Discussion

Acute kidney failure (AKF) is one of the major complications which increases the mortality rates after CABG.[1,15] Preoperative renal d ysfunction w as accepted as an independent predictor of long-term mortality after cardiac surgery.[16,17] Besides, close monitoring for early detection and prevention of AKF is of utmost importance in CABG perioperatively.[18] Although GFR measurement with urinary or plasma clearance of a standard marker such as inulin is accepted as the gold standard, it is not cost-effective in the daily clinical practice.[16] On the other hand, the creatinine clearance rate calculation via collecting urine output during a whole day can be accepted as the clinical standard.[18] In the present study, we preferred to use creatinine clearance rate measurements as the main standard in parallel with some recent literature study findings.[16] However, it is challenging to use it routinely for all patients. Another disadvantage of this technique is the risk of late detection and intervention. Blood urea nitrogen and creatinine levels can be used, but not enough to evaluate glomerular filtration rates alone. For this reason, estimated creatinine clearance and GFR equations were developed.[7-14] These equations are usually evaluated and used for overall patient populations.[6] In particular, in CABG patients, there are a number of major alterations in blood measures, including plasma albumin levels after on-pump surgery.[18,19] Cardiopulmonary bypass circuit and the oxygenator membrane itself may also induce a high amount of cytokines to release, increasing the membrane permeability.[19,20] Therefore, off-pump surgery is accepted superior to on-pump surgery in terms of the risk of acute kidney injury and failure.[19,20] Under these circumstances, estimated GFR calculations in on-pump CABG patients need to have special concerns. First, the majority of our CABG candidates suffered from several comorbidities such as diabetes mellitus, hypertension, and hyperlipidemia. It makes the first main difference between these patients and healthy population. Second, cardiopulmonary bypass itself was the major factor which affected renal functions in the early postoperative period. Perfusion also affected electrolyte and body fluid balances, resulting in body weight changes, eventually, estimated GFR equations. For these reasons, we made all calculations separately for different time periods using separate measurement tools. As we expected, creatinine clearance rates and blood urea nitrogen differed postoperatively compared to the baseline values. Besides, blood creatinine levels did not differ significantly in the perioperative period.

This finding can be interpreted as the lack of precision of conventional renal function tests reflecting the real glomerular filtration rates. After the estimated kidney function and glomerular filtration rate equation measurements were done, we adjusted all units according to the body surface areas. We did not use the race variable of MDRD formulas, as there was no black patient in our study. Consistent with the literature findings, correlation analyses showed us the Cockcroft- Gault formula had higher correlation coefficient and precision values for all the time periods perioperatively, compared to other equations.[6] Of note, among all time periods, the Cockcroft-Gault equation had the highest precision in the postoperative first-day with 50.6% (p<0.001). There were some literature findings suggesting that MDRD-4 formula was a more accurate measurement tool in predicting the long-term survival after cardiac surgery[16,17] and Cockcroft-Gault equation might result in an overestimated GFR in patients with end-stage kidney failure, particularly.[17] Furthermore, studies including a heterogeneous group of cardiac and vascular patients[17] were not consistent with our findings. We included a more homogenous patient population - only CABG patients. We also excluded patients with end-stage renal dysfunction and critical preoperative conditions. Cardiac risk scoring systems such as EuroSCORE-II utilize the Cockcroft-Gault equation for the evaluation of renal functions as well as in the risk prediction.[21] This also encourages that using the Cockcroft-Gault equation can be helpful in our daily clinical practice. In our study, the Bjornsson equation followed the Cockcroft-Gault formula with precision values of preoperative and postoperative first and fifth-day as 0.417; 0.503; 0.367, respectively (p<0.001). In contrast with the literature, we did not find MDRD-6 formula as precisely described.[6] This can be attributed to the impact of blood albumin levels in terms of postoperative changes after perfusion. We also found that blood albumin levels were significantly different between preoperative and postoperative first and fifth-day measurements (p<0.001). The scatter plots of all estimated renal function and glomerular filtration rate equations showed many different distributions. Overall, precision values were the highest in the postoperative first-day measurements. Furthermore, we observed a significant difference between preoperative and postoperative first and fifthday measurements in terms of creatinine clearance rate (p<0.001) . However, we were unable to find significant differences between the time groups of estimated GFR equations (p>0.05). Therefore, we should remember that predictive capabilities of estimated calculation formulas are still suboptimal compared to conventional standard methods.[6] Besides, the Cockcroft-Gault formula was also reported as the best predictor of early and late mortality after CABG compared to the other estimated renal function measurements.[22] This finding also supports the clinical use of the Cockcroft-Gault equation as a clinical practical evaluation method of kidney functions after CABG. Kidney dysfunctions still play an important role in the prediction of mortality and morbidity of CABG patients.[23-25] Therefore, practical and precise methods for the evaluation of glomerular filtration rates still stays as indispensable tools in our daily practices.

On the other hand, there are some limitations to this study. We used creatinine clearance rate measurements as the main standard for the evaluation in our study, as these tools are more practical and cost-effective compared to inulin clearance measurements, which is accepted as the gold standard. In addition, estimated GFR formulas which were compared in our study were originally described for different patient populations, but not specifically for the perioperative evaluation of CABG patients. Finally, our study was a single-center and retrospective study. Therefore, further multi-center and prospective studies with a larger sample size are required to confirm our findings.

In conclusion, although the efficacy of estimated glomerular filtration rate calculations is still suboptimal, they can be used instead of simple blood urea nitrogen and blood creatinine measurements. They are also more practical than standard creatinine clearance rate measurements by collecting whole 24-hour urine output. In the evaluation of renal functions of patients undergoing coronary artery bypass grafting surgery, the Cockcroft-Gault equation can be more reliably used for the calculation of estimated kidney functions, compared to the formulas.

Declaration of conflicting interests
The authors declared no conflicts of interest with respect to the authorship and/or publication of this article.

Funding
The authors received no financial support for the research and/or authorship of this article.

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