Urinary [TIMP-2]*[IGFBP7] for early prediction of acute kidney injury after coronary artery bypass surgery

Acute kidney injury (AKI) is a common and serious complication after coronary artery bypass grafting (CABG). According to current definitions, AKI occurs in up to 40 % of all patients undergoing cardiac surgery [1]. AKI requiring renal replacement therapy has an incidence of approximately 1–5 % [1]. The development of a postoperative AKI is associated with a more complicated clinical course and with an increased mortality of 15–30 %. Moreover, even slight changes of renal function are considered as an independent predictor of 30-day mortality [2]. Full recovery of renal function is only observed in 50 % of surviving patients [3, 4]. Thus, due to its significance, the term cardiac surgery-associated acute kidney injury (CSA AKI) was established. Risk assessment for AKI is recommended by clinical practice guidelines but remains imprecise mainly due to very limited sensitivity and specificity of early diagnostic tests available for AKI. Although serum creatinine is known to be an inadequate and delayed marker of acute changes in renal function, it is currently accepted as “gold standard” to diagnose AKI due to the lack of other reliable biomarkers. Obviously, there is a clear need for sensitive and specific biomarkers allowing early identification of patients with high risk for AKI to initiate preventive or therapeutic interventions [5–7].

Tissue inhibitor of metalloproteinases-2 (TIMP-2) and insulin-like growth factor binding protein 7 (IFGBP7) have recently been suggested as promising tools for the early detection of AKI in critically ill patients [8]. Both proteins are inducers of the G1 cell cycle arrest, considered as a key mechanism of AKI. In the Sapphire study enrolling 744 critically ill medical and surgical subjects with a moderate–severe AKI occurring in 14 % of patients, the combination of TIMP-2 and IGFBP7 was significantly superior to all other existing markers of AKI with an AUC of 0.80 and also better than TIMP–2 (AUC 0.76) and IGFBP7 (AUC 0.79) alone [9]. Others could confirm these findings showing that urinary [TIMP-2]*[IGFBP7] greater than 0.3 (ng/ml)²/1000 is able to identify patients at risk for imminent AKI [10]. In addition, urinary [TIMP-2]*[IGFBP7] is able to predict AKI and renal recovery early after cardiac surgery in a subgroup of patients with a high risk for the development of AKI defined by a high Cleveland Foundation Score [11]. Therefore, it was the aim of the present study to analyze the postoperative course of [TIMP 2]*[IGFBP7] to evaluate a new diagnostic approach for early detection of AKI in patients after isolated CABG.

In critically ill patients, cardiac surgery with the use of CPB is the second most common cause of AKI after sepsis, with approximately 1–5 % of patients requiring RRT [16]. AKI in the cardiothoracic population is associated with a significant increase of morbidity and mortality, as well as prolonged length of ICU and hospital stay [1].

In the absence of other appropriate alternatives, the use of serum creatinine as standard diagnostic test for acute kidney injury is commonly accepted among intensivists, despite its known major limitation: Renal function must be impaired more than 50 % before an elevation in serum creatinine can be detected [17]. In addition, serum creatinine does not accurately depict kidney function and rapidly changing glomerular fraction rate (GFR) until a steady state has been reached [17]. Among other shortcomings, serum creatinine levels do not only dependent on excretion through glomerular filtration, but also on production mainly by muscle cells and secretion via tubular cells. Because of this, conditions that affect muscle mass and tubular secretion (such as age, gender, race, and level of GFR) may influence the baseline level and the degree of rise in serum creatinine [17]. Moreover, other specific conditions of cardiac surgery with extracorporeal circulation, e.g., the CPB induced hemodilution with a decrease in serum creatinine concentration cumber the creatinine-based diagnosis of AKI [18]. Experimental studies indicate that specific preventive or therapeutic strategies might be able to reduce AKI-associated morbidity and mortality without showing comparable positive results in the clinical setting [19]. This discrepancy might at least partly be explained by the inability to identify AKI at a very early stage. Although Cystatin C and NGAL seem to be reliable in selected patient cohorts and automated assay methods are commercially available, the value of these two promissing biomarkers in the early diagnosis of AKI remains controversial [20–22].

Taken together, there is no biomarker available for early and reliable detection of AKI, in particular after cardiac surgery, with adequate sensitivity and specificity. Recent studies focused on combinations of biomarkers including urinary insulin-like growth factor-binding protein 7 (IGFBP-7) and tissue inhibitor of metalloproteinases-2 (TIMP-2). This takes into account different pathophysiological mechanisms and, thus, different time courses of biomarker release.

IGFBP7 and TIMP-2 are inducers of the G1 cell cycle arrest found in renal tubular cells being considered as a key mechanism of AKI. They have recently emerged as potential novel markers for risk stratification and early identification of kidney stress. In the multicenter observational Discovery study, a total of 300 potential biomarkers for the detection of AKI were evaluated in a heterogeneous group of 522 patients at high risk of AKI [9]. The results for IGFBP7 and TIMP-2 were better than for any other investigated biomarker (including NGAL, KIM-1, and IL-18) with increased sensitivity and specificity when used as combination compared to its single use (AUC of 0.80 vs. 0.76 and 0.79 alone). Whereas neither TIMP-2 nor IGFBP7 was elevated in patients without AKI being unaffected by important co-morbidities such as chronic kidney disease, diabetes, and sepsis, patients with a [TIMP-2]*[IGFBP7] greater than 0.3 had seven times the risk for AKI.

A follow-up trial—the Opal-trial—using identical enrolment criteria replicated the results of the Discovery study confirming TIMP-2 and IGFBP7 to have better performance than existing markers. Sensitivity and specificity of two cut-off values demonstrated the suitability of these cell cycle arrest markers for clinical application [23].

However, etiologies and pathophysiology of AKI differ substantially between patient populations and, therefore, the performance of every biomarker in different patient cohorts and clinical settings may vary significantly requiring careful and specific evaluation of the marker in different scenarios. This also applies for G1 cell cycle arrest marker: IGFBP7 is superior to TIMP-2 in surgical patients whereas TIMP-2 is better for prediction of septic AKI, demonstrating that these two biomarkers are involved in slightly different pathways. Therefore, it is not clear whether the excellent results of the Discovery, Sapphire and Opal study can be transferred to patients undergoing CABG. Studies investigating the role of [TIMP-2]*[IGFBP7] in the prediction of AKI and defining cut-off values in cardiovascular patient populations are rare and currently restricted to adult and pediatric open cardiac surgery. Meersch et al. investigated the diagnostic properties of these biomarkers for the detection of AKI in 50 patients undergoing CABG who were considered to be at high risk for impaired perioperative renal function assessed by the Cleveland Foundation Score [11]. Whereas diagnosis of AKI based on serum creatinine was delayed, TIMP-2 and IGFBP7 increased as early as 4 hours after surgery with a significant difference between patients with AKI and those without. The maximum urinary [TIMP-2]*[IGFBP7] value within the first 24 h following CABG showed excellent diagnostic accuracy with an area under the receiver operating characteristic curve of 0.84. Sensitivity was 0.92, and specificity was 0.81 for a cutoff value of 0.50. In addition, prediction of renal recovery from AKI was possible with urinary [TIMP-2]*[IGFBP7]. Comparable data were published from the same working group in a pediatric cardio surgical cohort (AUC 0.85, sensitivity 0.83, specificity 0.77 for cut-off 0.70 [(ng/ml)²/1000] [24].

The cut-off values defined in our study were slightly different from recently published studies In the Discovery, Sapphire and Opal study, best cut-off values for discrimination of patients with high risk for oncoming AKI was 0.3 (ng/ml)²/1000 whereas Meersch set the value of 0.5 (ng/ml)²/1000 for the adult and 0.70 (ng/ml)²/1000 for the pediatric cardiothoracic population. As mentioned above, etiologies and pathophysiology of AKI differ substantially between patient populations and therefore cut-off values from general surgical, septic, or medical patients might not be applicable to cardiothoracic patients.

Even comparing our results with other studies about the utility of urinary [TIMP–2]*[IGFBP7] for diagnosis of AKI after cardiac surgery may be difficult: Incidence of AKI was 52 % in the study from Meersch vs. 31.7 % in our study. In contrast, five patients suffered from AKI 3 in our series compared to one in the cited trial. The incidence of AKI after cardiac surgery depends on the type of surgery varying significantly between isolated coronary artery bypass grafting, valvular surgery and being the highest for combined CABG+ valvular surgery. Merrsch included all types of cardio surgical procedures not giving any detailed information about performed procedures. In contrast, only isolated CABG procedures were included in our study. In addition, type of CBP and myocardial protection strategy might influence incidence as well as time onset of AKI.

If AKI is recognized early, nephroprotective measures can be considered to reduce exposure to renal insults and potentially avoid the development of higher stage AKI. Although the discussion about preventive or therapeutic interventions in critically ill patients with AKI is controversial, there are some strategies that may be beneficial in the ICU setting: Although there are no specific therapies for AKI, the optimization of fluid balance and hemodynamics as well as a medication review with avoidance of nephrotoxic drugs can reduce the incidence and severity of AKI and improve long-term outcomes [7]. This applies in particular for the subgroup of high risk patients that might be identified better with the cell cycle arrest markers. Accordingly, delayed consultation of a nephrologist is associated with higher mortality and increased dialysis dependence rates in critically ill AKI patients at hospital discharge [25]. In addition, earlier commencement of RRT in critically ill patients with incipient AKI may be beneficial with a reduction of mortality, in particular for patients after cardiac surgery [6, 26]. Preoperative remote ischemic preconditioning significantly reduced the rate of AKI and use of RRT in patients undergoing cardiac surgery in a recently published multicenter randomized trial [27]. Therefore, our standard regime includes a bundle of interventions if [TIMP 2]*[IGFBP7] is higher than the reported cut-off value including consultation of a nephrologist, extensive chart review and stop of all nephrotoxic agents, goal-directed hemodynamic management, delay—if possible—of any exposure to contrast agent, discussion of early RRT and delay of transferring the patient from the ICU to the normal ward. Therefore, urinary [TIMP-2]*[IGFBP7] might help to initiate established preventive and therapeutic interventions and investigate new concepts to prevent AKI.

Urinary [TIMP-2]*[IGFBP7] may be considered as early predictor of AKI after CABG surgery. It allows the identification of patients at high risk for the development of severe AKI, even with need for RRT, within the postoperative course. Urinary G1 cell cycle arrest biomarkers allow the diagnosis of AKI earlier than creatinine-based definition of AKI.

There are some limitations to our study. Due to the relatively small sample size, the predicted cut-off values for [TIMP-2]*[IGFBP7] in the urine may be different from that obtained in other populations.

In the present study, the measurements of the biomarkers were conducted using the commercially available test kit, which only displayed the product of [TIMP-2] and [IGFBP7]. Therefore, we cannot draw any conclusions about the diagnostic properties of TIMP-2 and IGFBP7 alone in comparison to their multiplication.

In the current study, urinary creatinine was not measured to estimate the true creatinine clearance for these patients. There are three major errors that can limit the accuracy of the creatinine clearance as an estimate of GFR: errors in urine collection; increases in both creatinine secretion and extrarenal creatinine degradation as the GFR falls. Because of these limitations, we did not collect 24 h urine to calculate creatinine clearance but used the Cockcroft–Gault formula to estimate GFR.