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

Objectives In 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. Methods We 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 ® 2 (bioMérieux), of broth microdilution and of three different gradient strip test providers (Oxoid, Liofilchem, bioMérieux). For the latter we compared the standard procedure vs. a protocol with increased inoculum, a rich agar medium and a longer incubation time (“macromethod”). Results The sensitivity of Vitek ® 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. Conclusions The “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.