Background. Resistance to colistin, a last-reserve antibiotic used for treating drug-resistant infections, is increasing globally. This study evaluated six diagnostic tests designed to detect colistin-resistant pathogens. Methods. PCR and broth microdilution assays (BMD) were used to respectively characterize the molecular mechanisms and phenotypic colistin resistance of 142 Gram-negative bacterial isolates and controls. The sensitivity, specificity, positive- and negative-predictive values, major (ME) and very major errors (VME), categorical and essential agreements (EA) of ComASP Colistin, CHROMagar COL-APSE, Rapid NP Test, Sensititre, MicroScan, and Vitek 2 were determined with these isolates; the BMD was used as gold standard. Results. The Vitek 2, Sensititre, and ComASP tests were more efficient, albeit with concerning ME and VMEs and low EAs. Sensititre was 100% specific with 0% ME and 3.61% VME; Vitek 2 had the least VME (1.25% and 0%) and a low EA (57.50%). ComASP had an EA of 75.35%. MicroScan was highly sensitive (96.55%) but less specific (87.50%), with very below-accetable EAs (48.11%). The CHROMAgar COL-APSE efficiently identified the species with their unique colours but was the least specific (67.80%), with the highest ME (32.20%) and high VME (7.23%). The Rapid NP test had the highest VME (7.84%), producing results within 4 hours with 92.16% sensitivity and 96.08% specificity. Conclusion: Vitek 2, MicroScan, ComASP colistin, and Sensititre are good for determining colistin resistance; the latter two tests are recommendable for low-resourced laboratories. The in-house Rapid NP test has short turnaround time with high efficiency for initial resistance screening.

The authors have declared no competing interest.

This work was funded by a grant from the National Health Laboratory Service (NHLS) given to Dr. John Osei Sekyere under grant number GRANT004 94807 and GRANT004 94808.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The study was approved by the Research Ethics Committee, Faculty of Health Sciences, University of Pretoria, under reference number 550/2020. The study complied with the ICH-GCP guidelines and the Helsinki declaration.

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Yes

All data produced in the present work are contained in the manuscript