Iohexol for Measuring Renal Function

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.