Comparison of Diagnostic Tests for Fosfomycin Resistance Detection in Clinically Significant Enterobacterales

Multidrug resistance in Enterobacterales has triggered interest in old antibiotics like fosfomycin. Various methods to determine fosfomycin resistance are time-consuming and yield unpredictable results. Hence, the study was undertaken to compare the rapid fosfomycin NP test with the recommended agar dilution test and few other tests. In this cross-sectional, time-bound, prospective study, Enterobacterales (N = 110) were tested for their susceptibility to commonly used antibiotics by Kirby Bauer disc diffusion method. Susceptibility to fosfomycin was tested by agar dilution method, Vitek 2 system, rapid fosfomycin NP test and gene detection by PCR. The results of different tests were compared with fosfomycin MIC by agar dilution test. Of the110 Enterobacterales, sixty (54.5%) were resistant to fosfomycin by agar dilution and Vitek-2, of which twenty-three were extended spectrum beta-lactamase producers. The Vitek-2 system showed 100% sensitivity, specificity, positive and negative predictive values when compared with agar dilution method. The accuracy of rapid NP test and PCR for fos A gene were 81.8 and 91.1% respectively. fos A gene-mediated resistance was found to be common in Enterobacterales. Although the NP test is cost effective and rapid its interpretation is subjective, making it less accurate. Since antimicrobial susceptibility testing is crucial in clinical microbiology laboratories, the NP test cannot be used to detect fosfomycin resistance due to its false-positive nature. The findings of the study prompt us to use agar dilution or Vitek-2 system for detecting fosfomycin resistance in Enterobacterales.