Phase II clinical trial, open-labelled, prospective and single-center study directed to obtain blood samples in experimental detailed conditions in order to compare and optimize the dose of colistin in critically ill patients suffering from infections on which the indication of colistin would be accepted according to normal local protocols for severe infections treatment.

Inclusion Criteria:

• More than 60 Kg of weigh
• Patients with directed treatment with colistin as the recommended antimicrobial treatment protocols in the hospital to treat some of the following serious infections caused by carbapenems resistant A. baumannii: (i) bacteremia; (ii) nosocomial pneumonia or (iii) infection of skin and soft tissue (cellulitis, abscesses or infected ulcers).
• Written informed consent form.

Exclusion Criteria:

• Refractory shock or other illness with an expectative of life ˂ 48 hours after the recruitment;
• Patient declared not to resuscitation maneuvers;
• Suspicion or demonstration of endocarditis, osteomyelitis, or meningitis;
• Known hypersensitivity to polymyxins;
• Pregnancy.