Urinary TIMP‐2 and IGFBP‐7 protein levels as early predictors of acute kidney injury after cardiac surgery

Acute kidney injury (AKI) is a frequent complication associated with on‐pump cardiac surgery. Early recognition may alter their prognosis. Therefore, the urinary concentrations of TIMP‐2 (tissue inhibitor of metalloproteinases‐2) and IGFBP7 (insulin‐like growth factor‐binding protein) as predictors for AKI were studied.


Acute kidney injury (AKI) is one of the most frequent complications
following open cardiac surgery involving cardiopulmonary bypass (CPB). 1 Based on current AKI definitions, the incidence is about 30% where 2.2% of these require dialysis postoperatively. 1,2 Even a slight postoperative rise in serum-creatinine is an independent predictor of mortality and it increases with the increase in stage of AKI. 1,3 From an economic point of view, AKI is associated with a prolonged stay in the hospital that results in increased costs. 4 The pathogenesis of AKI is complex and multifactorial, where some include toxic, ischemic, oxidative, and inflammatory causes. 5 Because serum-creatinine is a limited biomarker in the perioperative setting, so the use of urinary proteins like TIMP-2 (tissue inhibitor of metalloproteinase) and IGFBP7 (insulin-like growth factor-binding protein) has been promoted. 6 Both are highly involved in the G1 cell cycle which is a known mechanism in AKI. The Sapphire study identified these two proteins and their products to be superior to the variety of biomarkers to predict AKI. The Nephrocheck™ test, which calculates the product of the TIMP-2 and IGFBP7 concentrations, was also developed.
In this pilot study, we investigated the function of the Nephrocheck™ test under clinical conditions. Additionally, we tested the assumption of urinary TIMP-2 and IGFBP7 can predict AKI in adult cardiac surgery patients at an earlier stage in comparison to the conventionally used biomarkers such as creatinine, urea, and glomerular filtration rate.

| Study design
The ethics committee of the Philipps-University Marburg approved this observational pilot study (Az 178/13). The Standards for Reporting of Diagnostic accuracy (STARD) were applied. Fifty male patients requiring elective coronary artery bypass grafting and/or valve surgery with the use of CPB were screened for study admission. Exclusion criteria included those patients with a Cleveland-Score of more than 7, younger than 35 years, acute infection, emergency surgery, and end-stage renal disease. Written informed consents were received from all patients for enrollment.
Urinary samples were taken just after anesthetic induction, intraoperatively 30 and 60 min after clamping of the aortic arch and 0,

| Statistical analysis
The statistical analysis was performed and graphics were created using the Microsoft ® Excel ® add-in XLSTAT (2016.7) for Mac (macOS

| Descriptive statistic and risk profile for AKI
For this study, 50 male patients were approached for participation and none were excluded from the analysis. Among these, 36% of patients received isolated coronary artery bypass grafting, 20% isolated valve surgery, and 44% combined coronary and valvular surgical interventions. Table 1 summarizes the descriptive statistic for the entire patient collective and for the subgroups with and without AKI.
Besides, it shows p-values for subgroup differences. Patients with postoperative AKI had a worse median baseline creatinine of 1.11 mg/dl compared to those without AKI with a baseline of 0.89 mg/dl. Most patients showed arterial hypertension, coronary heart disease, or heart failure as a comorbidity. The prevalence of patients with chronic kidney disease or severely reduced left ventricular function was 6%. Both the Cleveland-and Leicester-score showed a higher risk for AKI for the subgroup with postoperative AKI. The median Leicester-score for the patients with AKI was 29% compared to 18% for those without. For the entire patient collective, the Leicester-score anticipated an AKI incidence of 21%. The actual rate of AKI was 28%. Postoperatively the length of stay in ICU was a median of 6 days for those with AKI compared to 4 for those without postoperative AKI. No significant subgroup differences could be shown for age, CPB, or cross-clamp time. No patient in this study suffered an in-hospital death or required dialysis. Table 2 shows the distribution of the Cleveland-score for the entire study population and for the subgroups in absolute and relative frequencies. In the group without AKI, 50% of the patients had a Cleveland-score of zero opposed to only 21.4% in the group with AKI. A patient without AKI and a Cleveland-score of 7 due to chronic kidney injury, congestive heart failure, insulindependent diabetes, and a combined surgical intervention stand out.
T A B L E 1 Baseline characteristics for the entire patient collective and for the subgroups with and without acute kidney injury Time in ICU, days 4 (3-6) 6 (5.25-12.5) 4 (3)(4)(5) .005 Note: The p-value is shown for subgroup differences.
T A B L E 2 Distribution of the Cleveland-scores for the total study population and the subgroups and its dilution quotients showed no significant subgroup differences.

| Analysis of urinary dilution
The median urine osmolality for the entire population was 374 osmol/kg at admission to ICU compared to 572 osmol/kg 24 h later.
This difference was significant with a p-value below .0001 and is shown in Figure 7. Similar differences could be shown for the subgroups with and without AKI.

| DISCUSSION
AKI is a common postoperative complication of on-pump cardiac surgery. 1 Even minimal increase in serum-creatinine is associated with worse outcome and an increase in costs. 4