The New Kidney Allocation System

Ever since deceased donor kidney transplantation became practical and accessible to all, several competing factors have shaped the kidney allocation system (KAS) in the US. On one hand, scientific progress has allowed vastly improved preservation techniques, and cross-matching has made it possible to increase the allograft half-life significantly. On the other hand, there continues to be a moral obligation to achieve equitability and fairness in organ allocation practices.

Over the past several years, other issues have become increasingly important: the demand for deceased donor kidneys continues to increase as the supply remains at a plateau, the organ discard rate remains unacceptably high, and more already-treated patients are returning to the list for repeat transplantation. Furthermore, very highly sensitized patients (with preformed anti-HLA antibodies) are harder to match and tend to have very much longer waiting times. Over the past two decades, several minor changes were made to the KAS (e.g., removing HLA-B antigen matching in the match run) to address these issues, but the most significant new KAS was implemented by the Organ Procurement and Transplantation Network/United Network of Organ sharing (OPTN/UNOS) in December 2014.

The overarching goals for the new KAS are as follows:

  • Increase the life-years gained from each organ by matching donors and recipients on the basis of their health risk profiles. This is made possible by assigning an estimated posttransplant survival (EPTS) score to a recipient and matching it to the kidney donor profile index (KDPI) of the donor (Tables 1 and 2). This allows allocation of the best quality kidneys to the recipients with the highest predicted longevity.
  • Increase the chance for transplantation for highly sensitized patients (high calculated panel reactive antibodies [CPRA]). This is made possible by expanding the geographic area for organ sharing, allowing these patients to have access to more potential donors.
  • Improve procurement of organs from extended criteria donors that could potentially be used for patients with high (suboptimal) EPTS.
  • Decrease the organ discard rate of kidneys that were not used despite being procured for transplantation.
  • Standardize the waiting times: Patients with delays in evaluation for transplantation are to have their waiting time default to the dialysis initiation date.
  • Allow transplantation of blood type A2/A2B donor kidneys into B blood type recipients, who are considered to be at low risk for acute rejection.
  • Reduce the risk of listing for a second transplantation.

Table 1

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Table 2

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The OPTN/UNOS kidney transplantation committee released the early results of KAS in June 2015 and will continue to report detailed analysis 1 year and 2 years from implementation. Most of the observed transplantation trends were in keeping with the expectations of the KAS, but there were a few exceptions. The total number of transplantations and the number of patients added to the wait list remained stable over the 6 months after implementation of KAS.

Following are the four areas of significant gains from KAS:

  • A sixfold increase in transplantations for patients with the highest CPRA of 99 percent to 100 percent, from 2.5 percent to 13.5 percent.
  • An increase in nonlocal transplantations from 21 percent to 33 percent, indicating that more kidneys are being shared outside of the local area.
  • Thirty-eight percent of African Americans received transplants compared with 32 percent before the new KAS, whereas the percentage of these patients on the wait list remains the same. The increase in African Americans receiving transplants under KAS was statistically significant. Credit given for dialysis duration before wait list registration was likely the main contributing factor to this increase.
  • The proportion of longevity-mismatched transplants, defined as age difference between the donor and the recipient of more than 15 years, has decreased from 50 percent to 48 percent, as did the the proportion of high KDPI transplants to low EPTS candidates (3 percent to 1 percent).

A few unexpected trends to watch were also noted:

  • A significant drop in the zero-mismatch transplantations from 8 percent to 4.5 percent, probably because of the increased priority given to high CPRA patients
  • A higher organ discard rate of 20.3 percent compared with the pre-KAS era rate of 18.5 percent.

Overall, it is difficult to predict which of these early observations will be sustained over time. Because of the significant emphasis placed on equitability in allocation and increased organ use, most transplant recipients now have either more HLA mismatches or pre-existing anti-HLA antibodies. Whether this will lead to increased acute rejection or chronic alloantibody-mediated allograft injury remains to be seen. In other words, the gains made in terms of the equity and increased life-years of the allograft need to be significant in comparison with the downside of more transplantations with higher immunologic risk and higher dialysis vintage for the new KAS to be justified.