Screening for carbapenemase-producing Enterobacterales (CPE)-considering the practical implications of molecular results, the value of culture and deciding criteria for resistance

Background Carbapenemase-producing Enterobacterales (CPE) are of international concern. Screening for CPE can encompass single- or two-step approaches, using culture, PCR or a combination. Each approach has benefits, but none are without disadvantage.Objectives To reflect on the challenges and implications of PCR-positive/culture-negative CPE screening results and assess if PCR cycle threshold (Ct) value can be helpful in predicting positive culture results.Patients and methods Risk factor-based CPE screening swabs were tested using a two-step algorithm: PCR followed by culture of PCR-positive specimens. Data on all PCR-positive specimens between 1 August 2022 and 31 May 2024 were extracted. ORs were estimated using receiver operating characteristic (ROC) curves to compare Ct values and culture. Youden's index was calculated to establish the optimal Ct cut-off value. Compliance with the CPE screening pathway for newly identified CPE PCR-positive patients was assessed.Results Of 61 268 CPE screens, 292 were PCR positive (0.5%), with 298 genes identified. Of these, 81.5% were culture confirmed. ROC analysis showed an AUC of 0.82 and Youden's index yielding a Ct cut-off value of 33.7. Repeat CPE screens were obtained from 33 new PCR-positive, culture-negative inpatients. Further investigation was not possible for 17 new PCR-positive, culture-negative patients (11%).Conclusions Molecular platforms alone cannot detect species or antimicrobial resistance. A molecular-followed-by-culture algorithm can reduce workloads associated with culture, resulting in comprehensive data, including antimicrobial susceptibility results, to support informed clinical, policy and epidemiological decision-making.