Donor shortage remains the cardinal problem of modern transplant medicine, especially in kidney transplantation (KTX). To address this ever-growing issue, multiple approaches have been taken to increase the donor pool, but the number of patients waiting for a suitable organ still exceeds the number of potential donors. Hence, the acceptance of marginal organs continues to increase [1]. Undoubtedly, these kidneys have a higher susceptibility to ischemia-reperfusion injury, combined with an undeniable risk of inferior long-term graft function [2]. These developments highlight the importance of a patient-based allocation with the characterization of harmful and harmless donor conditions.

Proteinuria is a common diagnosis after KTX and has been identified as an independent risk factor for inferior graft function and reduced graft survival after KTX [3–5]. Proteinuria can be diagnosed by a quantitative measurement of urine albumin or protein-to-creatinine ratio, as well as by albumin or protein excretion (PE) rate. In addition, a semiquantitative measurement with urine dipsticks can be used, as described by Kidney Disease: Improving Global Outcomes (KDIGO) and Chronic Kidney Disease (CKD) guidelines [6]. The prevalence of proteinuria in kidney transplant recipients ranges from 7.5% to 45% [7]. However, proteinuria is much more prevalent in organ donors, with low- and high-grade proteinuria occurring in 35.1% and 74.1% of allocated kidneys, respectively [8]. Yet, there are no official guidelines regarding donor proteinuria (DP) in kidney allocation, and the long-term impact of DP as an independent risk factor has not yet been validated. In consequence, DP can influence allocation decisions, with the inherent risk of declining suitable organs. This is especially important as most countries are experiencing a shortage of organ donors, resulting in long waiting times for patients on transplant lists. Therefore, declining a potentially suitable organ is negligent. On the contrary, proteinuria may indicate chronic kidney disease [9], and in kidney recipients, proteinuria is associated with reduced graft function and impaired 5 years graft survival. Patients with proteinuria have a survival rate of only 69%, compared to 93% for patients without proteinuria [10]. In kidney recipients with proteinuria, a decreased overall survival was observed compared to KTX recipients without proteinuria [11]. Additionally, patients with proteinuria have a 2.45-times increased risk of a cardiovascular event, such as ischemic heart disease, cerebrovascular disease, or peripheral vascular disease [7]. Multiple risk factors for posttransplant proteinuria have recently been defined, including a female donor, a male recipient, patients with acute rejection, donor age, and donor cardiovascular death [12, 13]. However, the effect of DP on proteinuria in the recipient has not yet been evaluated. This study aims to analyze the impact of DP on long-term (36 months) outcomes after KTX, specifically focusing on patient and graft survival.

A total of 587 patients met the inclusion criteria. This cohort was further stratified based on the presence of proteinuria in the donor. Out of the total patients, 213 (36.3%) received a DP+ graft, while 374 patients (63.7%) received a DP− organ (Figure 1). Within the DP+ cohort, the majority had low grade proteinuria (55.4%) followed by mild grade proteinuria (39.4%). Only a small fraction had moderate proteinuria (3.3%) or high-grade proteinuria (1.9%), as indicated by semiquantitative measurement (Figure 2).

Both groups showed similar demographic donor characteristics (Table 1). However, DP− donors had a slightly higher BMI (26.0 vs. 27.0, p = 0.012) and higher sCr level at the time of procurement (0.760 μmol/L vs. 0.555 μmol/L, p = 0.007) (Table 1). In addition, the frequency of ECD donors was significantly higher in the DP+ cohort (57.75%) compared to the DP− cohort (41.71%) (Table 1) (p < 0.001). Moreover, DP+ donors had significantly lower diuresis during donor evaluation (DP+: 129.2 mL/h, DP−: 166.7 mL/h) (p < 0.001). When analyzing urine parameters, the frequency of positive findings for urine leukocytes, urine casts, urine erythrocytes, and urine glucose was significantly higher in the DP+ cohort (Table 1). There were no significant differences regarding baseline demographic recipient characteristics between the DP+ and DP− groups (Table 2).

The combined endpoint of patient and graft survival, specifically the probability of event-free survival, did not differ significantly between both groups (DP+: 83.5% event-free survival; DP−: 85.5% event-free survival; p = 0.379) (Figure 3A). This indicates that DP did not negatively affect long term outcomes after KTX. In addition, patient survival was comparable (p = 0.124), with 89.8% for DP+ patients and 93.3% for DP− recipients (Figure 3B). There was an equally low probability of graft loss in both cohorts, with 9.0% in the DP− group and 7.9% in the DP+ group (p = 0.642) (Figure 3C). Therefore, the results suggest that neither long-term patient survival, nor long-term graft loss was impaired by DP.

When analyzing post-transplant renal function, it was observed that the DP− and DP+ cohorts had similar eGFR at 3, 6, 12, 24, and 36 months after KTX (Figure 4A). However, longitudinal analysis within each cohort revealed a significantly higher eGFR (compared to baseline) 6 months after KTX in the DP− cohort (p = 0.005). Additionally, in the DP+ cohort, the eGFR at 24 months after KTX was significantly higher than at the 3 months baseline (p = 0.010) (Figure 4).

The comparison of post-transplant UPCR revealed decreasing values for both groups over time. Both the DP− and DP+ cohorts showed a significant decrease in UPCR at 12, 24, and 36 months after KTX compared to the 3 months baseline (Figure 4B). In addition, the overall urine PE in the DP− group was significantly lower at 6, 12, 24, and 36 months compared to the baseline at 3 months. In contrast, the DP+ group showed a significant decrease in urine PE 12 months after KTX compared to the 3 months baseline. Overall, the DP+ group showed lower values for PE and UPCR compared to the DP− group, but these differences were not statistically significant (Figure 4C).

Analysis of secondary endpoints showed no significant difference between the DP+ and DP− cohorts for the incidence of DGF, PNF, biopsy-proven rejection, new onset of diabetes after transplantation, or cardiovascular events after transplantation (Table 3).

To explore independent donor-associated risk factors, univariate and multivariate Cox regression models were used for the following endpoints: event-free survival (including patient and graft survival), patient survival, graft survival, and marginal renal function (eGFR <30 mL/h/1.73 m²). Regarding event-free survival, both univariate and multivariate analyses showed no significant association with proteinuria, cold ischemia time, CPR, sCr at procurement, or diabetes mellitus (Table 4). However, donor age was found to be significantly associated with a reduced probability of event-free survival in both univariate analysis (HR: 1.05 [1.03–1.08], p < 0.001) and multivariate analysis (HR: 1.05 [1.03–1.08], p < 0.001) (Table 4). Donor age was also found to negatively affect patient survival (HR: 1.04 [1.01–1.06], p = 0.002) (Table 5), graft survival (HR: 1.04 [1.03–1.06], p < 0.001) (Table 6), and renal function (HR: 1.03 [1.02–1.05], p < 0.001) (Table 7), all in the multivariate analysis, respectively. In addition, hypertension was found to be associated with a reduced probability of event-free survival (HR: 1.92 [2.00–3.36], p = 0.022) in the univariate analysis (Table 4). It was also associated with reduced graft survival (HR: 1.62 [1.06–2.5], p = 0.025) (Table 6) and marginal renal function (HR: 1.60 [1.06–2.40], p = 0.025) (Table 7).

To further investigate whether the severity of DP would impact the outcome after KTX, a subgroup analysis was conducted within the DP+ group. When stratified for DP severity, the probability of event-free survival did not differ significantly among the DP ⁽⁺⁾, DP⁺, and DP⁺⁺ groups (83.7%, 84.5%, and 100%, respectively) (Figure 5A). However, the DP⁺⁺⁺ cohort showed a tendency towards decreased event-free survival compared to the other subgroups, although this difference remained statistically insignificant (50.0%, p = 0.151) (Figure 5A). In addition, the overall patient survival of the DP ⁽⁺⁾, DP⁺, and DP⁺⁺ cohorts were comparable (89.6%, 92.4%, and 100%) (Figure 5B). A significant decrease in overall patient survival was observed in the DP⁺⁺⁺ cohort compared to the other subgroups (50.0%, p = 0.017) (Figure 5B). The probability of graft loss was equally low in the DP ⁽⁺⁾, DP⁺, DP⁺⁺, and DP⁺⁺⁺ groups (p = 0.709) (Figure 5C).

With respect to the excretory renal parameters, a similar range was observed within the subgroups over time. The DP⁺⁺⁺ cohort showed an overall trend of reduced eGFR. However, this reduction was only significant 24 months after KTX (p < 0.0001). In addition, PE was increased in the DP⁺⁺⁺ group at three and 6 months after KTX (536.7 mg/gr Cr and median = 443.2 mg/gr Cr, respectively), but this increase did not reach statistical significance (Supplementary Figure S1). The analysis of UPCR showed relatively low parameters in the DP ⁽⁺⁾, DP⁺, and DP⁺⁺ groups.

Proteinuria is a well-described feature after KTX, but its impact on graft and patient outcomes remains uncertain, making it an undefined factor in the kidney allocation process. Therefore, this study investigated long-term outcomes in KTX patients, stratified based on donors with and without proteinuria. Additionally, the underlying investigation aimed to evaluate DP as a potential risk factor for post-transplant proteinuria. Proteinuria is known to be a prognostic factor for poor long-term outcomes, including reduced patient and graft survival as well as an increased risk for cardiovascular events after KTX [7]. This study established that within a 36 months post-transplant follow-up period, DP was not associated with impaired patient or graft survival or impaired graft function. Our results affirm the previous findings of Kuhn et al. [8], who demonstrated that there was no effect of DP in KTX on graft survival or function within 12 months after surgery [8].

It has been thoroughly established that KTX is associated with improved survival, reduced morbidity, and increased quality of life when compared to long-term dialysis [14]. However, KTX is facing an ever-growing obstacle due to the declining number of donated organs. Thus, the shortage of donors demands the optimal utilization of every potentially suitable organ. Critical assessment of donor organ quality in deceased donor KTX includes evaluating urine findings. Among the challenges of analyzing urine findings in deceased donors is that pathological findings may not always indicate preexisting chronic kidney disease. This is also true for proteinuria in donors, which could be caused by trauma, intense exercise, dehydration, fever, or a urinary tract infection. At the same time, DP could be the result of a glomerular disorder, including focal segmental glomerulosclerosis, glomerulonephritis, diabetic or hypertensive nephropathy, and vasculitis [15]. Thus, when DP is included in the decision-making process of donor selection, the involved surgeons and nephrologists are at risk of either declining a suitable graft or accepting a graft with structural kidney damage. To address this dilemma, this study aimed to investigate the impact of DP on outcomes after KTX. For this purpose, 578 patients were enrolled in the study and closely monitored at our interdisciplinary KTX clinic. As this investigation was conducted at a single center, we were able to utilize nearly complete datasets for analyzing the long-term effects.

Both donors and recipients showed similar baseline demographic variables in the DP+ and DP− groups. Nevertheless, some differences were observed with less favorable features, including a higher rate of ECD grafts in the DP− donor group compared to the DP+ cohort. On the other hand, DP− donors showed a lower BMI and a higher eGFR prior to KTX compared to DP+ donors. The higher rate of ECD kidneys in the DP− cohort may have influenced the results of this study in favor of DP+ donors. However, since the KDPI and KDRI were similar in both groups and the eGFR rates were initially higher in DP− grafts, the difference may be less significant.

DP was not associated with impaired event-free survival and did not affect patient survival or the likelihood of graft loss. This demonstrates that DP did not negatively affect long term outcomes after KTX and thus, transplantation of grafts from donors with low-grade DP is safe with regard to short- and long-term outcomes. In both the univariate and multivariate analyses of donor characteristics, DP was not identified as a risk factor for any of the three defined endpoints.

As indicated by both UPCR and PE parameters, DP was not associated with post-transplant proteinuria over time after KTX. Both UPCR and PE parameters concordantly decreased within both experimental groups over the observed time. However, the increment in PE development was stronger in the DP− group compared to the DP+ group. Nevertheless, DP was not associated with high PE and UPCR values. In fact, the DP+ group showed even smaller PE and UPCR values compared to the DP− group. Therefore, this study indicated that DP was not associated with post-transplant proteinuria after KTX. The analysis of eGFR in the DP+ group over 36 months showed significantly higher values at 12 months after KTX compared to the 3 months time point.

Concordantly with findings of previous studies, donor age was associated with a higher risk of impaired overall patient survival, death-censored graft survival, and event-free survival within this investigation [16, 17]. In addition, the underlying study confirmed that donor hypertension is a risk factor for impaired graft survival and event-free survival [18, 19]. According to current literature, these results show that higher donor ages and hypertension negatively affect overall patient survival [20]. Therefore, the validity of the underlying results can be assumed.

The semiquantitative measurement of proteinuria within this study, using urine dipsticks, undoubtedly represents one main limitation of this investigation. It is important to note that with proper quantification in the donor, one could better extrapolate how a high degree of UPE would impact post-transplant graft function. Furthermore, urine dipstick measurements should be interpreted with caution because they correlate poorly with the albumin-to-creatinine ratio (ACR), have low sensitivity and specificity, and have not yet been evaluated in renoprotective randomized controlled trials [19]. On the other hand, this method is used for kidney allocation, and specialized transplant surgeons select suitable kidney grafts based on semiquantitative measurements of proteinuria. Despite its drawback, dipstick analysis correlates with end-stage renal disease and is a widely used screening parameter [19]. In addition, this study confirmed recent findings on DP in KTX, even for a long-term period of up to 36 months after transplantation. Similar to the previous investigation on DP KTX [8], the underlying study did not investigate DP as a combined risk factor in ECD kidneys. Therefore, future investigations should outline the possibility that DP could be a combined risk factor in elderly donors (e.g., ≥60 years) with diabetes and nicotine abuse. In addition, it would be helpful to correlate the degree of DP with pre-transplant or implantation biopsies to better test the hypothesis that severe DP is indicative of structural kidney damage in the donor.

Interestingly, the subgroup analysis of the DP+ group revealed a significant decrease in overall patient survival in the group with high DP (p = 0.017). The results indicate an adverse effect of high-grade DP on long-term patient survival and are further supported by the observation of a reduced eGFR in the DP group. As stated earlier, proteinuria after KTX is a well-known risk factor for impaired graft survival [10]. This study indicates that donors with high levels of proteinuria might have an impact on the long-term graft performance in KTX.

However, our findings suggest that low-grade DP does not imply a risk of long-term complications or an influence on graft survival. Nevertheless, we highlight the need for further research on DP with respect to high proteinuria in quantitative urine measurements. Additionally, there is a need for further testing of donors at risk, particularly those who are older (e.g., ≥60 years) or have diabetes.

In conclusion, this retrospective cohort study of 587 patients investigated the impact of DP from a 36 months perspective. No effect on patient or graft survival was observed in low-grade DP. This indicates that transplantation of grafts from donors with low-grade DP is safe with regard to short- and long-term outcomes. Nevertheless, differences in the secondary endpoint analysis revealed a trend towards decreased patient survival and eGFR values in DP+ patients, especially in subgroups with severe proteinuria. Therefore, the underlying results suggest caution when allocating kidneys from donors with high levels of proteinuria.