Evaluation of an In-House Colistin NP Test for Use in Resource-Limited Settings.

Colistin has become increasingly important in the treatment of multidrug-resistant Gram-negative bacteria. Resistance to colistin has emerged globally, necessitating the need for an accurate method to detect colistin resistance. The colistin NP test has shown promise as a rapid screening assay for colistin resistance. This study compared the performance of an in-house-prepared colistin NP test against broth microdilution (BMD) as the gold standard and against Etest (bioMérieux, Marcy l'Etoile, France) as an alternative method. A total of 215 stored isolates were evaluated, of which 159 were resistant and 56 susceptible to colistin by BMD. The categorical agreement of the colistin NP test with BMD was found to be 98.1%, compared to 87.9% for the Etest. One major error was detected with both the colistin NP test and the Etest. Three very major errors were detected with the colistin NP test compared to 25 with the Etest. This resulted in a major error rate of 1.8% for both the colistin NP test and the Etest and a very major error rate of 1.9% and 15.7% for the colistin NP test and the Etest, respectively. The colistin NP test compared satisfactorily to the BMD reference method in determining colistin susceptibility. The colistin NP test is a rapid, inexpensive screening method for colistin resistance, especially in resource-limited settings.