The EDTA-based disk-combination tests are unreliable for the detection of MCR-mediated colistin-resistance in Enterobacteriaceae

We evaluated several EDTA-based combined-disk tests to detect 25 mcr producers among 48 Enterobacteriaceae. Colistin disks plus EDTA (292/584 μg) on MH and CAMH agar were used. Results were positive if with chelator there was an inhibition zone increase ≥3 mm compared to colistin alone. All tests resulted unreliable (sensitivity ≤68%).