9.2 Carbapenemase confirmation and identification

Meropenem is the best indicator antimicrobial to screen for carbapenemases in Gram-negatives. Resistance is implied if the MIC is raised or the inhibition zone size reduced.

It is then necessary to determine whether any resistance detected is due to a carbapenemase and not to a combination of resistance mechanisms. For example:

• ESBL or AmpC enzymes combined with decreased permeability as the result of alteration or down-regulation of porins, can also lead to carbapenem resistance
• OXA-48-like enzymes are another challenge. Organisms producing these can look susceptible to cephalosporins so it is necessary to rely on the meropenem as an indicator here. Most other carbapenemases hydrolyse all β-lactams; the OXA-48-like enzymes are unusual and easy to miss because of this unexpected sensitivity.