It is well established that kidney transplantation is the treatment of choice for patients with end stage renal disease (ESRD), as this treatment modality has been shown to provide improved patient survival and quality of life compared with dialysis (1). In an ideal system, patients in need of a kidney transplant would receive one as soon as the need arises. Unfortunately, the well-described mismatch between a limited number of available organs and the larger number of patients in need of a transplant makes this impossible, necessitating policies for the allocation of this limited resource. Available organs had been, until recently, allocated to potential recipients based on an algorithm that primarily weighed “time on the waitlist from the moment of listing.”
Under this system there were several limitations: The system did not take into account matching organs and recipients for graft longevity posttransplant, leading to a significant re-transplant rate. There was variability in access to transplantation depending on level of anti-HLA antibody sensitization, blood type, and geographical location. And there was a higher than desired organ discard rate leading to underutilization of kidneys that could have been potentially transplanted.
To address these limitations and to increase the efficiency of kidney allocation, the United Network for Organ Sharing (UNOS) and the Organ Procurement and Transplantation Network (OPTN) implemented a new kidney allocation system (KAS) in December 2014.
The KAS incorporated new parameters to allocation algorithms, previously described in Kidney News (2). Major changes included:
■ The adoption of new donor and recipient quality metrics: Kidney Donor Profile Index (KDPI) and Estimated Post Transplant Survival (EPTS). These metrics are the basis for longevity matching between recipients and donors: Donors with EPTS scores of 0–20% are prioritized for kidneys with a KDPI of less than or equal to 20%.
■ Increasing the priority assigned to highly sensitized recipients (as defined by calculated panel reactive antibody [cPRA] score). The KAS assigns points on the basis of a sliding scale points system for cPRA. Whereas previously candidates received an absolute 4 additional points for a cPRA at or above 80% KAS, candidates now receive approximately 4 points at a cPRA of 85 to 89 and rapidly increase afterward. Candidates with cPRA scores of 99% and 100% receive 50 and 202 points, respectively.
■ Modifying the blood type eligibility of candidates with blood types associated with longer wait times by allowing ABO type non-A1 and non-A1B kidneys to be allocated to type B candidates.
■ Modification of the waiting time calculation by adding the pre-registration dialysis time into a candidate’s waiting time.
December 2017 marked the three-year anniversary of the new KAS. Reports have been published describing six- and 12-month outcomes analyzing incident transplant changes pre- and post-KAS implementation. The two-year KAS implementation data were reported during the summer of 2017. This report included new data points not available in earlier reports, including stratified delayed graft function (DGF) rates, one-year survival outcomes, and re-listing rates. The two-year data also revealed the development of interesting trends and patterns (3).
■ Among the encouraging trends, data show that pre-KAS differences in the rates at which African American, Hispanic, and Caucasian transplant candidates received kidneys from deceased donors have attenuated post-KAS; hence the percentage of kidney transplants performed by recipient ethnicity now reflects the ethnic composition of candidates on the waitlist. Longevity matching continues to function as designed; over half (56%) of EPTS 0–20% adult recipients received a KDPI 0–20% kidney, while only 1% received a KDPI 86–100% kidney. There is a continued increase in the number of blood type A2/A2B subtype to blood type B recipients (0.2% pre-KAS vs. 1.4% post-KAS). Finally, although not statistically significant, re-listing rates within one year of transplant decreased from 1.64% to 1.38%.
■ There appears to be an attenuation of the “bolus effects” initially observed in candidates who received increased transplant priority under KAS (highly sensitized candidates and long dialysis times prior to listing). Prior to KAS, the percentage of transplants to candidates with greater than 10 years of dialysis was 4.5%, sharply increasing to 18.6% immediately after implementation. Newer data show that these rates have decreased substantially and appear to have leveled off at approximately 6%. Likewise, the increased rates of DGF initially observed immediately after KAS have improved. Prior to KAS the rate of DGF was 24.4%, rising to 29.6% 1-year post-KAS, and subsequently decreasing to 27.7% 2 years post-KAS.
■ Unfortunately, the kidney discard rate post-KAS remains higher than prior to implementation. As expected, there is an association between higher KDPI scores and discard rates: 3% of kidneys with a KDPI of 0 to 20 are discarded vs. 60% of kidneys with a KDPI between 86 and 100. Although overall one-year patient and graft survival remain very high post-KAS, two-year data show a slight decrease compared with pre-KAS. The underlying etiology for these observations is equivocal, but may be a sequel to the earlier observed bolus effect. Longer outcomes data may aid in elucidating this observation.
Based on limited data this new report is certainly compelling, but it begs the question, “Has the new KAS benefited patients?” This question is particularly contentious because when the allocation criteria to a limited resource are modified, the waiting time might become shorter for some patients and longer for others. The Equity in Access Report released in August 2017 may provide some insight to help answer this question (4). This report relies on a recently developed metric, Access-to-Transplant Score (ATS), a numerical measure developed to quantify the variability in expected waiting times for receiving a deceased donor kidney transplant among waitlisted patients. After implementation of KAS, the overall ATS among waitlisted candidates has not just decreased, but also remained relatively stable suggesting that KAS has improved equity in access to deceased donor kidney transplants. Long-term data will provide further clarification to this question.
Wolfe RA, et al.. Comparison of mortality in all patients on dialysis, patients on dialysis awaiting transplantation, and recipients of a first cadaveric transplant. N Engl J Med 1999; 341:1725–30.