Colistin terminology

At the First International Conference on Polymyxins, held in Prato, Italy, from 2 to 4 May 2013, the terminology of colistin was one of the important issues raised as requiring urgent attention (R. Nation et al 2013). Colistin is administered parenterally as its inactive prodrug, colistin methanesulfonate (CMS); the prodrug is also known as colistimethate. Two primary conventions are used in different global regions to describe the contents of parenteral vials and corresponding doses for CMS products. Both conventions rely on in vitro microbiological standardization assays. The conventions are as follows:

  1. Number of international units (IU). This convention is used in Europe, India, and a relatively small number of other global regions. Summary of product characteristics/product information documents for some of the many (generic) brands available in these areas have introduced mention of vial contents and/or dose in terms of the number of milligrams of the chemical CMS, in addition to number of IU.

  2. Number of milligrams of colistin base activity (CBA). This convention is used in the remaining global regions where parenteral colistin is available, including North and South America, Southeast Asia, and Australia.

One million IU is equivalent to approximately 30 mg of CBA. This corresponds to approximately 80 mg of the chemical CMS. Thus, a given number of milligrams of CBA corresponds to approximately 2.7 times (ie, 80 รท 30) that number of milligrams of CMS. Clearly, the existence of 2 ways of describing a colistin dose in milligrams (ie, milligrams of CBA and milligrams of the chemical CMS) creates significant potential for errors, thereby jeopardizing patient safety.

Read more on reporting of CMS doses: R. Nation et al 2013

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