Evaluation of the β-CARBA™ test, a colorimetric test for the rapid detection of carbapenemase activity in Gram-negative bacilli

Objectives: There is an urgent need for accurate and fast diagnostic tests to identify carbapenemase-producing bacteria. Here, we have evaluated a novel colorimetric test (the beta-CARBA (TM) test; Bio-Rad) to detect carbapenemase-producing Gram-negative bacilli from cultured colonies. Methods: The performance of the beta-CARBA (TM) test was compared with that of the Carba NP test (or the CarbAcineto NP test) and RAPIDEC (R) CARBA NP (bioMerieux) using a collection of 290 isolates with characterized beta-lactamase content. This collection included 199 carbapenemase producers (121 Enterobacteriaceae, 36 Pseudomonas and 42 Acinetobacter baumannii) and 91 non-carbapenemase producers (55 Enterobacteriaceae, 20 Pseudomonas and 16 A. baumannii). Results: The beta-CARBA (TM) test correctly detected 84.9% of the carbapenemase producers, including all KPC and IMP, 96.4% of VIM, 85.3% of NDM, 80.5% of OXA-48-like and 91.2% of A. baumannii-related OXA carbapene-mases (OXA-23, OXA-40, OXA-58, OXA-143 and overexpressed OXA-51). All rare metallo-beta-lactamases (SPM, AIM, GIM, DIM and SIM) were detected. Importantly, all non-KPC Ambler class A carbapenemases were not detected, including GES variants with carbapenemase activity (n = 6), IMI (n = 3), NMC-A (n = 1), SME (n = 2), FRI-1 (n = 1) and BIC-1 (n = 1). All non-carbapenemase producers gave a negative result except with OXA-163-, OXA-405- and one TEM-3-producing Citrobacter freundii. The overall sensitivity and specificity of the beta-CARBA (TM) test were 84.9% and 95.6%, respectively. This test is easy to perform and to interpret by non-specialized staff members. Conclusions: Despite lack of specificity towards non-KPC Ambler class A and OXA-48-like carbapenemases, the beta-CARBA (TM) test could complete the existing panel of tests available for the confirmation of carbapenemases in Gram-negatives.