Table 4.
Distribution of MICs for isolates tested against colistin using the reference BMD method (recommended by the CLSI) and Vitek 2
| Organism (total isolates) | Method | Isolates with indicated colistin MICs (mg/L) determined by two methods, n (%) | Agreementa | Isolates with VME,b n (%) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ″0.5 | 1 | 2 | 4 | 8 | 16 | ≥32 | Essential | Categorical | ||||
| Escherichia coli (121) | BMD | 14 (11.7) | 103 (85.8) | 3 (2.5) | 0 | 0 | 0 | 0 | 118 (97.5) | 121 (100) | 0 | |
| Vitek 2 | 120 (100) | 0 | 0 | 0 | 0 | 0 | 0 | |||||
| Klebsiella pneumoniae (137) | BMD | 6 (4.4) | 124 (90.5) | 4 (2.9) | 1 (0.7) | 0 | 2 (1.5) | 0 | 131 (95.6) | 135 (98.5) | 2 (66.7) | |
| Vitek 2 | 136 (99.3) | 0 | 0 | 0 | 0 | 1 (0.7) | 0 | |||||
| Proteus mirabilis (38) | BMD | 0 | 0 | 0 | 0 | 0 | 0 | 38 (100) | NA | NA | NA | |
| Morganella morganii (12) | BMD | 0 | 0 | 0 | 0 | 0 | 0 | 12 (100) | 12 (100) | NA | 0 | |
| Vitek 2 | 0 | 0 | 0 | 0 | 0 | 12 (100) | 0 | |||||
| Citrobacter freundii (10) | BMD | 2 (20.0) | 8 (80.0) | 0 | 0 | 0 | 0 | 0 | 10 (100) | NA | 0 | |
| Vitek 2 | 10 (100) | 0 | 0 | 0 | 0 | 0 | 0 | |||||
| Enterobacter cloacae (51) | BMD | 1 (2.0) | 32 (62.7) | 2 (3.9) | 0 | 1 (2.0) | 1 (2.0) | 14 (27.5) | 35 (68.6) | 37 (72.5) | 14 (87.5) | |
| Vitek 2 | 49 (96.1) | 0 | 0 | 0 | 0 | 2 (3.9) | 0 | |||||
| Serratia marcescens (42) | BMD | 0 | 1 (2.4) | 1 (2.4) | 0 | 0 | 0 | 40 (95.2) | NA | NA | NA | |
| Pseudomonas aeruginosa (128) | BMD | 0 | 14 (10.9) | 108 (84.4) | 4 (3.1) | 2 (1.6) | 0 | 0 | 14 (10.9) | 122 (95.3) | 6 (100) | |
| Vitek 2 | 128 (100) | 0 | 0 | 0 | 0 | 0 | 0 | |||||
| Acinetobacter baumannii complex (138) | BMD | 2 (1.4) | 60 (43.5) | 62 (44.9) | 7 (5.1) | 5 (3.6) | 2 (1.4) | 0 | 62 (44.9) | 124 (89.9) | 14 (100) | |
| Vitek 2 | 133 (96.4) | 3 (2.2) | 2 (1.4) | 0 | 0 | 0 | 0 | |||||
| Burkholderia cepacia (19) | BMD | 0 | 0 | 0 | 0 | 0 | 0 | 19 (100) | 19 (100) | NA | 0 | |
| Vitek 2 | 0 | 0 | 0 | 0 | 0 | 19 (100) | 0 | |||||
| Stenotrophomonas maltophilia (61) | BMD | 0 | 0 | 2 (3.3) | 3 (4.9) | 2 (3.3) | 8 (13.1) | 45 (73.8) | NA | NA | NA | |
Notes:
a
Categorical agreement between susceptibility-testing methods measured as percentage of isolates with concordant test results when determining susceptible or WT and resistant or NWT to colistin;
b
major errors for Vitek 2 defined as colistin-resistant or NWT isolate determined using BMD method interpreted as colistin-susceptible or WT isolate.
Abbreviations: MICs, minimum inhibitory concentrations; BMD, broth microdilution; CLSI, Clinical and Laboratory Standards Institute; VME, very major error; NA, not available; WT, wild type; NWT, non-WT.