Another question remaining to be solved is the optimal doses of colistin for the treatment of these infections.Current recommendations are between 3 and 9 MU/day, in two or three daily administrations, based on previous pharmacokinetic and pharmacodynamic (pK/pD)
studies conducted in patients with cystic fibrosis [8]. In arecent study in patients with severe MDR-GNB infections without cystic fibrosis, the use of a 9 MU/day dose was an
independent factor for positive outcome compared to doses of 3 and 6 MU/day [6]. Recent pK/pD data in critically ill patients recommend the administration of aloading dose, from 9 to 12 MU, followed by doses of 4.5 MU every 12 hours in order to rapidly reach therapeutic
concentrations [20].