Evaluation of the simplified Carbapenem Inactivation Method (sCIM) for detection of carbapenemase-producing Gram-negative bacilli

Abstract

Background: Currently, prompt phenotypic detection of carbapenemase-producing Enterobacterales by clinical laboratories is a challenge. Available techniques are expensive and strenuous. Results are delayed and the techniques are unsuitable for high- throughput laboratories.

Objective: This study evaluates the performance of the sCIM against the Modified Hodge and Imi-EDTA tests as an alternative method to detect carbapenemase enzymes in Gram-negative bacilli.

Methods: This is a prospective laboratory-based study, comprising 137 well-characterised stored isolates. These isolates include: 96 Enterobacterales, 23 Acinetobacter baumannii and 18 Pseudomonas aeruginosa tested with the sCIM. The performance of the sCIM was compared to the Modified Hodge and Imi-EDTA tests with the multiplex PCR used as a reference gold standard.

Results: Overall, we report 95.8% accuracy of the sCIM when testing Enterobacterales, 95.6% for Acinetobacter baumannii, and 77.8% for Pseudomonas aeruginosa using PCR as the gold standard. There was a significant correlation of 95.4% sensitivity and 100% specificity in testing both Enterobacterales and Acinetobacter baumannii isolates with the sCIM. Contrary to expectation, both Enterobacterales and Acinetobacter baumannii showed a low negative predictive value (NPV) of 69.8% and 50%, respectively, while the positive predictive value (PPV) was 100% for both. Pseudomonas aeruginosa isolates showed 40% sensitivity, 92.3% specificity, 66.6% PPV and 80% NPV.

Conclusion: Notwithstanding the lack of agreement with our low NPV, the sCIM demonstrated an acceptable performance, as described in previous studies. It is inexpensive, less tedious and suitable for use in high-throughput laboratories with reliable and consistent results.