The pharmacokinetics of flucloxacillin are expected to be different in ICU patients compared to non-ICU patients. The investigators will determine total and free flucloxacillin concentrations in 30 ICU patients, who will get continuous (n=10) or intermittent infusion (n=20) of flucloxacillin as standard care.

Full pharmacokinetic curves will be taken for individual patients on the intermittent dosing regimen and limited sampling will be taken for individual patients on the continuous dosing regimen on day 2 and 4.

The pharmacokinetics of flucloxacillin are expected to be different in ICU patients compared to non-ICU patients. There is a substantial risk that present standard dosing regimens of antibiotics lead to suboptimal outcomes for patients on the ICU. To prevent the risk of inadequate dosing in critically ill patients, it is important to optimize dosing regimens in ICU patients.

With this study the investigators will define pharmacokinetics of flucloxacillin in ICU patients and search for variables influencing pharmacokinetics. By using population modeling the investigators will simulate different dosing regimens, intermittent and continuous, and compare probability of target attainment between continuous and intermittent infusion.

To be able to include 30 patients within the study duration, a multi-centre approach is necessary.

Patients will receive standard care, as stated in the product characteristics or according to local protocols. Minimally invasive blood sampling for pharmacokinetic analysis will be retrieved through a central venous catheter or an arterial line. Full pharmacokinetic curves will be taken for individual patients on the intermittent dosing regimen and limited sampling will be taken for individual patients on the continuous dosing regimen on day 2 and 4.

Inclusion Criteria:

1. Patient is admitted to an ICU
2. Subject is at least 18 years old on the day of the first dosing
3. Is managed with a central venous catheter or arterial line
4. Patient is treated with flucloxacillin

Exclusion Criteria:

1. Has previously participated in this trial

All patients receiving flucloxacillin for treating a suspected or proven bacterial infection at the ICU will be included.