Evaluation and readout optimization of the rapid polymyxin NP test for the detection of colistin-resistant Enterobacteriaceae

Purpose. In an increasing number of cases the last therapeutic option for treatment of carbapenem-resistant Enterobacteriaceae is colistin. As the detection of colistin resistance is problematic and time-consuming, it is desirable to find a rapid and reliable test. The rapid polymyxin NP test developed by Nordmann et al. addresses this problem and has a sensitivity of 99.3 % and a specificity of 95.4 %, as described by the authors. Methods. In this study, we aimed to evaluate the NP test and tested the effect of measuring the absorbance of the test with an enzyme-linked immunosorbent assay (ELISA) reader at 430 nm as an alternative objectified readout. We performed a study with 120 carbapenemase-producing Enterobacteriaceae isolates, including 40 colistin-resistant and 23 colistin-susceptible Klebsiella pneumoniae, 4 resistant and 23 susceptible Escherichia coli, and 20 susceptible and 10 resistant Enterobacter species, respectively. Results. Our data showed lower values for sensitivity and specificity than previously, namely only 91 % and 70 %, respectively, due to visual inspection. Furthermore, the results revealed a weakness in the correct detection of colistin-susceptible Enterobacter species. With the measurement of the absorbance we optimized the results to prevent misinterpretations of weak or inconclusive colour changes and enhanced the accuracy and objectivity of the rapid polymyxin NP test results. Conclusion. We reinforced the rapid polymyxin NP test as a rapid and valuable tool for detecting colistin-resistant K. pneumoniae isolates, although false-positive results were obtained for several colistin-susceptible Enterobacter spp. By using the optimized method, we were able to increase the sensitivity and specificity values to 94 % and 95 %, respectively.