Comparative evaluation of rapid colorimetric methods for detecting carbapenemase enzymes in Gram-negative bacilli

Most carbapenem-resistance in Gram-negative bacilli (GNB) is mediated by carbapenemase enzymes. It is therefore important to evaluate methods for their detection.

Comparing Carba NP with Blue Carba test in Enterobacteriaceae and Pseudomonas aeruginosa, and Carb Acineto NP with Blue Carba test in Acinetobacter calcoaceticus baumannii complex (ACBC). Also, assessing modifications of test protocols for possible improvement of performance.

Total 184 carbapenem-resistant GNB were subjected to Carba NP, Blue Carba, and Carb Acineto NP tests as applicable. Variations were attempted in i) Purity of imipenem, and ii) carbapenemase induction by substrate exposure, to improve test performance. Isolates were also screened for carbapenemase genes by PCR.

Carba NP and Blue Carba tests performed equally well with a sensitivity of 91.8% in Enterobacteriaceae and 61.2% in P. aeruginosa. Carb Acineto NP and Blue Carba tests were equivalent with 84.6% sensitivity in ACBC. Imipenem purity did not affect test performance. Carbapenemase induction by substrate exposure increased sensitivity by 6.25% only in Enterobacteriaceae. All strains carried the blaNDM-1 gene; some were positive for blaVIM or blaOXA-48 genes too.

Our study is the first to demonstrate the equivalence of the Carba NP and Blue Carba tests in Enterobacteriaceae and P. aeruginosa. It is also the first to demonstrate the equivalence of the CarbAcineto NP and Blue Carba tests in ACBC. Minor modifications to test protocols did not improve performance, except for slightly improved sensitivity by enzyme induction in Enterobacteriaceae.