Patients hospitalized for sepsis or septic shock that are treated with Piperacillin-Tazobactam or Meropenem and meet all the inclusion criteria and none of the exclusion criteria.

The plasmatic levels determinations will always be made in state of equilibrium (after the fourth dose) and two samples will be collected:

1. Just before the antibiotic infusion (T0)
2. In the midpoint of time (T50). If the antibiotic is administered every 8 hours this determination will be made at 4 hours. If the antibiotic is administered every 6 hours, the determination will be made at 3 hours.

A second complete determination will be made (before the infusion, in the midpoint of time and before the next dose) after the administration of 4 doses as long as a posology modification is made as a consequence of the information received from the plasmatic levels.

The chosen efficiency parameter is that 100 % of the time the concentration remains 4 times above the minimum inhibitory concentration (MIC), the exact minimum inhibitory concentration (MIC) of the used antibiotic will be determined by E-test by the Microbiology Service which means that the seric concentrations asociated with the maximum therapeutic efficacy are:

• For Meropenem 2mg/dl, which means reaching a concentration in T0 >8 mg/dL.
• For Piperacillin-Tazobactam 16 mg/dL, which means reaching a concentration in T0 >64 mg/dL.

The seric concentrations asociated with the maximum therapeutic efficacy are the same as the described for the description of the Primary Outcome Measure and it´s considered that a patient has renal hyper clearance when they have a creatinine clearance above 130 ml/min.