A retrospective analysis of the performance of Vitek®2 compared to manual broth microdilution for colistin susceptibility testing of acinetobacter baumannii clinical isolates

Abstract

Studies comparing the performance of Vitek®2 against manual broth micro-dilution (mBMD) for colistin susceptibility testing have yielded conflicting results. Although the latter is the recommended reference method, it is not readily available. A retrospective analysis was performed to assess the performance of Vitek®2 against mBMD on colistin minimum inhibitory concentration determination in extensively-drug resistant A. baumannii isolates and to determine the epidemiology of these isolates at Charlotte Maxeke Johannesburg Academic Hospital. Vitek®2 performance as compared to mBMD was as follows: categorical agreement 89%, essential agreement 56%, major error rate 8% and very major error rate 55%. With these results Vitek®2 cannot be recommended as an alternative to mBMD for colistin susceptibility testing. Of the isolates analysed, 71% were from adult patients, 18% from neonates and the remainder (11%) from older paediatric patients. Differences in ward distribution between the age groups were noted. These require further investigation