Doripenem (DRPM) is a broad-spectrum antibacterial agent often used as empirical therapy for critically ill patients, although there is a lack of studies validating the recommended dosage regimen for patients admitted to intensive care unit (ICU), based on pharmacokinetic (PK)/pharmacodynamic (PD) index. In this study, we estimated the free time above minimum inhibitory concentration (fT>MIC (%)) of DRPM using population PK analysis of 12 patients in ICU, and evaluated the validity of the dosage regimen stratified by creatinine clearance. Using a 2-compartment population PK model reported previously, the mean total clearance or distribution volume of DRPM estimated by Bayesian estimation was significantly lower or higher than that of based on population PK model. The estimated fT>MIC (%) of the recommended standard (normal renal function: 0.5 g every 8 h, moderate: 0.25 g every 8 h, severe renal impairment: 0.25 g every 12 h) and higher doses (normal: 1.0 g every 8 h, moderate: 0.5 g every 8 h, severe: 0.25 g every 8 h) against MICs of 0.5, 1 and 2 µg/mL exceeded 40% in all patients. When stratified by creatinine clearance, the PK/PD breakpoints estimated by Monte Carlo simulation in three grades of renal function tended to be higher than the previously reported PK/PD breakpoints for patients with urinary tract infection, an infection of lesser severity than ICU patients. These results suggest that the dosage regimen stratified by renal function derived from Japanese package insert may be sufficient to achieve effective treatment in ICU patients.Key words doripenem; pharmacokinetic/pharmacodynamic analysis; intensive care unit; breakpoint Doripenem (DRPM) is a newer carbapenem antimicrobial agent with potent and broad spectrum activities against Grampositive, Gram-negative and anaerobic bacteria, and is more potent against Pseudomonas aeruginosa than other carbapenems.1-3) Doses of 0.5-3.0 g daily in two or three divided doses administered by 0.5 or 1 h infusion are used commonly to treat infectious diseases such as pneumonia, complicated urinary tract infection and intra-abdominal infections. In order to provide optimal antimicrobial therapy, selecting an effective and well-tolerated dosage regimen based on pharmacokinetic (PK)/ pharmacodynamic (PD) index is important. For DRPM and other β-lactams with time-dependent antimicrobial activity, an effective dosage regimen requires that the blood concentrations exceed the minimum inhibitory concentration (MIC) of the infecting bacteria for at least 40% of the dosing interval.4) Therefore, the appropriate dosage regimen based on time above MIC (%T>MIC) is of great importance when using DRPM.Critically ill patients in the intensive care unit (ICU) are often affected by infections, and are given antimicrobial agents with a broad spectrum. 5) In particular, since severe sepsis and septic shock are major causes of morbidity and mortality in the ICU, early use of carbapenems including DRPM as an empirical therapy is recommended by international guidelines for the manageme...