Table 1. Colistin AST methods compared in this study.

VITEK 2 cards were incubated and evaluated automatically by the device, all other tests were incubated at 36 ± 2°C for 16–20 hours in ambient air. McF, McFarland standard; CAMHB, cation-adjusted Muller-Hinton II broth; QC, quality control; MHA, Muller-Hinton agar; BMD, broth microdilution; FSCA, Field Safety Corrective Action.

Test	Description	MIC range (mg / L)	Inoculum	Medium used in this study	Comment	Reference
Broth microdilution	In-house prepared serial dilution of colistin sulfate in untreated polystyrene 96-well plates	≤0.25 –>128	5 × 105 CFU / mL	CAMHB (Sigma-Aldrich)	EUCAST and CLSI reference method; comparably labor intensive; single isolate test runs possible;	ISO 20776–1[15]
Agar dilution	In-house prepared serial dilution of colistin sulfate in untreated polystyrene 96-well plates	≤0.5 –>16	1 × 104 CFU / spot	MHA (CAMHB + 1.7% Agar)	Comparably labor intensive; single isolate test runs possible by using 8-well microtiter strips;	Wiegand et al. [16]
Etest, MIC Test Strip	Agar gradient diffusion	≤0.016 –>256	McF 0.5 streaked with sterile swab	MHE (bioMérieux), MHA (Oxoid), MHA (BD)	Suitable for routine application; single isolate test runs possible; For Etest, use of MHE plates is mandatory; For Etest, only Enterobacteriaceae must be tested for diagnostic use; Comparably expensive;	FSCA 3061, FSCA 3206
VITEK 2	Card-based, semiautomated AST (manual inoculum preparation, automated incubation and reading)	≤0.5 –>16	McF 0.5–0.63	Contained in the test card	Suitable for routine application; single isolate test runs possible; Colistin susceptible test results require confirmation;	Dafopoulou et al.[17] FSCA 3490
SensiTest	Commercially available 4-test BMD panel containing dried colistin in 7 two-fold dilutions	≤0.25 –>16	5 × 105 CFU / mL	CAMHB (Liofilchem, provided with assay)	Suitable for routine application; Comparably expensive; simultaneous AST of four isolates per panel required for optimum cost-efficiency;	Matuschek et al.[18]