Approximately 25-35% of all children admitted to the paediatric intensive care unit (PICU) or neonatal intensive care unit (NICU) will develop Acute Kidney Injury (AKI) during the first seven days after admission. AKI is associated with a worse outcome, including an increased risk of mortality compared to patients without AKI. However, this AKI prevalence estimation is based on serum creatinine based glomerular filtration rate (eGFR), which is known to be inaccurate. The investigators postulate that measured GFR (mGFR) based on iohexol clearance in critically ill children will detect a higher prevalence of children with AKI than currently used methods based on endogenous markers. This study will additionally provide mechanistic knowledge on the relative contribution of GFR and renal transport to renal function in critically ill children.

Primary objective: To determine the prevalence of AKI in critically ill children based on clearance of iohexol.

Secondary objectives:

1. To determine the prevalence of AKI in critically ill children using serum creatinine, creatinine clearance, cystatin C and/or blood urea nitrogen based eGFR equations as well as urinary iohexol clearances.
2. To determine serum Proenkephalin (PENK) levels in critically ill children.
3. To compare the prevalence of AKI when this diagnosis is based on plasma iohexol clearances with the prevalence of AKI based on serum creatinine, creatinine clearance, serum cystatin C, PENK and/or Blood Urea Nitrogen (BUN) based eGFR and to assess agreement between those methods
4. To determine risk factors for the development of AKI when based on iohexol clearance.

Exploratory endpoint: To explore the relationship of genetic variation with the development of AKI.

Inclusion Criteria:

• 0-18 years of postnatal age
• >37 weeks of gestational age (for infants < one year postnatal age)
• Bodyweight >2500g
• Patients admitted to pediatric or neonatal intensive care unit
• PELOD-II (pediatric logistic organ dysfunction score, 2nd version) of 1 or higher (= at least one failing organ)
• Indwelling central line or arterial line in place for clinical purposes, or scheduled regular blood work for clinical reasons (at least once a day)
• Informed written consent

Exclusion Criteria:

• Known medical history of allergic reaction to injection of iodinated contrast material
• Receiving renal replacement therapy
• Language or cognitive inability of parents/caregivers to understand written and oral informed consent.

105 critically ill children admitted to the pediatric intensive care unit or neonatal intensive care unit from the Radboudumc with at least one failing organ.