 The investigators aim was to determined the pharmacokinetic profile of piperacillin 4g every 8 hour in 22 patients treated empirically for sepsis and severe sepsis. A PK population model was be established with the dual purpose to assess current standard treatment and to simulate alternative dosing regimens and modes of administration. Time above the minimal inhibitory concentration TMIC predicted for each patient was evaluated against clinical breakpoint MIC for Pseudomonas Aeruginosa 16 mgL. Pharmacokinetic-pharmacodynamic PK-PD targets evaluated were 100 f TMIC and 50 fTMIC. 