The urgent need for clear and accurate information on the polymyxins.

TO THE EDITOR—We read with interest the recent invited article on polymyxins (colistin and polymyxin B) by Kassamali et al [1] and agree that wisdom does not always come with age. As discussed at the recent international conference on poly-myxins in Prato, Italy [2], there is much to be done to improve the official phar-macopoeial monographs for the poly-myxins, an important goal supported by Kassamali et al [1]. However, as also stated at the conference [2], it is essential that clinicians be provided with accurate and unambiguous information to promote the optimal use of these important last-line antibiotics. Unfortunately, the article by Kassamali et al [1] adds to existing confusion and provides incorrect information in several very important areas. We shall comment on our most serious concerns. In both the text and Table 2, Kassamali et al [1] have provided very confusing information relating to the different labeling conventions fortheparenteral products of colistin methanesulfonate (CMS; the inactive prodrug of colistin [3]) used in various parts of the world and the corresponding daily dose ranges. The 2 conventions for CMS products used in different global regions are (1) number of international units (IU) and (2) milligrams of colistin base activity (CBA); 1 mg of CBA is equivalent to approximately 2.7 mg of the chemical CMS [4]. Both the IU and CBA conventions rely on in vi-tro microbiological standardization assays. It is extremely unhelpful, as in Table 2 of the invited article [1], to report dosages of CBA-labeled products not only in terms of milligrams of CBA per kilogram per day (ie, the preferred approach because the vials and doses are expressed as milligrams of CBA) but also as units of CBA per kilogram per day and milligrams of CMS per kilogram per day. The introduction of " units of CBA " clouds an already extremely confusing situation for clinicians. Indeed, even the current multiple ways of expressing dose have caused substantial confusion among clinicians and led to the issuing of an alert for " risk of serious or fatal medication error " arising from the CMS-to-CBA conversion factor of 2.7, mentioned above [5]. Doses for the CBA-labeled and IU-labeled products should be prescribed in terms of the respective labeling conventions , and authors and journals should cease reporting doses in milligrams of CMS [2] and should certainly not adopt " units of CBA. " It has been reported elsewhere that the …