Comparison of Vitek(R) 2, three different gradient strip tests and broth microdilution for detecting vanB-positive Enterococcus faecium isolates with low vancomycin MICs

Abstract

ObjectivesIn 2018, EUCAST issued a warning regarding unreliable results of gradient strip tests for confirming vancomycin resistance in enterococci. We compared the performance of various diagnostic standard and confirmatory tests to identify and determine vanB-type vancomycin resistance.\n\nMethodsWe analysed a collection of vanB-positive E. faecium isolates (n = 68) with low level vancomycin minimal inhibitory concentrations (MICs) and compared the performance of Vitek(R) 2 (bioMerieux), of broth microdilution and of three different gradient strip test providers (Oxoid, Liofilchem, bioMerieux). For the latter we compared the standard procedure vs. a protocol with increased inoculum, a rich agar medium and a longer incubation time (\"macromethod\").\n\nResultsThe sensitivity of Vitek(R) 2 was 81% of vanB-isolates compared to 72% for broth microdilution and 61 - 63% for the three gradient strip tests using standard conditions. The \"macromethod\" substantially improved performance of all strip tests resulting in a sensitivity of 89 - 96% at 48h readout.\n\nConclusionsThe \"macromethod\" provided the overall best performance for recognition of vanB E. faecium. We therefore propose to adapt the EUCAST warning and to recommend the \"macromethod\" with an additional 48h readout or a confirmation by a vanB-specific PCR from culture.