TABLE 1.
Pre-discrepant analysis result interpretation of NG-Test Carba 5 compared to the composite reference method
| Composite reference method result |
NG-Test Carba 5 result | Pre-discrepant NG-Test Carba 5 interpretation | No. of results by pre-discrepant analysisa | |
|---|---|---|---|---|
| Xpert Carba-R | mCIMb | |||
| + | + | + | True positive | 169 |
| − | − | + | False positive | 0 |
| + | − | + | False positive | 0 |
| − | + | + | False positive | 7c |
| + | + | − | False negative | 0 |
| + | − | − | True negative (lack of or low-level expression) | 1d |
| − | + | − | True negative (other off-target carbapenemase) | 15e |
| − | − | − | True negative | 121f |
The results on blood and MacConkey agars were the same. Results are shown by enzyme type and not by isolate. The 172 on-target carbapenemase-producing isolates demonstrated a total of 176 positive NG-Test Carba 5 results due to four isolates that coproduced two target carbapenemases.
mCIM, modified carbapenem inactivation method.
Of the 7 false-positive results, results for IMP for 6 isolates (3 Enterobacterales and 3 Pseudomonas aeruginosa isolates) were positive by NG-Test Carba 5 and negative by Xpert Carba-R. The remaining false-positive result was for a Klebsiella oxytoca isolate that was positive for both KPC and NDM by NG-Test Carba 5 but negative for NDM by Xpert Carba-R. PCR and bidirectional sequencing confirmed all 7 NG-Test Carba 5 results.
One Enterobacter cloacae prospective isolate was resistant to ertapenem (zone diameter, 18 mm) but susceptible to imipenem and meropenem and harbored the blaKPC gene as determined by Xpert Carba-R. The isolate was negative by both mCIM and NG-Test Carba 5, indicating a lack of or low-level expression.
These 15 true-negative results were found for 7 Enterobacterales isolates and 8 P. aeruginosa isolates. Of the Enterobacterales isolates, four harbored blaSME. A carbapenemase gene was not detected in the remaining three isolates, and these isolates likely had false-positive mCIM results due to ESBL and/or AmpC expression combined with permeability defects. Among the P. aeruginosa isolates, one harbored the blaGES gene and the remaining seven harbored non-carbapenemase β-lactamase genes.
One P. aeruginosa isolate and one Klebsiella aerogenes isolate led to indeterminate mCIM results and were negative by Xpert Carba-R and NG-Test Carba 5.