Urinary Biomarkers TIMP-2 and IGFBP7 Early Predict Acute Kidney Injury after Major Surgery

Objective To assess the ability of the urinary biomarkers IGFBP7 (insulin-like growth factor-binding protein 7) and TIMP-2 (tissue inhibitor of metalloproteinase 2) to early predict acute kidney injury (AKI) in high-risk surgical patients. Introduction Postoperative AKI is associated with an increase in short and long-term mortality. Using IGFBP7 and TIMP-2 for early detection of cellular kidney injury, thus allowing the early initiation of renal protection measures, may represent a new concept of evaluating renal function. Methods In this prospective study, urinary [TIMP-2]×[IGFBP7] was measured in surgical patients at high risk for AKI. A predefined cut-off value of [TIMP-2]×[IGFBP7] >0.3 was used for assessing diagnostic accuracy. Perioperative characteristics were evaluated, and ROC analyses as well as logistic regression models of risk assessment were calculated with and without a [TIMP-2]×[IGFBP7] test. Results 107 patients were included in the study, of whom 45 (42%) developed AKI. The highest median values of biomarker were detected in septic, transplant and patients after hepatic surgery (1.24 vs 0.45 vs 0.47 ng/l2/1000). The area under receiving operating characteristic curve (AUC) for the risk of any AKI was 0.85, for early use of RRT 0.83 and for 28-day mortality 0.77. In a multivariable model with established perioperative risk factors, the [TIMP-2]×[IGFBP7] test was the strongest predictor of AKI and significantly improved the risk assessment (p<0.001). Conclusions Urinary [TIMP-2]×[IGFBP7] test sufficiently detect patients with risk of AKI after major non-cardiac surgery. Due to its rapid responsiveness it extends the time frame for intervention to prevent development of AKI.


Introduction
Postoperative AKI is associated with an increase in short and long-term mortality. Using IGFBP7 and TIMP-2 for early detection of cellular kidney injury, thus allowing the early initiation of renal protection measures, may represent a new concept of evaluating renal function.

Methods
In this prospective study, urinary [TIMP-2]×[IGFBP7] was measured in surgical patients at high risk for AKI. A predefined cut-off value of [TIMP-2]×[IGFBP7] >0.3 was used for assessing diagnostic accuracy. Perioperative characteristics were evaluated, and ROC analyses as well as logistic regression models of risk assessment were calculated with and without a [TIMP-2]×[IGFBP7] test.

Results
107 patients were included in the study, of whom 45 (42%) developed AKI. The highest median values of biomarker were detected in septic, transplant and patients after hepatic surgery (1.24 vs 0.45 vs 0.47 ng/l 2 /1000). The area under receiving operating characteristic Introduction Postoperative acute kidney injury (AKI) is one of the most common postoperative complications and is associated with an increase in hospital mortality and decreased survival for up to 15 years after surgery [1][2][3][4][5]. The prognosis of patients with AKI is still poor, intervention for prevention and therapy of AKI are currently only initiated in the late phase of already established injury; therefore, benefits remain limited [6].
Using biomarkers for the early detection of cellular injury, thus allowing the early initiation of renal protection measures, may represent a new concept of evaluating renal function in critically ill patients [7,8]. Combination of two novel urinary cell-cycle arrest biomarkers, i.e. the insulin-like growth factor-binding protein 7 and the tissue inhibitor of metalloproteinase-2 ([TIMP-2]×[IGFBP7] panel) was described and validated in two multicenter studies for prediction of risk of moderate and severe AKI (AKI stage 2 and 3 according to KDIGO 2012 classification) in critically ill patients. Both TIMP2 and IGFBP7 are markers of cellular stress in the early phase of tubular cell injury caused by a wide variety of insults (inflammation, ischemia, oxidative stress, drugs, and toxins) [9][10][11][12]. Therefore, both markers are involved in the process of G1 cell-cycle arrest that prevents cells from dividing in the case of damage to the DNA until such damage can be repaired [13]. Importantly, both biomarkers appear as "alarm" proteins for other nearby cells in a paracrine fashion [14,15]. These two biomarkers performed better in prediction of AKI than NGAL (AUC 0.64), KIM-1 (AUC 0.69), IL-18 (AUC 0.76), L-FABP (AUC 0.66), or Cystatin C (AUC 0.63) [16,17]. In cardiac surgery patients act [TIMP-2]×[IGFBP7] as a sensitive predictor of AKI and may help to predict renal recovery after AKI using cutoff of 0.5 [18].
The aim of the current study was to evaluate, how the biomarker would perform in non-cardiac surgical patients, if assessed in very early phase after surgery by using a validated cutoff of > 0.3. Moreover, we investigated the performance of the [TIMP-2]×[IGFBP7] test across different severities of AKI (including or excluding stage 1). In addition to perioperative risk factors, we assessed the risk stratification in conjunction with bedside clinical parameters at the time of biomarker assessment. Finally we tested the relation between early cell cycle arrest in the kidney and outcome in surgical patients.

Methods
Patients 120 patients were evaluated between May 2013 and November 2013 in the multidisciplinary surgical ICU of a tertiary care university hospital; 13 patients were excluded because they had already developed AKI before biomarker assessment (based on an increase in SCr >0.3 after surgery) (Fig. 1). The study was approved by the local Institutional Review Board (Ethics Committee, University of Regensburg, no. 13-101-0191). Written consent was obtained from all patients or from their next of kin. All patients over the age of 18 years, who had received major non-cardiac surgery, were transported to the ICU immediately after the completion of surgery. Additionally, the patients had at least one risk factor for AKI, such as trauma, sepsis, critical  1 High Risk for AKI-major surgery and one additional risk factor-critical illness, sepsis, major trauma, chronic renal disease or use of radiocontrast agent. 2  illness, chronic renal disease, and use of an intravenous radiocontrast agent and were thus eligible for inclusion into the study. Critical illness was defined as requirement of inotropic support or mechanical ventilation at the time of admission to the ICU. Patients were excluded if they had end-stage renal disease or developed AKI during the period of time until biomarker assessment.

Measurements
Urine samples for biomarker assessment were taken from the urinary catheter of eligible patients soon after the transfer from the operating theatre to the ICU.

Baseline and clinical characteristics; ROC analysis
The baseline characteristics of the patients are shown in Table 1

Discussion
Our study evaluated the use of the novel urinary cell-cycle arrest biomarkers [TIMP-2]×[IGFBP7] in patients with a high risk of AKI (at least one additional risk factor according to the KDIGO recommendation [19]) after major non-cardiac surgery.
The mean findings of our study were: 1) In addition to good prediction for moderate and severe AKI, as showed in the validation study, we found that any AKI (including and excluding stage 1) can be predicted in non-cardiac surgery patients with a predefined cutoff of 0.3. 2) Compared to the Sapphire study 16 , in which biomarkers were tested up to 12 hours after admission, risk stratification in our study occurred very early after surgery within the mean time of 245 minutes. These results illustrate the rapid responsiveness of the test. 3) In the multivariable model with perioperative risk factors and bedside clinical parameters, [TIMP-2]×[IGFBP7] test was the strongest predictor and significantly improved the detection of surgical patients with an increased risk of AKI. 4) Early cell cycle arrest after surgery may be associated with adverse outcome. 5) Patients after hepatic surgery showed increased median values of biomarkers, and only one patient developed AKI within 48 hours after surgery. These findings suggest that the early correction of hypovolemia, which is routinely performed postoperatively in this group of patients, may reverse cell cycle arrest and prevent the development of AKI.
In the present study, higher median values of [TIMP-2]×[IGFBP7] after admission were associated with an increased degree of renal injury within 48 hours after surgery. Patients with AKI stage 1 had the lowest median values, and patients requiring RRT had the highest median [TIMP-2]×[IGFBP7] test results. This association was crucial and showed that the degree of early cellular damage was associated with the severity of the functional defect occurring at a later stage. Our results provide a new perspective on the performance of biomarkers in combination with perioperative parameters. Established risk factors for AKI are severity of the illness score, creatinine at admission, and age. In addition, we tested for the first time clinical parameters that help clinicians identify high-risk patients in the daily routine and that are associated with increased incidence or severity of AKI (urine output, hemoglobin level, mean arterial pressure, fluid balance, and use of vasopressors) [20][21][22][23][24]. In both models, use of the [TIMP-2]×[IGFBP7] test significantly improved AKI prediction. Moreover, the AUC for predicting AKI with urinary biomarker alone was higher than the prediction with perioperative risk factors in both clinical models (0.85 for biomarker alone vs. 0.72 for perioperative and 0.81 for  Table 3. Multivariable logistic regression model of perioperative parameters at the time of biomarker assessment with and without the (TIMP-2)* (IGFBP7) for risk assessment of AKI, AKI Stage 2 and 3, RRT and 28-day mortality. test is primarily an AKI risk assessment tool. However, these findings support recent data suggesting that early cell cycle risk arrest in the kidneys may be associated with adverse outcome [25].
Interestingly, in the subgroup of patients after major hepatic surgery, relatively high median values of biomarkers of cellular injury contrasted with only one consecutive case of AKI within 48 hours after surgery. Application of a low CVP strategy is an effective and recommended method to reduce blood loss during liver surgery [26]. However, restrictive volume anesthesia is associated with an increased risk of acute renal failure in a postoperative setting [27]. Therefore, patients after liver resection are given higher amount of fluids in the early postoperative period to prevent prolonged perioperative hypoperfusion. In our study, the hepatic surgery subgroup had an unproportionally high mean positive fluid balance within first 24 hours postoperatively if compared to patients after cancer surgery with very similar baseline characteristics (1419 vs. 939 ml/24h). This fact may indirectly indicate that the early correction of hypovolemia may reverse early cell cycle arrest and prevent development of AKI.
Our study has several limitations. We conducted the study in one single surgical ICU. However, we believe that we examined a representative cohort of non-cardiac surgical patients. Our results showed higher rates of AKI (>42%) as well as early use of RRT (>10%). The high incidence of AKI − that may influence the predictive performance of biomarkers − was due to the study design that only included patients with a high risk of AKI. The previous two multicenter studies, however, have shown the ability of the [TIMP-2]×[IGFBP7] test to provide risk stratification for moderate and severe AKI in over 1100 critically ill patients with a prevalence of AKI, which was similar to that described in other literature reports. Moreover, another recent study has shown a very similar incidence of postoperative AKI of 39% in the cohort of 50 314 patients after major surgery [28]. Finally, we did not evaluate the long-term outcome (>28 days) in patients with cellular injury detected with [TIMP-2]×[IGFBP7] test. These issues need to be investigated in future studies.
In summary, we showed that the [TIMP-2]×[IGFBP7] test significantly improves the early prediction of any type of AKI in high-risk surgical patients. Due to its rapid responsiveness, the test extends the therapeutic window for renal protection measurements or future therapeutic interventions to prevent the development of AKI. Together with the known risk factors and bedside clinical parameters, this test helps clinicians more accurately identify patients who should consequently receive renal protective measures and escalation of care. Particularly postoperative hemodynamic optimization seems to decrease the risk of renal impairment in surgical patients [29]. This process could be started very early after surgery in patients with cellular stress diagnosed by means of urinary biomarkers. Future studies are required to determine whether early and more accurate risk stratification is consistently associated with better outcome for surgical patients with AKI.