Postoperative acute kidney injury (AKI) is a common complication after heart surgery, with an incidence of between 5% and 42%. Postoperative AKI is associated with a variety of adverse events, such as prolonged intensive care unit (ICU) stay and increased mortality [1, 2]. The development of AKI is influenced by many clinical factors, such as advanced age, congestive heart failure, hyperglycemia, pre-existing kidney disease, and emergency surgery [1, 2]. Therefore, adequate assessment of preoperative renal function is particularly important for identifying high-risk patients and predicting postoperative AKI.

Estimated glomerular filtration rate (eGFR) is a convenient tool for assessing renal function in patients. In addition to the commonly used Cockcroft Gault (CG) equation [3], the modification of diet in renal disease (MDRD) equation [4], and chronic kidney disease-epidemiology (CKD-EPI) equation [5], there are also the Schwartz equation [6] for children and the Berlin Initiative Study (BIS) equation [7] for people over 70 years of age. Recently, Hans Pottel [8, 9] proposed a full age spectrum (FAS) equation that could cover all ages. Studies have shown that the FAS equation is also suitable for the Chinese general population [10, 11, 12]. Besides assessing renal function, the above-mentioned equations are also widely used to assess the risk of death, AKI, and other adverse events [13, 14, 15, 16, 17].

Xiaoyun Wu et al. [17] study demonstrated that the CKD-EPI equation, which was based on serum creatinine, has a better predictive power than the CG and MDRD equations on the incidence of postoperative AKI in on-pump heart surgery. However, their study did not include eGFR calculated by cystatin C. Cystatin C, a 13-kDa cysteine proteinase inhibitor protein, is freely filtered by the kidney with near-complete reabsorption and catabolism in the proximal tubule and no significant urinary excretion. Serum cystatin C is much less affected by patient characteristics such as gender, age, nutritional status, and sarcopenia than serum creatinine [18, 19, 20]. Therefore, equations developed based on cystatin may have a wide range of applications. It is therefore necessary to evaluate the prediction value of eGFR calculated by cystatin C.

With the improvement of surgical techniques, more and more patients are receiving off-pump coronary artery bypass grafting. Since the conclusions of the previous study were based on patients who underwent on-pump surgery [17, 21], it is necessary to explore the predictive power of each of the eGFR equations after off-pump surgery. The aim of our study was to compare the predictive power of preoperative eGFR calculated by each equation (including CG, MDRD, CKD-EPI${}_{\text{Creatinine}}$, CKD-EPI${}_{\text{Cystatin C}}$, FAS${}_{\text{Creatinine}}$, and FAS${}_{\text{Cystatin C}}$) for AKI after off-pump coronary artery bypass grafting.

This study found that preoperative eGFRs calculated based on cystatin C have more accurate predictive power for AKI after off-pump CABG. Moreover, in patients younger than 65 years, all equations had better performance than patients over 65 years. Compared with each eGFR, we also found that the average value of eGFR calculated by the FAS equation based on cystatin C was the lowest, while the average value of the MDRD equation was the highest. The predictive power of preoperative eGFR based on cystatin C was better than creatinine. This phenomenon may be related to the following two reasons: ① cystatin C is better than creatinine in evaluating renal function. It was much less affected by patient characteristics, such as gender, age, body size and composition, and nutritional status [18, 19, 20]. ② Due to SPS, eGFRs calculated by the creatinine could be overestimated. In this study, the incidence of SPS was significantly higher in the AKI group than in the non-AKI group. Therefore, we suggest using equations based on cystatin C to calculate eGFR [22, 23].

Xiaoyun Wu et al. [17] compared the predictive value of eGFRs after cardiac surgery. The main conclusion of their paper is that the CKD-EPI equation has a better predictive effect than the CG and MDRD equations. Additionally, the body surface area adjusted CG equation performed better in their cohort. In the comparison of the predictive power of CKD-EPI, CG, and MDRD equations, the results of our study are consistent with their conclusions. It is important to note that the decrease in perfusion pressure during on-pump surgery can alter renal function. Hence, the occurrence of postoperative AKI, in their study, is more than 2-fold of our cohort. Differences were also seen in sample size, average age, and the incidence of males/females. Wu’s study also explored the predictive effect of eGFR within 2 days after surgery. Patients were being treated in the cardiac intensive care unit at this time. Hence, AKI can be directly and conveniently diagnosed based on urine volume or creatinine levels. Therefore, it is not necessary to calculate eGFR to predict AKI alone. In summary, Wu’s conclusion cannot be directly applied to all cardiac surgery patients.

In the subgroup analysis, we also found that the predictive effect of the eGFR equations was enhanced in patients less than 65 years. This phenomenon may be related to the patient’s condition when the equation was established. Younger patients have a lower incidence of heart failure, atrial fibrillation, and other co-morbidities. Therefore, renal function may have a greater impact on postoperative AKI in younger patients. In other studies, the equations were more accuracy for the diagnosis and stratification in patients with advanced kidney disease or older age [24, 25]. But none of the patients in these studies underwent cardiac surgery.

Notably, in our study, eGFR based cystatin C had better predictive power in the diabetic population. Studies have also demonstrated that serum cystatin C has better predictive power for adverse outcomes in diabetic patients. Kati Jarvela et al.’s [26] study prospectively enrolled 200 patients who underwent elective CABG, and measured their serum creatinine and cystatin C levels. Study point out that cystatin C and cystatin C-based estimation of GFR may be useful and more sensitive than creatinine in detecting mild acute renal insufficiency in diabetic patients. In addition, Caroline Pereira Domingueti and colleagues [27] also demonstrated that cystatin C-based equations present the best accuracy to detect macroalbuminuria in Cystatin type 1 diabetes mellitus patients. This phenomenon may be related to the fact that diabetes not only increases serum cystatin levels, but also significantly increases the risk of postoperative AKI [26, 28].

By comparing each equations, we identified two equations that were suitable for preoperative assessment of renal function. Accurate assessment of preoperative renal function not only could predict the risk of postoperative AKI, but also be used to guide preoperative clinical strategy to avoid AKI. In addition, AKI is influenced by multiple perioperative risk factors, which may explain the limited predictive power of eGFR. Despite improvement in bias compared with equations based on creatinine, some studies suggest that a few biases remain in equations based on cystatin C, especially in patients who are chronically ill. Among them, measured GFR may be necessary for the accurate assessment of GFR in these populations [20, 29]. On the other hand, some studies found that further modification and the addition of the Chinese racial factor could improve the predictive and stratification ability of preoperative eGFR [12, 30]. Hence, further research is needed.

Preoperative eGFR calculated by FAS${}_{\text{cys}}$ and CKD-EPI${}_{\text{cys}}$ equations have better performance in predicting AKI after off-pump CABG than other equations, especially in diabetics.

The data analyzed in this study are available from the corresponding author or first author upon reasonable request.

WQ and JT were responsible for the conception and design of the work. JT and CZ were responsible for data collection and article drafting. WQ, JT, WH were responsible for data analysis and substantial revision. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

The Ethics Committee of the Affiliated Hospital of Qingdao University approved the study [QYFY WZLL 28075]. All procedures performed in this study involving human participants were by the Declaration of Helsinki. The need for consent was waived by the Ethics Committee of the Affiliated Hospital of Qingdao University due to the retrospective nature of the study. Patient data confidentiality was protected.

Thanks to Dr. Yangyan Wei from Wuhan Asia General Hospital for his help in study design and statistics.

This research received no external funding.

The authors declare no conflict of interest.