Evaluation of Six Commercial and Noncommercial Colistin Resistance Diagnostics.

Resistance to colistin, a last-reserve antibiotic used for treating drug-resistant infections, is rising globally. We compared six commercial and in-house diagnostics-ComASP colistin, CHROMagar COL-APSE, rapid polymyxin NP (Nordmann/Poirel) test, Sensititre, MicroScan, and Vitek 2-against ISO-standard broth microdilution (BMD) using 142 Gram-negative isolates. The isolates (Enterobacterales = 110, Acinetobacter baumannii = 21, Pseudomonas aeruginosa = 11) underwent BMD and conventional multiplex PCR screening for mcr-1-mcr-5. Sensitivity, specificity, categorical agreement (CA), major error (ME), and very major error (VME) were calculated for each test. Vitek 2 and sensititre yielded the highest CA (≥ 98%) and the lowest VME (≤ 0.0%) across taxa. ComASP showed excellent performance for A. baumannii (100% sensitivity/specificity) but slightly lower CA for P. aeruginosa (73%). CHROMagar COL-APSE demonstrated acceptable sensitivity (92%) but low specificity (69%) in Enterobacterales. MicroScan had reduced specificity in Enterobacterales (87.80%). The CHROMAgar COL-APSE efficiently identified the species with their unique colours but was the least specific (68.63%), with the highest ME in Enterobacterales. The rapid NP test provided rapid results within 4 h but showed a relatively high VME (7.84%), despite maintaining an acceptable sensitivity (92.16%) and specificity (96.08%). For laboratories with automated platforms, Vitek 2 remains optimal for colistin MIC testing; Sensititre and ComASP are suitable low-cost BMD alternatives. The Rapid NP test provides a same-day screen, but confirmatory MIC testing is advised. CHROMagar COL-APSE should be used with a ≤ 1 CFU mL⁻¹ inoculum to minimise false resistance calls. Knowing the comparative performance of these different tests will assist in choosing the best test for every species, improving on efficient diagnosis and healthcare outcomes.