Prognostic values of blood urea nitrogen/creatinine and cystatin C in patients with radical nephrectomy for renal cell carcinoma

Background To evaluate the prognostic value of blood urea nitrogen/creatinine ratio (BUN/SCr) and cystatin C (Cys C) in patients with renal cell carcinoma (RCC) after radical nephrectomy. Methods The study analysed 348 patients with RCC who underwent radical nephrectomy. The optimal cut-off was obtained based on the ROC of specific survival outcomes and the maximum Youden index. The patients were divided into four groups: Group 1 (low BUN/SCr-low Cys C), Group 2 (low BUN/SCr-high Cys C), Group 3 (high BUN/SCr-low Cys C), and Group 4 (high BUN/SCr-high Cys C). The primary endpoint was cancer-specific survival (CSS), and the secondary endpoint was disease-free survival (DFS).


Introduction
Renal cell carcinoma (RCC) accounts for over 90% of kidney cancers (1).According to the World Health Organization's new classification of kidney tumours, clear cell RCC (ccRCC) accounts for most RCC cases (2).Operative radical nephrectomy (ORN) has been the gold standard for malignant renal masses for decades (3).In addition, ORN is the standard surgical procedure for T3 and T4 tumours (4).ORN involves the entire kidney and surrounding perirenal fat, ipsilateral adrenal glands, and regional lymph nodes (5).The mortality rate of RCC still accounts for a high proportion of malignant urinary system tumours, and the course and prognosis of individual diseases are uneven and difficult to predict (6).The ability to accurately predict the prognosis of cancer patients concerning death is critical to determining treatment and monitoring strategies for patients.An accurate understanding of disease progression and risk of death is essential to guide patients, plan individualized surveillance protocols, and select appropriate treatment plans.In this case, tumour size, histological subtype, pathological stage, and nuclear grade have been identified as prognostic factors for RCC (7).
The blood urea nitrogen (BUN)/creatinine (SCr) ratio is associated with disease severity and survival.A survey of 139 COVID-19 patients shows that high BUN/SCr predicts severe disease and poor survival (8).Epidemiological studies have found a bidirectional relationship between renal cancer and impaired renal function (9,10).A bidirectional causal relationship has been determined between kidney cancer and renal function biomarkers, including BUN/SCr ratio (11).Cystatin C (Cys C) is widely expressed in early organisms and involved in immune-related processes under pathological conditions (12).Urinary Cys C is a potential early marker of acute kidney injury (13) and a valuable prognostic indicator in malignant tumours (14,15).However, due to eradicating the primary tumour, major surgery may affect the host's immune response and kidney function.No studies have investigated BUN/SCr ratio and Cys C when evaluating RCC patients' prognosis after ORN.In this case, we investigated the prognostic significance of BUN/SCr ratio and serum Cys C in patients undergoing ORN.

Study population
The Institutional Review Board of the Heping Hospital, affiliated with Changzhi Medical College, approved the current study.We reviewed the medical records of 462 patients with RCC diagnosed and treated in Heping Hospital, affiliated with Changzhi Medical College, between January 2012 and January 2018.We included only 368 patients with no lymph nodes and distant metastases and who had under-gone ORN.Patients with the following history were excluded: (1) prior history of any malignancy, a second primary cancer diagnosed before treatment; (2) Postoperative SCr, Cys C, and BUN data were incomplete.Finally, 348 patients were enrolled in the study.

Research design
Clinicopathologic variables included age at surgery, gender, body mass index (BMI), tumour location, tumour size, histological type, pT stage, and Fuhrman nuclear grade.The eighth edition of UICC/AJCC categorizes pathology according to the tumour-node-metastasis (TNM) system.Postoperative blood samples were collected within 3 months after surgery.All patients underwent a disease history consult, physical examination, routine laboratory tests, and imaging tests such as chest X-rays and computed tomography of the kidneys.Follow-up assessments of all patients were conducted every 3 months for the first 2 years and then every 6 months according to our institution's protocol until January 2023.
The primary endpoint was cancer-specific survival (CSS), and the secondary endpoint was diseasefree survival (DFS).Survival results were examined via the utilization of receiver operating characteristic (ROC) curve analysis, wherein the optimal cut-off value is determined through ROC analysis (Table I).We divided the patients into 4 groups based on the threshold value: Group 1 (low BUN/SCr-low Cys C), Group 2 (low BUN/SCr-high Cys C), Group 3 (high BUN/SCr-low Cys C), and Group 4 (high BUN/SCrhigh Cys C).

Sample collection and measurement
The experimental participants were instructed to collect venous blood from the elbow vein after fasting (12), allow it to coagulate at room temperature, and separate the serum within 4 hours of collection.The serum was then tested at a constant temperature of 4 using a fully automated biochemical analyser (ADVIA2400, SIEMENS AG, Berlin, Germany) to analyse BUN, SCr, and Cys C. Shanghai Kehua Biotechnology Co., Ltd., manufactured the Cys C kit used in the analysis and was strictly operated in accordance with the instructions provided with the kit.The normal range for BUN was determined to be 3.8 to 7.0 mmol/L; for SCr, it was 45 to 105 mmol/L; and for Cys C, it was 0.70 to 1.15 mg/L.

Data analysis
All statistical analyses were conducted by SPSS software 22.0.The chi-square test was used for classification parameters, and the Mann-Whitney U test for continuous parameters.Spearman correlation coef ficient was applied to analyse the correlation between serum factors.The AUC was calculated using ROC analysis, and the optimal critical value was selected based on the maximum Youden index.The Youden index is sensitivity plus specificity minus 1, and the larger the Youden index, the higher the accuracy of diagnosis.The survival rate was analysed by Kaplan-Meier and tested by logarithmic rank test.In addition, univariate and multivariate Cox regression models were employed to analyse the correlation between variables and survival further.Bilateral P-values represented statistical significance at <0.05.

Patient characteristics
This study summarized the demographics of 348 patients with RCC who underwent ORN (not shown in the table).The median age was 56 years (IQR, 47-66 years), and 66.7% were male.The me -     Table II shows the baseline characteristics of patients according to BUN/SCr values and serum Cys C cut-off.The number (proportion) of patients in each classification is as follows: There were 300 cases (15.41%) (BUN/SCr ratio<17.41),48 cases (13.8%) (BUN/SCr ratio≥17.41),267 cases (76.7%) (Cys C<3.98 mg/L), and 81 cases (23.2%) (Cys C≥3.98 mg/L).We observed that patients with higher BUN/SCr ratio (>17.41) and high Cys C level (>3.98 mg/L) had a higher proportion of tumour size over 7 cm than patients with lower BUN/SCr ratio and Cys C (P=0.010, P=0.008).
For other clinicopathological variables, no significant differences were observed.It was worth noting that serum Cys C significantly differed between the group with high BUN/SCr and low BUN/SCr (P<0.001).Serum BUN significantly differed between the groups with high and low Cys C levels (P<0.001).This suggests that there seems to be a correlation between serum BUN and Cys C, which was confirmed by Spearman correlation analysis (r=0.764,P<0.001) in Figure 1A.We also found a stronger positive correlation between BUN/SCr ratio and Cys C (r=0.978, P<0.001) (Figure 1A, B).
Next, groups were set according to the critical value, and Group 3 (high BUN/SCr-low Cys C) had only one patient, so this group was not analysed.Kaplan-Meier survival analysis showed (Figure 3) that patients in Group 4 (high BUN/SCr-high Cys C) had poorer CSS and DFS than those in Group 1 (low BUN/SCr-low Cys C) and Group 2 (low BUN/SCr-low Cys C).No significant difference in survival outcomes was observed between Groups 1 and 2. After adjusting for various clinicopathological factors, we finally determined that BUN/SCr ratio and serum Cys C were predictors of CSS and DFS in multivariate Cox analysis, in addition to tumour size and pT stage (Tables III and IV).

Discussion
This was the first study to show that survival outcomes in RCC patients undergoing ORN were significantly associated with BUN/SCr and Cys C. In particular, patients with higher BUN/SCr and Cys C had worse CSS and DFS results than patients with lower BUN/SCr and Cys C.More importantly, among patients with high BUN/SCr and Cys C, 19 deaths occurred, all of which were cancer-specific, accounting for 40.4%.However, no significant differences were found in CSS and DFS results between group 1 (low BUN/SCr and low Cys C) and group 2 (low BUN/SCr and high Cys C).BUN/SCr and Cys C were predictors of survival outcomes.
Patients with RCC have decreased renal function after surgery (16,17).Although the mechanism is unclear, ORN is potentially associated with chronic kidney disease in patients with renal cancer (18).In a follow-up of 500 patients with ORN, the cumulative 5-year incidence of chronic kidney disease was 43.4% (19).Chronic kidney disease can develop into endstage renal disease, and patients have an increased risk of death (20).After ORN, the glomerular filtration rate is significantly reduced, suggesting impaired renal function (21).BUN/SCr is associated with the prognosis of patients with heart failure or renal impairment caused by acute infarction.For example, BUN/Cr exceeding the optimal threshold could predict the disease condition and poor survival of more severe COVID-19 patients (8).Moreover, BUN/Cr is associated with long-term mortality from heart failure (22).
Cys C has been determined as a marker of renal insufficiency or kidney injury (23).The use of Cys C avoids limitations related to diet, nutrition, and muscle mass that affect BUN and SCr (24).In addition, Cys C can provide reliable prognostic information for diseases after treatment, such as kidney transplanta-tion (25), heart transplantation (26), and percutaneous coronary intervention for acute myocardial infarction (27).
Our results showed that BUN/SCr had a strong positive correlation with Cys C. The combination of low BUN/SCr and low Cys C accounted for 76.4%, high BUN/SCr and high Cys C 13.5%, low BUN/SCr and high Cys C 9.8%, and high BUN/SCr and low Cys C 1 case.This suggests that combining the two may have strong prognostic information for patients after surgery.In our study, some patients below the threshold survived, while some above it died, and vice versa.Nevertheless, the patient-specific mortality in Group 4 (high BUN/SCr with high Cys C) was 40.4%, compared to 7.1% in Group 1.This is similar to some previous reports that the detection of BUN/SCr or Cys C is associated with a relatively poor prognosis (15,28).High BUN/SCr values and high Cys C serum levels after ORN were also highly predictive and independent predictors of mortality risk.Our analysis showed that the threshold of 17.41 (BUN/SCr), 3.89 mg/L (Cys C) is the most appropriate level to distinguish between specific survival and death.When this threshold is exceeded, the specific mortality of the population increases by 1.8 to more than 5 times.Similarly, exceeding the threshold is a risk factor for DFS.Although we did not analyse whether BUN/SCr combined serum Cys C was an independent risk factor for death, these data suggest that high BUN/SCr values and high serum Cys C levels may strongly predict patient outcomes even after surgery.
The current study has some potential limitations.First, this is a retrospective study and has inevitable limitations, including the lack of consistent data collection, relatively small sample size, and high percentage of missing data in some groups.Second, our data are based on single-centre trials.Therefore, external validation is needed to substantiate the results of the current study further.Finally, we could not provide mechanistic clues to support our new findings on the prognostic effects of postoperative BUN/SCr values and Cys C levels on survival outcomes in patients with RCC.Nevertheless, we demonstrated independence from established prognostic factors, including tumour size and pathological T stage.We reported that high BUN/SCr values combined with high Cys C levels were associated with a risk of poor prognosis in patients with RCC undergoing ORN.

Funding
Not applicable.

Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.

Table I
Cut-off and corresponding sensitivity and specificity based on maximum Youden Index.

Table II
Cut-off and corresponding sensitivity and specificity based on maximum Youden Index.

Table IV
Univariate and multivariate analyses of predictors of Disease free survival for patients with renal cell carcinoma who underwent radical nephrectomy