ISSN : 1301-5680

e-ISSN : 2149-8156

The reliability of estimated glomerular filtration rate in coronary artery bypass grafting

Ozan Onur Balkanay, Deniz Göksedef, Suat Nail Ömeroğlu, Gökhan İpek

Department of Cardiovascular Surgery, İstanbul University, Cerrahpaşa Medical Faculty, İstanbul, Turkey

**Methods:** A total of 167 patients (124 males, 43 females;
mean age 60.9±9.4 years; range, 35 to 83 years) who
underwent on-pump coronary artery bypass grafting in our
clinic between January 2009 and January 2011 were enrolled
in the study. Demographic characteristics of the patients,
creatinine clearance rates measured with 24-hour urine
collection, and estimated renal function, and glomerular
filtration rates according to the formulas of Cockcroft-Gault,
Modification of Diet in Renal Disease-four variables and
six variables, Jelliffe-1972, Jelliffe-1973, Mawer, Bjornsson,
and Gates of preoperative and postoperative first and fifthdays
were retrospectively analyzed.

**Results:** Among all estimated renal function and glomerular
filtration rate equations, the Cockcroft-Gault formula
yielded the most significant results for the creatinine
clearance rate for all periods.

**Conclusion:** Our study results suggest that the Cockcroft-
Gault equation can be reliably used in the evaluation of
kidney functions of patients undergoing coronary artery
bypass grafting.

Kidney dysfunction in the perioperative period
of coronary artery bypass grafting (CABG) has
been significantly associated with the mortality.[1-3] Timely detection of the dysfunction, particularly
in the early postoperative period, can be, therefore,
life-saving.[4,5] The common methods such as blood urea nitrogen and creatinine level measurements
are usually incapable to meet all the requirements
and a standard method of creatinine clearance
rate calculation including the collection of all
the urine output during a whole day is usually
accepted as cumbersome to use.[6] For these practical
reasons, a number of estimated kidney function
and glomerular filtration rate (GFR) calculation
formulas using different numbers of variables have
been developed to date.[7-14] The predictive values
of these equations have been also investigated
among different patient populations.[6] However, the
reliability of estimated GFR equations needs to be
evaluated for CABG patients, as well. Therefore,
in our study, we investigated the reliability of the
estimated creatinine clearance and GFR in patients
undergoing on-pump CABG.

The exclusion criteria were as follows: preoperative renal dysfunction, re-operation rates and simultaneous surgical procedures, critical preoperative conditions such as endocarditis, and emergency or salvage operation.

All the operations were performed on-pump and by a single surgical team. Mild hypothermia was used during surgery. Anastomoses were performed under a single aortic cross-clamp. Myocardial protection was achieved via both antegrade and retrograde cold blood cardioplegia.

**Statistical analysis**

Statistical analysis was performed using IBM SPSS
software package version 20.0 (IBM Corporation,
Armonk, NY, USA). All data were expressed in
mean ± standard deviation or percentage (n, %).
For the normality assumptions, the Kolmogorov-
Smirnov, Shapiro-Wilk tests, and histograms were
used. Parametric variables with normally distributed
data were compared among the groups by using
repeated measures analysis of variance. Otherwise,
the Friedman test was used. The Pearson correlation
coefficients (r) were calculated for the normally
distributed data; otherwise, the Spearman’s correlation
coefficients (r) were calculated. The precision values
(r^{2}) were calculated after the correlation analyses.
The scatter plots of estimated GFR calculations were
created for both preoperative and postoperative first
and fifth-days. A p value of <0.05 was considered
statistically significant.

**Table 1: Patient characteristics and perioperative variables**

**
****Table 2: Renal function tests, estimated creatinine clearance and glomerular filtration rates, and related variables**

**
Estimated glomerular filtration rate calculation formulas we used for female patients were as follows:
• Cockcroft-Gault equation:^{[10]}
[[(140-age) x weight) / (72 x CreatinineSerum)] x 0.85]
• MDRD-4 equation:^{[13]}
(186 x CreatinineSerum^{-1.154} x age^{-0.203} x 0.742)
• MDRD-6 equation:^{[14]}
(170 x CreatinineSerum^{-0.999} x age^{-0.176} x 0.762 x UreaSerum^{-0.170} x AlbuminSerum^{0.318})
• Jelliffe-1972 equation:^{[8]}
[(80 / CreatinineSerum)-7]
• Jelliffe-1973 equation:^{[9]}
[[[98-[0.8 x (age - 20)]] / CreatinineSerum] x 0.9]
• Mawer equation:^{[7]}
[[[weight x [25.3-(0.175 x age)] x [1-(0.03 x CreatinineSerum)]] / [(14.4 x CreatinineSerum) x (70 / weight)]
• Bjornsson equation:^{[11]}
[[[25-(0.175 x age)] x weight x 0.07] / Creatinine_{Serum}]
• Gates equation:^{[12]}
[(60 x CreatinineSerum^{-1.1}) + (56 - age) x (0.3 x CreatinineSerum^{-1.1})]
Estimated GFR calculation formulas we used for male patients were as follows:
• Cockcroft-Gault equation:^{[10]}
[(140 - age) x weight) / (72 x CreatinineSerum)]
• MDRD-4 equation:^{[13]}
(186 x CreatinineSerum^{-1.154} x age^{-0.203})
• MDRD-6 equation:^{[14]}
(170 x CreatinineSerum^{-0.999} x age^{-0.176} x UreaSerum^{-0.170} x AlbuminSerum^{0.318})
• Jelliffe-1972 equation:^{[8]}
[(100 / Creatinine_{Serum})-12]
• Jelliffe-1973 equation:^{[9]}
[[98 - [0.8 x (age - 20)]] / CreatinineSerum]
• Mawer equation:^{[7]}
[[[weight x [29.3-(0.203 x age)] x [1-(0.03 x CreatinineSerum)]] / [(14.4 x Creatinine_{Serum}) x (70 / weight)]
• Bjornsson equation:^{[11]}
[[[27-(0.173 x age)] x weight x 0.07] / Creatinine_{Serum}]
• Gates equation:^{[12]}
[(89.4 x CreatinineSerum^{-1.2}) + (55 - age) x (0.447 x CreatinineSerum^{-1.1})]**

**
After these calculations were made, all equations were adjusted according to the body surface areas and all units
were standardized to mL/min/1.73 m ^{2}.**

** **

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.