitals. Re-evaluation is required of the methods available to address the numerous technical challenges associated with colistin susceptibility testing, and to determine which method yields the most meaningful results. These studies will provide critical information on the appropriate selection of colistin therapy, as well as evaluating novel and upcoming compounds with structure and properties similar to the Polymyxin.