Target Attainment of Continuous Infusion Flucloxacillin and Cefazolin Coupled With TDM vs. Standard of Care Treatment in Patients With Complicated S. Aureus Infection

Inclusion Criteria: * Informed Consent as documented by signature. For patients, who are not able to sign consent, a physician not involved in the current study has to confirm that patient's interest and rights are guaranteed during participation in the current study. Subsequently, informed consent will be obtained as soon as possible from the patient or his/her legally authorised representative. * Age ≥ 18 years * CSAI which is defined as (i) blood stream infection (BSI) with S. aureus or (ii) deep-seated infections caused by S. aureus (e.g. osteoarticular infections, deep-seated abscesses) without BSI. * Intended or active (less than 24 hours) treatment with FLU or CZO Exclusion Criteria: * Patients on hemodialysis or eGFR\<10 ml/min as these patients have a special pharmacokinetic * Patients on Cytosorb® therapy * Patients with liver cirrhosis CHILD B and C * Patients who are very likely to stop treatment with FLU or CZO in the next 48 hours as per treating physician (because of treatment failure, switch to oral medication, palliative care, allergy etc.) or who are very likely to be discharged or transferred to another hospital in the next 48 hours as per treating physician. * Polymicrobial infection except concomitant isolation of a likely contaminant (e.g. Staphylococcus epidermidis or Cutibacterium acnes). If an additional pathogen is identified after inclusion of the patient into the study, the patient will remain in the study. * CSAI caused by methicillin-resistant S. aureus (MRSA) * Participation in another study with investigational drug within the 30 days preceding and during the present study * Previous enrolment into the current study * Any uncontrolled or significant concurrent illness that would put the patient at a greater risk or limit compliance with the study requirements at the discretion of the investigator