TABLE 2.
Performance of clinical and challenge isolates, Vitek 2 automatic dilution methodd
| Ceftazidime-avibactam treatment | Total tested, n | EA, n | EA, % | Eval |
CA, n | CA, % | R, n | Major, n | Very major, n | ||
|---|---|---|---|---|---|---|---|---|---|---|---|
| EA total, n | EA, n | EA, % | |||||||||
| Enterobacterales, ≤8 susceptible, none intermediate, ≥16 resistant | |||||||||||
| Clinical | 819 | 770 | 94.0 | 265 | 216 | 81.5 | 813 | 99.3 | 10 | 6 | 0 |
| Challenge | 47 | 46 | 97.9 | 20 | 19 | 95.0 | 46 | 97.9 | 27 | 1 | 0 |
| Combined | 866 | 816 | 94.2 | 285 | 235 | 82.5 | 859 | 99.2 | 37 | 7a (0.8%) | 0 (0%) |
| P. aeruginosa, ≤8 susceptible, none intermediate, ≥16 resistant | |||||||||||
| Clinical | 161 | 152 | 94.4 | 145 | 136 | 93.8 | 154 | 95.7 | 10 | 7 | 0 |
| Challenge | 46 | 46 | 100 | 18 | 18 | 100 | 46 | 100 | 28 | 0 | 0 |
| Combined | 207 | 198 | 95.7 | 163 | 154 | 94.5 | 200 | 96.6 | 38 | 7b (4.1%) | 0 (0%) |
| Enterobacterales plus P. aeruginosa | |||||||||||
| Clinical | 980 | 922 | 94.1 | 410 | 352 | 85.9 | 967 | 98.7 | 20 | 13 | 0 |
| Challenge | 93 | 92 | 98.9 | 38 | 37 | 97.4 | 92 | 98.9 | 55 | 1 | 0 |
| Combined | 1,073 | 1,014 | 94.5 | 448 | 389 | 86.8 | 1,059 | 98.7 | 75 | 14c (1.4%) | 0 (0%) |
aOut of seven major errors, four major errors were one dilution apart from the reference method and, as such, fall within essential agreement.
bAdjusted major error rate for FDA criteria for P. aeruginosa was calculated due to lack of an intermediate category. Before adjustment, major error rate as presented in the table was 4.1% (7/169). After adjustment, major error rate was 1.2% (2/169). Five P. aeruginosa isolates were within essential agreement compared to the reference method.
cThe overall categorical major error rate for Enterobacterales and P. aeruginosa combined was 1.4% (14/998). Nine major errors were one dilution apart from the reference method and, as such, fall within essential agreement. Based on the essential agreement and the lack of an intermediate breakpoint for ceftazidime-avibactam, the adjusted categorical major error rate for FDA criteria is 0.5% (5/998). For resolved errors per ISO standards, refer to Table S4.
Essential agreement (EA; ±1 doubling dilution) occurs when there is agreement between the result of the reference method and that of the Vitek 2 test card within one serial twofold dilution of the antibiotic. Evaluable results are those that are on scale for both the Vitek 2 test card and the reference method. Category agreement (CA) occurs when the interpretation of the result of the reference method agrees exactly with the interpretation of the Vitek 2 test card. Eval, evaluable isolates; major, major discrepancies; n, number of isolates; R, resistant isolates; very major, very major discrepancies.