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OBJECTIVES This study evaluated the association between the pharmacokinetic/pharmacodynamic index and treatment response to amikacin in critically ill patients. METHODS An observational prospective study was designed. Critically ill adult patients with infection due to amikacin-sensitive Gram-negative bacteria treated with amikacin were included. Amikacin maximum (Cmax) and minimum (Cmin) plasma concentration samples were taken during the first 48-96h after the beginning of treatment. The impact of Cmax/MIC ratio and area under the concentration-time curve (AUC)/MIC ratio on early and final clinical response, microbiological eradication, development of resistant strains and renal toxicity was analysed using a multivariate model. RESULTS A total of 85 patients received amikacin treatment, of whom 71 (83.5%) achieved a Cmax/MIC >6, 66 (77.6%) a Cmax/MIC >8, 64 (75.3%) a Cmax/MIC >10 and 72 (84.7%) an AUC/MIC >65. Clinical response at the end of treatment was significantly greater in patients with Cmax/MIC >6 [OR=5.48 (95% CI 1.28-11.40)], Cmax/MIC >8 [OR=6.01 (2.41-12.2)] and Cmax/MIC >10 [OR=8.02 (2.21-14.2)]. Cmax/MIC >10 was associated with a non-significant increase in microbiological eradication [OR=2.84 (0.76-10.61)]. Achieving Cmax/MIC >6 was associated with a lower proportion of patients with selection of resistant strains or with an increase in amikacin MIC (27.8% vs. 10.2%). Amikacin AUC was associated with development of nephrotoxicity [ROC curve 0.77 (0.66-0.87)]. CONCLUSIONS The Cmax/MIC ratio of amikacin in critically ill patients is directly related to the response to treatment and the selection of resistant strains.