Evaluation of phenotypic methods for the detection of carbapenemases applicable to low complexity laboratories

The spread of carbapenemase-producing gram-negative bacilli is a global public health problem. Several authors have proposed phenotypic assays to presumptively detect these enzymes applicable to low and medium complexity laboratories. In the present study, we have developed and compared different phenotypic techniques using strains genetically identified as carbapenemase-producing. All the tested methods detected the presence of carbapenemases. The carbapenem inactivation method (MIC) and the modified carbapenem inactivation method with and without EDTA (mMIC-eMIC) were the simplest and easiest to interpret but their disadvantage was on the time required to obtain results. The direct Carba NP and Carba-Blue colourimetric methods were the fastest but they depend on reagent preparation and accurate pH adjustment of the solutions. Synergy methods with EDTA discs, boronic acid and the Triton Hodge Test (THT) require technical expertise to evaluate true synergism. Whereas, the Disk Carbapenemase Test (DCT) was the method that presented the greatest technical difficulties.