Acute kidney injury (AKI) develops in up to 40% of patients after cardiac surgery. The soluble urokinase plasminogen activator receptor (suPAR) has been identified as a biomarker for incident chronic kidney disease (CKD). Proenkephalin (proENK) also has been shown to be a biomarker for renal dysfunction. We hypothesized that pre-surgery suPAR and proENK levels might predict AKI in patients undergoing cardiac surgery. Consecutive patients (n = 107) undergoing elective cardiac surgery were studied prospectively. Clinical data, laboratory parameters, suPAR and proENK serum levels were assessed before operation, after operation and days one and four post-operatively. A total of 21 (19.6%) patients developed AKI within the first four days after elective surgery. Serum levels of suPAR and proENK, but not of creatinine, were significantly higher before surgery in these patients compared to those patients without AKI. This difference remained significant for suPAR, if patients with or without AKI were matched for risk factors (hypertension, diabetes, CKD). If cardiac surgery patients with pre-existing CKD (n = 10) were excluded, only pre-operative suPAR but not proENK serum levels remained significantly elevated in patients with subsequent AKI. Thus, our findings indicate that suPAR may be a predictive biomarker for AKI in the context of cardiac surgery, even in patients without underlying CKD.

Elective cardiac surgery has low procedural complications. However, about 40% of patients develop extracardiac complications including delirium and acute kidney injury. We hypothesized that inflammatory processes and immune cell activation might be associated with these complications. We therefore prospectively included 104 patients undergoing cardiac surgery in our study. We assessed peripheral blood leukocyte populations by flow cytometry and circulating cytokines before operation, after surgery and at days one and four post-operatively. Patients undergoing cardiac surgery showed significantly elevated leukocytes and neutrophils after surgery. On the contrary, monocytes decreased after surgery and significantly increased at days 1 and 4, particularly classical (Mon1,CD14++CD16−) and intermediate (Mon2,CD14++CD16+) monocytes. While peripheral leukocyte subsets were unaltered in patients with infectious (n = 15) or cardiac complications (n = 31), post-operative leukocytes (p = 0.0016), neutrophils (p = 0.0061) and Mon2 (p = 0.0007) were clearly raised in patients developing extracardiac complications (n = 35). Using multiple logistic regression analyses, patient's age, ICU days, number of blood transfusions and elevated post-surgery Mon2 independently predicted extracardiac complications. Our findings demonstrate that elevated Mon2 after cardiac surgery are associated with an increased risk for extracardiac complications. These findings might improve the risk estimation after cardiac operations and the role of Mon2 for inflammation in cardiac surgery.Despite advances in surgical and anesthetic techniques as well as enhanced postoperative care, cardiac surgery is still associated with a high risk of postoperative complications 1 . Besides typical cardiac (e.g., arrhythmia, myocardial infarction) and infectious complications (e.g., wound infection, pneumonia and sepsis), extracardiac non-infectious complications like acute kidney injury (AKI, 30%) and delirium (up to 26-52%) are frequent in patients after cardiac surgery 2,3 . It has been speculated that a large part of these complications could be explained by excessive inflammation caused by extracorporeal cardio-pulmonary bypass (CPB), hypothermia, myocardial ischemia and reperfusion and tissue damage due to the surgical procedure 4,5 . The influence of distinct immune cells and their subpopulations on the occurrence of post-operative complications is still ambiguous and needs further investigations. Particularly monocytes seems to play in important role within this context 6,7 .Circulating monocyte subsets represent a continuum of differentiation stages 8,9 . Using the surface markers CD14 (LPS receptor) and CD16 (FcγRIII), blood monocytes can be subdivided into three subpopulations, CD14++CD16− "classical" monocytes (Mon1), CD14++CD16+ "intermediate" (Mon2) and CD14-16+