Can we use imipenem and meropenem Vitek 2 MICs for detection of suspected KPC and other-carbapenemase producers among species of Enterobacteriaceae?

Abstract

Imipenem and meropenem Vitek 2 MICs were evaluated for a panel of 104 Enterobacteriaceae for identification of carbapenemase producers. The sensitivity and specificity values for the new CLSI interpretative criteria (CLSI document M100-S20-U, 2010) were 98% and 83% for imipenem and 76% and 83% for meropenem, respectively. We propose an algorithm that is highly sensitive (98%) and specific (94%) for carbapenemase screening based on the combined use of imipenem and meropenem MICs.

Fig. 1.

Scattergrams showing numbers of isolates at the indicated carbapenem MICs (averages of duplicate values, in μg/ml, of imipenem [IPM] and meropenem [MEM]) obtained by Vitek 2 for carbapenemase producers and carbapenemase nonproducers. (a) The sensitivity (SN), specificity (SP), positive predictive value (PPV), and negative predictive value (NPV) were calculated for carbapenemase screening using the imipenem M100-S20-U CLSI nonsusceptibility breakpoint (a MIC of ≥2 μg/ml) (members of the Proteeae tribe were excluded from the group of carbapenemase nonproducers for the calculation of SP, PPV, and NPV). (b) Meropenem M100-S20-U CLSI nonsusceptibility breakpoint (a MIC of ≥2 μg/ml [dashed lines]). (c) Cutoff values recommended in this work for a combination of imipenem (MIC ≥ 2 μg/ml) and meropenem (MIC ≥ 1 μg/ml). The mechanisms of resistance are indicated for undetected carbapenemases or isolates with interfering mechanisms. Prot, Proteeae; CTX-M, production of the CTX-M-2 ESBL plus porin loss; AmpC, derepressed expression of chromosomal cephalosporinase plus porin loss.

Fig. 2.

Proposed flowchart for screening Enterobacteriaceae suspected of producing carbapenemases with Vitek 2. IPM, imipenem; MEM, meropenem; R, resistant; I, intermediate; S, susceptible (as defined by CLSI document M100-S20-U [8]); APB, aminophenylboronic acid; DPA, dipicolinic acid.