Optimal understanding of ciprofloxacin pharmacokinetics in critically ill patients is lacking resulting in large variation of achieved exposure and possible inadequate therapy. The investigators hypothesize that drug dosing based on CKD-EPIcr-cys provides a useful method to individualize and optimize therapy for ciprofloxacin and eventually improve outcome.

In a multi-centre, observational, open-label study the investigators aim to define : the model for estimation of renal function that most accurately predicts ciprofloxacin clearance in critically ill patients.

Correct estimation of glomerular filtration rate (GFR) is necessary in critically ill patients in order to asses renal function. GFR is subsequently used to derive and appropriate drug dosing of renally excreted drugs and warrant adequate dose adaptations.

It is known that estimation of GFR based on creatinine clearance is not precise, especially in populations with altered muscle mass or instable renal function, such as the Intensive Care Unit (ICU) population.

The use of combined filtration markers together, cystatin C and creatinine, can improve precision in estimating GFR (eGFR). Studies confirmed that eGFR based on both creatinine and cystatin C is more precise than eGFR creatinine or eGFR cystatin C. The equation based on both creatinine and cystatin C, the Chronic Kidney Disease Epidemiology Collaboration creatinine-cystatin C (CKD-EPIcr-cys), may therefore improve eGFR and thus drug dosing in ICU patients, a population that does not reach PK/PD targets frequently.

So far little is known about drug dosing based on CKD-EPIcr-cys. Currently optimal understanding of ciprofloxacin pharmacokinetics in critically ill patients is lacking, resulting in large variation of achieved exposure and possible inadequate therapy. The investigators hypothesize that drug dosing based on CKD-EPIcr-cys provides a useful method to individualize and optimize therapy for ciprofloxacin and eventually improve outcome.

In a multi-centre, observational, open-label study the investigators aim to define the model for estimation of renal function that most accurately predicts ciprofloxacin clearance in critically ill patients.

Inclusion Criteria:

1. Patient is admitted to an ICU
2. Subject is at least 18 years on the day of the first dosing
3. Is managed with an arterial line or central venous catheter
4. Is managed with an urinary catheter
5. Is already treated with ciprofloxacin as part of routine clinical care

Exclusion Criteria:

1. Has previously participated in this study
2. Is on renal replacement therapy (RRT)

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