Abstract:
Colistin has become a critical antibiotic for fatal Gram-negative infections owing to the proliferation of
multidrug-resistant carbapenemase-producing bacteria. Thus, cheaper, faster, efficient and easier-touse
colistin diagnostics are required for clinical surveillance, diagnoses and therapeutics. The sensitivity,
specificity, major error (ME), very major error (VME), categorial agreement, essential agreement,
turnaround time (TAT), average cost, and required skill for four colistin resistance diagnostics viz.,
CHROMagar COL-APSE, ComASP Colistin, MicroScan, and Colistin MAC Test (CMT) were evaluated
against broth microdilution (BMD) using 84 Gram-negative bacterial isolates. A multiplex PCR (M-PCR)
was used to screen all isolates to detect the presence of the mcr-1 to mcr-5 genes. A 15-point grading
scale was used to grade the tests under skill, ease, processing time etc. mcr-1 was detected by both
M-PCR and CMT in a single E. coli isolate, with other PCR amplicons suggestive of mcr-2, -3 and -4 genes
being also observed on the gel. The sensitivity and specificity of CHROMagar COL-APSE, MicroScan, and
ComASP Colistin, were 82.05% and 66.67%, 92.31% and 76.92%, and 100% and 88.89% respectively.
The MicroScan was the most expensive at a cost (per sampe tested) of R221.6 ($15.0), followed
by CHROMagar COL-APSE (R118.3; $8.0), M-PCR (R75.1; $5.1), CMT (R20.1; $1.4) and ComASP
Colistin (R2.64; $0.2). CHROMagar was the easiest to perform, followed by ComASP Colistin, M-PCR,
MicroScan, CMT and BMD whilst M-PCR and MicroScan required higher skill. The ComASP Colistin
was the best performing diagnostic test, with low VME and ME, making it recommendable for routine
colistin sensitivity testing in clinical laboratories; particularly, in poorer settings. It is however limited by
a TAT of 18–24 hours.
