Tubular and not glomerular damage affects cardiac surgery associated acute kidney injury

Introduction Cardiac surgery associated acute kidney injury (CSA-AKI) is a major complication associated with a high mortality.(1) Early prediction of AKI using biomarkers has become increasingly popular in recent years.(2) The localisation of the damage in the kidney has not yet been investigated. Hence, we aim to differentiate whether the damage is glomerular or tubular using two brand new biomarkers: Podocin(3), a biomarker that selectively reflects glomerular damage, and Nephrin, which reflects both glomerular and tubular damage.(4) Methods In this prospective cohort study, we analysed 63 patients who underwent elective cardiac surgery on cardiopulmonary bypass (CPB) at the Medical University of Vienna. Urine samples were collected at the beginning of the operation, 30 min after start of CPB, at the end of surgery and on postoperative day 1 (POD1). Quantitative measurement of Podocin as well as Nephrin was performed by an enzyme-linked immunosorbent assay (ELISA) with a 96-well plate human Podocin ELISA (Human Podocin ELISA Kit, Abcam, Cambridge, UK) and human Nephrin ELISA (Nephrin ELISA Kit, Lifespan Biosciences, Seattle, USA). Patients were grouped in two groups: patients without postoperative AKI and with AKI (noAKI, AKI) according to KDIGO-criteria.(5) Differences between groups were analysed using a student's t-test. Results We report 63 patients (25 female), with a mean age of 67.1±11,6 years. In 12.7% (n=8) CABG, in 57.1% (n=36) a valve, in 28.6% (n=18) a combined procedure and in one case (1.6%) a Bentall procedure was performed. According to the KDIGO-criteria 5, 19 patients (30.2%) suffered from acute kidney injury, none of them needed renal replacement therapy. The time on CPB was not significantly different (150.75±55.91min for noAKI vs. 153.47±58.27min for AKI, p=0.8642). The SAPS3-Score tended to be higher in patients with AKI (45.74±10.76) than in patients without AKI (42.53±7.63), but the difference was not significant (p=0.2513). The baseline of Podocin was 25.57±76.10pg/ml vs. 12.99±8.20pg/ml (noAKI vs. AKI). For Podocin, we did not find any significant difference between groups at any timepoint. The baseline of Nephrin was 31146.8±16602.52 pg/ml vs. 34786.3±24489 pg/ml (noAKI vs. AKI). For Nephrin, there were no significant differences between the two groups at baseline, 30min after start of the CPB or on POD1, but we found a significant difference at the end of the surgery (32356.6±34226,7 pg/ml for noAKI vs. 54120.5±33725.2pg/ml for AKI; p=0.02515). Discussion As there is only a difference in Nephrin-levels but not in Podocin-levels between patients with AKI and without AKI, our data suggests that patients with CSA-AKI have a damage to the renal tubule and not to the glomerulum.