Evaluation of the Direct MacConkey Method for Identification of Carbapenem-Resistant Gram-Negative Organisms from Rectal Swabs: Re-evaluating Zone Diameter Cutoffs

The optimal method to screen for gastrointestinal colonization with carbapenem-resistant organisms (CRO) has yet to be established. The Direct MacConkey plate method (Direct MAC) demonstrates high sensitivity for CRO detection, but established zone diameter (ZD) criteria for ertapenem (≤27 mm) and meropenem (≤32 mm) result in high rates of false-positives upon confirmatory testing. To increase specificity, we screened for CRO in two high-risk wards using the Direct MAC method, recorded ZDs for each sample, and generated receiver operating characteristic (ROC) curves to evaluate the optimal ZD cutoff criteria. Of 6,868 swabs obtained over an 18-month period, 4,766 (69%) had growth on MAC plates and 2,500 (36%) met criteria for further evaluation based on previously established ZDs around the carbapenem disks. A total of 812 (12%) swabs were confirmed positive for at least one CRO, including 213 (3 %) carbapenemase-producing organisms (CPO), resulting in a specificity of 78% for the Direct MAC plate. Reducing the ertapenem and meropenem ZDs to ≤25 mm improved specificity to 83%, decreasing the confirmatory testing workload by 32%. The sensitivity with the lower ZD criteria were 89% for CRO and 94% for CPO, respectively. The Direct MAC plate method criteria for CRO testing can be modified to balance the sensitivity and specificity of CRO while reducing the burden on clinical microbiology laboratories. These modifications can be particularly helpful in regions with a low CRO prevalence.