Kidney transplant from donors with expanded criteria (ECD) is under debate for two reasons: (1) the imbalance between the available organs and patients on the waiting list for kidney transplantation and (2) the changes experienced in the donor's profile. Hemorrhagic or ischemic stroke has become the leading cause of death, this is associated to an increase in both the mean age and the risk factors of cadaveric donors.1 Using the UNOS 20012 criteria for the selection of donors, more than 50% of the donors generated annually in Spain would be labeled as ECD donors, which sometimes requires preimplant biopsy in order to assess the viability of the graft.

In the last decade, several scales have been developed to measure the prognosis of the graft trying to eliminate the dichotomy of standard donor (SCD) versus the ECD donor. The idea is to perform an individualized assessment of graft quality and graft survival, considering the characteristics of both donor and recipient. None of these scales has been validated for prediction of graft function in ECD donors in the different countries.3 The Kidney donor risk index (KDRI)4 and its adaptation to the Kidney donor risk profile (KDPI)5 have been developed by the American Registry of Transplants. The KDRI calculates the relative risk of graft failure and the grades are from 0.5 and 3.5. The KDPI score are from 0 to 100 points with a table of assigned values to the score obtained from the donor parameters of the KDRI score, in which a score of 85 means that 85% of the donors are of better quality. Both scores have been associated to the survival of the transplanted organ.

In Spain, ECD grafts are evaluated by preimplantation biopsy. In Andalusia, the biopsy is performed as part of the transplant protocol in patients with UNOS criteria and it is evaluated following the Andalusian protocol for the evaluation of renal biopsy.6 Currently there is no validated clinical tool to make decision beyond the UNOS 2001 criteria. However, recently the KDPI score has been proposed as an alternative to preimplantation renal biopsy.

Objectives of the present study are:

• 1.

  To evaluate the correlation between the KDPI and histological scores (preimplantation renal biopsy) in ECD donors.

• 2.

  To assess the relationship between KDRI, KDPI, histological score and graft survival.

In our cohort of ECD patient, we found a moderate correlation between KDPI score and the preimplantation renal biopsy. Both the KDPI and KDRI scores are related to survival and both are potentially valid for donor selection in the ECD graft group.

The utility of the KDPI and the preimplantation biopsy to assess grafts from ECD donors are not established. In the present study, the KDPI score and the preimplantation renal biopsy show a moderate concordance. The mean KDPI score in this study was 1.47/86%: it is above 85%, which is the cut-off point for recommendation of biopsy in other countries10 since the percentage of discarded organs could reach 38% and exceed 60% for scores of 90 or higher.11 The percentage of discarded grafts in our cohort of patients was 10, 26 and 16% for KDPI values lower than 80, between 80 and 90, and greater than 90, respectively. Gandofini et al.11 performed a study with similar design in a large cohort of patients. The preimplantation biopsy allowed to reduce the percentage of organs that would have been discarded in KDPI scores over 80 and 90, with excellent survival results. These results make it unwise to use the KDPI in our setting as the only tool to accept or reject a kidney graft, as Pascual and Pérez state in a recent editorial in this journal12 and as other authors have proposed in other countries.13

We agree that the value of the biopsy has been overestimated and the time has come to reach a balance between clinical and histological parameters, optimizing the resources without increasing the percentage of discarded organs. The histological scores combined with KDPI have advantages over the classical SCD/ECD division, however it is necessary to establish a cut-off point to indicate the preimplantation biopsy that allows individualized decisions about the donors. We find it unlikely that a clinical trial could be conducted to assess the usefulness of the biopsy as proposed by other authors,14 given the ethical problems that could be posed by the implantation of all grafts after being aware of the biopsy results.

In our study, the KDPI and KDRI values are correlated with graft survival. This relationship has been described by others.4,5,15,16 In our cohort, a 0.1 point increase in the KDRI score increases the risk of graft failure by 9.5% per year. As far as the KDPI score, an increase in 1 point is associated with a decreased survival of 2.5% per year. The predictive power of KDRI to estimate survival at 5 years is 64%, superior to that of KDPI, and it is identical to the values published by Rao et al.4 In our study the predictive power of the KDPI score is low, although its predictive capacity could be comparable, since acute rejection is a confounding variable in the relationship KDPI and graft survival. In any case, performing the biopsy causes a delay in the transplant and allows to have all the data from the recipients, so that the calculation of the complete KDRI score (including the data of potential recipients) can be accomplished which improves the predictive value. The information from the biopsy plus the KDRI score in grafts of higher KDPI score (and supposedly of worse quality) would allow to select the ideal recipient with greater possibilities of survival for each graft. Other articles17 have shown a significant association between KDRI and 4-year follow-up graft survival in a cohort of patients with short cold ischemia; this confirm that KDRI is a useful tool for assessing the donor ECD, with independence of the period of cold ischemia. The hazard ratios (HR) found in our study are very high and have a very wide confidence interval (CI). The elevation of one point in the scores increases the risk of graft failure between 23 and 25 times. In clinical practice the graft score is usually distributed in 0.1 point changes, which means HR between 2.3 and 2.53 per 0.1 points per year. The high IC is due to the sample size and the large number of variables that are introduced in each of the scores. By dichotomizing the variables, the magnitude of the HR and the CIs is reduced, which gives validity to our model.

The presence of renal lesions in the donor's biopsy is associated with acute rejection and worse kidney function and reduced survival.18–22 Our group has demonstrated the validity of the renal biopsy to identify the presence of histopathological lesions that condition a worse outcome.23 In a recent study, the biopsy score correlated with renal function during the first year after transplantation.24 However, in the present work we have not been able to demonstrate a relationship between the biopsy score and graft survival. This result should be interpreted adequately, since the donor biopsy was used to select the suitability of the organ, eliminating grafts with high scores, which makes difficult to compare survival. By contrast, Han et al.17 have shown the relationship between biopsy score on graft survival, although their cohort of patients had not been selected by biopsy and the histological score was obtained a posterior with a score distribution very different from ours. The majority of the grafts analyzed by Han et al.17 have a score of 0, which corresponds to grafts from standard donors. A comparison of survival of score of 0 versus score greater than 0, is really a comparison between survival of ECD donors against donors without expanded criteria. In addition, the proportion of ECD patient was 11%, which is very different from the distribution of donors in Spain.

Although our study has limitations because it is retrospective and from a single center, it also has some strengths. The volume of patients is relatively high considering that it is a single-center study and the follow-up period is much longer than the existing in the literature. It focuses on the analysis a concordance between the biopsy findings and the KDPI score, which could help the different renal transplant groups to select a cut-off point of KDPI to indicate the performance of renal biopsy as a complementary tool. In addition, the being performed in a single center can be considered a strength, since both the biopsy assessment and histological processing have been performed by a homogeneous group of experts in the assessment of preimplantation biopsies, which minimizes interobserver variability. In fact, a recent study24 has shown the importance of interobserver variability, as well as the superiority of paraffin processing in this type of studies, especially for organs with higher scores in preimplant biopsy.

In conclusion, (1) the KDPI and histological scores (preimplantation biopsy) show a moderate concordance. The value of both scores combined is to be determined. It is necessary to carry out prospective studies to select an adequate KDPI cut-off point, although with the accumulated evidence it could be reasonable to set a KDPI score between 85 and 91; (2) the KDPI scores, and especially KDRI, are valid to estimate the survival of the grafts and can be used in combination with the biopsy to make decisions in the graft group from ECD.

The authors declare that they have no conflicts of interest.