Each patient received 4.5 g every 6 h of piperacillin/tazobactam within 24 h of severe sepsis or septic shock, blood samples (approximately 3 ml) will be obtained by direct venipuncture at the following time: 0, 0-0.5, 0.5-2, 2-4 and 4-6 h after piperacillin/tazobactam therapy.