TABLE 3.
Performance characteristics of colistin and polymyxin B susceptibility testing methods in comparison to broth microdilutiona
| Drug | Method | No. of isolates that were susceptible | No. of isolates that were resistant | No. (%) of isolates exhibiting EA | No. (%) of isolates exhibiting CA | No. of isolates exhibiting VMEs (%) | No. of isolates exhibiting MEs (%) | Spearman's coefficient |
|---|---|---|---|---|---|---|---|---|
| Colistin | Vitek 2 | 60 | 16 | 71 (93.4)b | 67 (88.2) | 9 (36.0) | 0 (0) | 0.873c |
| Sensititre | 46 | 30 | 68 (89.5) | 69 (90.1)b | 1 (4) | 6 (11.8) | 0.863c | |
| Etest | 51 | 25 | 57 (75.0) | 70 (92.1)b | 3 (12.0) | 3 (5.9) | 0.600c | |
| MicroScan | 44 | 32 | NAd | 67 (88.2) | 1 (4) | 8 (15.8) | NAd | |
| Polymyxin B | BMD | 49 | 27 | NA | NA | NA | NA | NA |
| Vitek 2 | 47 | 29 | 73 (96.1)b | 72 (94.7)b | 1 (3.7) | 3 (6.1) | 0.917c | |
| Sensititre | 47 | 29 | 73 (96.1)b | 72 (94.7)b | 1 (3.7) | 3 (6.1) | 0.877c | |
| Etest | 53 | 23 | 38 (48.7) | 68 (89.5) | 6 (26.1) | 1 (1.9) | 0.534c |
a
BMD, broth microdilution; EA, essential agreement; CA, categorical agreement; VME, very major error; ME, major error; NA, not applicable. Spearman's coefficient indicates concordance of MICs with those of BMD.
b
The testing method-drug combination meets CLSI M52 recommendations for acceptable EA or CA performance.
c
P value < 0.001.
d
EA and Spearman's correlation coefficient were not determined for MicroScan due to a narrow MIC range.