Evaluation of Xpert Carba-R Assay v.2 to Detect Carbapenemase Genes in Two Hospitals in Korea

Background: As the spread of carbapenemase-producing Enterobacteriaceae poses a critical threat to public health, rapid detection of carbapenemase genes is urgently required for prompt initiation of appropriate antimicrobial therapy and infection control. We evaluated the performance of Xpert Carba-R v.2 (Cepheid, USA) compared with that of culture-based conventional PCR.

Methods: Using the results of 5,479 consecutive clinical rectal swabs, discrepant analysis (enriched culture followed by PCR) was performed for all discordant samples (N=100), which were Carba-R v.2-positive and culture-negative.

Results: Among the samples, 206 carbapenemase genes (3.6%) were detected by Carba-R v.2. The sensitivity and specificity were 95.0% and 98.1%, respectively. The positive predictive value (PPV) and negative predictive value (NPV) were 49.0% and 99.9%, respectively. Following discrepant analysis, the PPV increased to 73.5% and the low PPV (8.1%) of the 86 non-KPC improved to 48.8%. Among the 105 discrepancies, NDM was the most frequently observed (N=56), followed by KPC (N=26), VIM (N=10), IMP (N=8), OXA-48 (N=5). The threshold cycle values between discordant vs. concordant and resolved groups were significantly different (P<0.001).

Conclusions: Carba-R v.2 is a rapid and sensitive method for detecting carbapenemase-encoding genes compared with culture-based conventional PCR. Most of our discrepant results were non-KPC genes. Thus, the clinical significance of the non-KPC positive cases detected by Carba-R v.2 should be investigated. This assay would be useful for deciding whether to isolate pre-exposed patients in hospital settings, based on the high specificity and NPV.