 Colistin is administered intravenously as colistimethate sodium CMS, an inactive prodrug that lacks antibacterial activity, and is hydrolyzed into its active form in plasma as colistin A and colistin B. Prior pharmacokinetic studies have demonstrated a wide range of pharmacokineticpharmacodynamics PKPD findings for colistin. Variability in results have been attributed to an unreliable differentiation between colistin and CMS plasma concentrations with the use of microbiological assays, in addition to procedures that resulted in substantial in vitro hydrolysis of CMS to colistin during sample preparation. Refined laboratory methods such as high-performance liquid chromatography HPLC have circumvented such issues and provided opportunities for further pharmacokinetic profiling. 