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 ORIGINAL ARTICLE
Year : 2020  |  Volume : 38  |  Issue : 1  |  Page : 58-65

Colistin susceptibility testing of gram-negative bacilli: Better performance of vitek2 system than E-test compared to broth microdilution method as the gold standard test


1 Department of Bio-Medical Laboratory Science and Management, Vidyasagar University, Midnapore, West Bengal, India
2 Department of Microbiology, Tata Medical Center, Kolkata, West Bengal, India
3 Department of Statistics, Tata Medical Center, Kolkata, West Bengal, India
4 Department of Laboratory Medicine, All India Institute of Medical Sciences, New Delhi, India
5 Division of Epidemiology and Communicable Diseases, Indian Council of Medical Research, New Delhi, India

Correspondence Address:
Dr. Sanjay Bhattacharya
Tata Medical Center, 14 MAR (E-W), New Town, Rajarhat, Kolkata - 700 160, West Bengal
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijmm.IJMM_19_480

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Introduction: Unavailability of optimal susceptibility testing (ST) challenges the clinical use of colistin. Broth microdilution (BMD), which is the reference for colistin ST, is inconvenient for diagnostics. Vitek2 and E-test although technically easier, are no longer recommended. Materials and Methods: For the evaluation of Vitek2 and E-test in reference with BMD, a total of 138 Gram-negative bacilli (GNB) especially carbapenem-resistant isolates from Tata Medical Center, Kolkata, India, were included during 2017–2018. The evaluation was performed only for Enterobacteriaceae (n = 102), but not for non-fermentative GNB (n = 36) due to lack of colistin-resistant (COLR) isolates. Results and Conclusion: Of 138 isolates, meropenem, colistin and dual resistance were detected in 110 (79.7%), 31 (22.5%) and 21 (15.2%) of isolates, respectively. Using the European Committee on Antimicrobial Susceptibility Testing guidelines (susceptible, ≤2 μg/ml), Vitek2 performed better than E-test (essential agreement, 92.2% vs. 63.7%; categorical agreement, 94.1% vs. 93.1%; very major error [VME], 10% vs. 23.3%). However, Vitek2 overcalled resistance than E-test (major error, 4.2% vs. 0%). Considering Chew et al. proposed breakpoints (susceptible, ≤1 μg/ml), VMEs declined for both test (6.7% vs. 10%), but still remained unacceptable. Of eight colistin-heteroresistant isolates, two VME were categorised by Vitek2, one VME was by E-test, and two were uninterpretable. Both Vitek2 and E-test are unreliable. Further studies correlating minimum inhibitory concentrations with clinical outcome are needed to determine the accurate breakpoints for better patient management.






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