The investigators seek to determine whether a reduced preoperative renal functional reserve predicts postoperative acute kidney injury in patients with normal estimated glomerular filtration rates undergoing elective cardiac surgery.

Although acute kidney injury (AKI) frequently complicates cardiac surgery, methods to determine AKI risk in patients without underlying kidney disease are lacking. Renal functional reserve (RFR) can be used to measure the capacity of the kidney to increase glomerular filtration rate under conditions of physiological stress and may serve as a functional marker that assesses susceptibility to injury.

The investigators seek to determine whether a reduced preoperative RFR predicts postoperative AKI in patients with normal estimated glomerular filtration rates undergoing elective cardiac surgery. All centres will measure RFR with creatinine clearance, except University Hospital Giessen where in addition iohexol plasma-clearance will be used.

Inclusion Criteria:

1. Subjects older than 18 years
2. Subjects undergoing elective cardiac surgery (with or without cardiopulmonary bypass)
3. Subjects with an estimated GFR ≥60 ml/min/1.73 m2 (CKD-Epidemiology Collaboration equation)
4. Subjects who signed informed consent forms

Exclusion Criteria:

1. Preexisting acute kidney injury (as determined by all available serum creatinine values from hospital and outpatient medical records within the previous 90 days)
2. Chronic kidney disease ≥ stage III (KDIGO)
3. Subjects undergoing transcatheter aortic valve implantation (TAVI)
4. Pregnancy
5. Solitary kidney
6. Diabetes mellitus type 1
7. Recent cardiac arrest (within last 3 months)
8. Liver failure or cirrhosis
9. Total parenteral nutrition
10. Hemoglobin <11 g/dl
11. Sepsis
12. History of malabsorption, chronic inflammatory bowel disease, short bowel, or pancreatic insufficiency
13. Transplant donor or recipient
14. Active autoimmune disease with renal involvement
15. Rhabdomyolysis
16. Prostate hypertrophy with International Prostate Symptom Score ≥20
17. Active neoplasm
18. Decompensated heart failure / inability to pause angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers minimum 2 days before protein load
19. Known iodine allergy (exclusion criteria only for those centres who use iohexol plasma clearance for determination of GFR)
20. TSH <0.3 µU/l (exclusion criteria only for those centres who use iohexol)
21. Subjects who received intravenous radiocontrast agents within the 72 hours before the protein load
22. Subjects who received NSAIDs within 48 hours before the protein load

Adults undergoing elective cardiac surgery (with or without cardiopulmonary bypass) with an estimated GFR ≥60 ml/min/1.73 m2 (CKD-Epidemiology Collaboration equation)

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