Evaluation of the Carbapenem Detection Set™ for the detection and characterization of carbapenemase-producing Enterobacteriaceae

The objective of this study was to assess the performance of the Carbapenemase Detection Set™ (CDS; Mast Diagnostics) in association with i) the EUCAST meropenem screening cut-off and ii) the faropenem-temocillin algorithm (FTa) for the screening of carbapenemase-producing Enterobacteriaceae (CPE). A total of 200 well-characterized enterobacterial isolates with reduced susceptibility to at least one carbapenem including 63 non-CPEs and 137 CPEs belonging to different Ambler classes were initially screened for CPEs using i) the EUCAST meropenem cut-off (diameter <25 mm) and ii) the FTa. Highly suspected CPEs underwent further testing using the CDS, which is based on the inhibition zone diameters determination of combined disks (A: meropenem, B: meropenem + dipicolinic acid, C: meropenem + cloxacillin, and D: meropenem + boronic acid). With the FTa, 66.7% of the non-CPE isolates were correctly identified. Most OXA-48-like producers (90.5%) were detected with 98.6% specificity. The FTa discriminates CPE from non-CPE with 100% sensitivity, but complementary tests were still needed for 59 % (118/200) of the strains. The EUCAST cut-off led to 3 false-negative results (2 OXA-181 and 1 NMC-A producer) resulting in a sensitivity of 97.8% for the discrimination between CPE and non-CPE, and 75.5% (151/200) of the strains still required complementary test. The CDS reduced the number of isolates requiring additional tests from 59% to 22%, and from 75.5% to 38% for FTa and EUCAST cut-off, respectively. FTa possesses very good specificities for the detection and classification of Ambler class A and most class B carbapenemase-producers, except for IMP producers, which were almost not detected (10/11). In conclusion, the association of the CDS with the FTa presented only 22% of inconclusive results, while this number was 38% with the EUCAST meropenem CPE screening cut-off.