INTRODUCTION — Organ sharing is a nationwide system by which deceased-donor kidneys are allocated to potential recipients. The deceased-donor kidney allocation policy of the United Network for Organ Sharing (UNOS)/Organ Procurement and Transplant Network (OPTN) was extensively revised and implemented December 4, 2014 and is presented in this topic review.
The evaluation of potential transplant recipients, including the optimal timing of referral, and issues related to the transplant waiting list, as well as live kidney paired donation, are discussed elsewhere:
●(See "Kidney transplantation in adults: Evaluation of the potential kidney transplant recipient".)
●(See "Kidney transplantation in adults: The kidney transplant waiting list in the United States".)
●(See "Kidney transplantation in adults: Kidney paired donation".)
NATIONAL DECEASED-DONOR KIDNEY ALLOCATION POLICY
Overview — A national kidney allocation policy was established in the United States in 1987 for the purpose of the equitable and utilitarian distribution of deceased-donor kidneys.
The policy was initially devised and agreed upon by the transplant programs that comprise the United Network for Organ Sharing (UNOS)/Organ Procurement and Transplant Network (OPTN).
A revised UNOS/OPTN policy was implemented in 2014 in order to increase the utilization of available kidneys, reduce regional variability in access to transplantation, and improve the outcomes for individual kidneys that are transplanted [1].
According to this policy, the following parameters are considered whenever a kidney donor is entered into the matching system:
●The estimated posttransplant survival (EPTS) score estimates for each waitlisted candidate >18 years of age the number of years of potential benefit from a kidney transplant.
●The kidney donor profile index (KDPI) incorporates more information about the quality of the donor and classifies kidneys on the basis of a clinical formula that estimates how long the kidney is likely to function once transplanted. A KDPI calculator is provided by OPTN (figure 1) [2].
●Additional priority is given to candidates with less common blood types (that is, type B and type O) and high immune system sensitivity (estimated by the calculated panel reactive antibody [CPRA] score) since such candidates have fewer opportunities for transplantation.
●As previously, candidates are prioritized based upon waiting-list time.
In general, kidneys are allocated locally first, then regionally, and then nationally, although exceptions are made for zero-mismatched kidneys and for highly sensitized candidates. (See 'Mandatory sharing' below.)
The allocation policy is discussed in more detail below.
Estimated posttransplant survival (EPTS) score — The EPTS score estimates for each waitlisted candidate >18 years of age the number of years of benefit from a kidney transplant. The EPTS score is based upon candidate age, time on dialysis, absence or presence of diabetes, and history of prior solid organ transplant. The score is calculated for each candidate upon registration on the waiting list. The score is updated daily and any time the transplant hospital reports a change in any EPTS factor.
Twenty percent of kidneys that are estimated to function the longest are allocated to candidates that are expected to need the allograft for the longest time.
Kidney donor profile index (KDPI) — The KDPI is derived from the kidney donor risk index (KDRI) score and is the percentage of donors in a reference population that have a KDRI score less than or equal to the donor's KDRI score.
The KDRI estimates the relative risk of posttransplant graft failure and is calculated from the donor age, ethnicity, creatinine, history of hypertension or diabetes, cause of death, height, weight, hepatitis C virus (HCV) status, and whether the kidney was donated after circulatory death. The reference population is reviewed annually by the OPTN Kidney Transplantation Committee. The 15 percent of kidneys with the highest KDPI scores, estimated to have the shortest potential lifespan, are offered over a wider geographic area. It is expected that this feature will increase the utilization of donated kidneys and minimize differences in local transplant waiting times. The policy does not affect the decision process between the potential recipient and his/her transplant team; potential recipients may still turn down kidneys that are deemed unsuitable. Transplant programs must have informed consent from potential recipients in order to accept kidneys with a KDPI score >85 percent (figure 1).
Waiting time — Candidates are ranked both by score and by the date and time of registration on the waiting list. Candidates who have waited the longest have a higher priority compared with those with shorter waiting times. (See "Kidney transplantation in adults: The kidney transplant waiting list in the United States".)
For candidates ≥18 years old, the calculation of waiting-list time is based upon the earliest of the following:
●The registration date with measured or calculated creatinine clearance ≤20 mL/min
●The date after registration (ie, at which the measured or calculated creatinine clearance ≤20 mL/min)
●The date that the candidate starts dialysis
For candidates <18 years old, waiting-list time begins either the date of registration or the date of initiation of dialysis, whichever is earliest.
Predialysis patients who are referred for transplantation with a glomerular filtration rate (GFR) that is already ≤20 mL/min are not retroactively assigned waiting time to the date that their GFR became ≤20 mL/min. As a result, early referral of predialysis patients for transplant is important in order to maximize their waiting time.
Blood type — Kidneys are allocated based upon blood type as follows:
●Type O kidneys may be given to type O candidates unless there is zero-human leukocyte antigen (HLA) mismatch, in which case kidneys may go to any type blood.
●Type A kidneys may be given to type A or AB candidates.
●Type B kidneys may be given to type B candidates, unless there is zero-HLA mismatch, in which case kidneys may be given to other blood type candidates.
●Type AB kidneys may be given to type AB candidates.
●Type non-A1 and non-A1B kidneys may be given to type B candidates, provided that the type B candidate provides written, informed consent and the transplant center has an established written policy regarding acceptance of non-A1 and non-A1B kidneys by type B candidates. Specific candidate eligibility must be confirmed every 90 days.
Sensitized candidates — Highly sensitized waiting-list candidates are awarded incremental points based upon the degree of sensitization. The degree of sensitization is defined by the candidate's unacceptable antigens, reported at the time of listing. Unacceptable antigens are HLA antigens on a donor kidney to which a potential candidate has antibodies. Antibodies are determined using at least one solid-phase immunoassay (eg, enzyme-linked immunosorbent assay [ELISA] or Luminex using purified HLA molecules) and expressed as the CPRA. Each candidate is assigned a CPRA score, which expresses the percentage of deceased donors in the donor pool expected to have one or more of the unacceptable antigens. The CPRA is derived from HLA antigen/allele group and haplotype frequencies for the different racial/ethnic groups in proportion to their representation in the national deceased-donor population.
Each center may define the criteria for unacceptable antigens that are considered as contraindications for transplantation. It is the prerogative of the transplant center to establish criteria for additional unacceptable antigens, such as repeat transplant mismatches. A CPRA will be calculated automatically when the unacceptable antigens are listed or updated on the waiting list.
For candidates with CPRA >99 or 100 percent, written approval of unacceptable antigens must be provided by the HLA laboratory director and the transplant physician and surgeon.
Mandatory sharing — Zero-mismatched kidneys and kidneys for candidates with CPRA >99 percent must be shared nationally. A zero-antigen mismatch occurs when the patient and donor have zero nonequivalent HLA-A, B, and DR antigens and compatible blood types. Such kidneys are shared because allograft survival has been shown to be superior to all other match grades [3-10].
Double kidney allocation — Kidneys from adult donors must be offered singly, unless the donor meets at least two of the following conditions and the Organ Procurement Organization (OPO) would not otherwise use the kidneys singly:
●Donor age greater than 60 years
●Estimated donor creatinine clearance <65 mL/min based upon serum creatinine concentration upon admission
●Rising serum creatinine concentration (>2.5 mg/dL [221 micromol/L]) at time of retrieval
●History of either longstanding hypertension or diabetes mellitus in the donor
●Glomerulosclerosis greater than 15 and less than 50 percent
Medical urgency — No points are assigned to patients based upon medical urgency for national allocation of kidneys. Locally, the patient's physician has the authority to use medical judgment in assignment of medical urgency points if there is only one kidney transplant center. When there is more than one local kidney transplant center, a cooperative medical decision is required prior to assignment of medical urgency points.
IMPACT OF REVISED POLICY — A few months after the revised policy was implemented, the following trends in kidney allocation were observed [11]:
●There was a sixfold increase in the number of transplants for highly sensitized candidates (defined as calculated panel reactive antibody [CPRA] >99 percent).
●There was an increase in nonlocal transplants from 20 to 30 percent.
●There was a decrease in the number of age-mismatched kidneys (defined as donor/recipient age difference >15 years) from 50 to 44 percent.
●There was an increase in transplants for candidates ages 18 to 49 years and a decrease in transplants for candidates >50 years.
●There was a decrease in pediatric transplants from 5 to 3.6 percent.
●There was a decrease in transplantation of zero-mismatched kidneys from 8 to 5 percent.
Subsequent studies have further assessed the impact of the revised kidney allocation system (KAS) on organ distribution. As examples:
●One study that compared Organ Procurement and Transplantation Network (OPTN) data one year before and after implementation of the revised KAS found a 23 percent reduction in transplants in which the donor and recipient age differed by more than 30 years [12]. There was an initial sharp increase in transplants for recipients with a CPRA of 99 to 100 percent and those with at least 10 years on dialysis, followed by a tapering of transplants to these groups suggesting a bolus effect. Kidneys were more frequently shipped long distance, with a consequent increase in cold ischemia times. Although higher delayed graft function rates were noted, six-month graft survival rates were unchanged.
●A second study compared Scientific Registry of Transplant Recipients (SRTR) data for the two years pre-KAS with data for the first nine months post-KAS [13]. Key findings included an increase in both regional (12.5 percent post-KAS versus 8.8 percent pre-KAS) and national (19.1 percent post-KAS versus 12.7 percent pre-KAS) imports. The proportion of recipients >30 years older than their donor decreased from 19 to 15 percent, while the proportion of recipients with a CPRA of 100 percent increased from 1 to 10 percent. Although the overall rate of deceased-donor kidney transplants (DDKTs) did not change, DDKT rates increased for Black transplant candidates and Hispanic transplant candidates, as well as for candidates aged 18 to 40 years; DDKT rates decreased for candidates aged >50 years. Rates of delayed graft function increased from 25 percent pre-KAS to 30 percent post-KAS.
PROPOSED CHANGES TO KIDNEY ALLOCATION — The United Network for Organ Sharing (UNOS) will implement substantive changes to the way that organs are allocated nationally toward the end of 2020. This change has triggered the need to bring organ allocation into full alignment with the Final Rule, which requires that available organs for transplantation be medically prioritized among potential recipients regardless of geographic location. The goal is to ensure that policy is legal, equitable, maximizes lives saved, and minimizes the effect of geography [14]. The change will distribute kidneys within 250 nautical miles of the donor hospital and assigns points based upon recipient proximity.
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Kidney transplantation".)
SUMMARY
●Organ sharing is a nationwide system by which deceased-donor kidneys are allocated to potential recipients based upon human leukocyte antigen (HLA) matching between donor and recipient as well as multiple other donor kidney and recipient characteristics. A national kidney allocation policy was established in the United States in 1987 for the purpose of the equitable and utilitarian distribution of deceased-donor kidneys. A revised United Network for Organ Sharing (UNOS)/Organ Procurement and Transplant Network (OPTN) policy was implemented in 2014 to increase the utilization of available kidneys and maximize the number of years that individual recipients may have a functioning allograft. (See 'Introduction' above and 'Overview' above.)
●The allocation of kidneys is based upon donor kidney and candidate characteristics and on the candidate's waiting-list time. The estimated posttransplant survival (EPTS) score estimates the number of years of benefit from a kidney transplant, and the kidney donor profile index (KDPI) classifies kidneys according to their predicted survival once transplanted. Priority is also accorded to candidates with less common blood types and high immune system sensitivity since such candidates have fewer opportunities for transplantation. (See 'Overview' above.)
●The EPTS score is based upon candidate age, time on dialysis, absence or presence of diabetes, and history of prior solid organ transplant and is calculated for each candidate upon registration on waiting list. (See 'Estimated posttransplant survival (EPTS) score' above.)
●The KDPI is derived from the kidney donor risk index (KDRI) score, which is the percentage of donors in a reference population that have a KDRI score less than or equal to the donor's KDRI score. The KDRI is calculated from the donor characteristics. (See 'Kidney donor profile index (KDPI)' above.)
●For candidates ≥18 years old, the calculation of waiting-list time is based upon the earliest of the following:
•The registration date with measured or calculated creatinine clearance ≤20 mL/min.
•The date of registration (ie, at which the measured or calculated creatinine clearance ≤20 mL/min).
•The date that the candidate starts dialysis.
•For candidates <18 years old, waiting-list time begins either the date of registration or the date of initiation of dialysis, whichever is earliest. (See 'Waiting time' above.)
●Kidneys are allocated based upon blood type first. (See 'Blood type' above.)
●Highly sensitized waiting-list candidates are awarded points based upon the degree of sensitization, which is reported for each candidate at the time of listing. Unacceptable antigens are HLA antigens on a donor kidney to which a potential candidate has antibodies that preclude transplantation. Each center may define the criteria for unacceptable antigens that are considered as contraindications for transplantation. (See 'Sensitized candidates' above.)
●Zero-mismatched kidneys and kidneys for candidates with a calculated panel reactive antibody (CPRA) >99 percent must be shared nationally. A zero-antigen mismatch occurs when the patient and donor have zero nonequivalent HLA-A, B, and DR antigens and compatible blood types. Such kidneys are shared because allograft survival has been shown to be superior to all other match grades. (See 'Mandatory sharing' above.)
●Kidneys from adult donors are usually offered singly, except in defined circumstances. (See 'Double kidney allocation' above.)
●No priority is assigned on the basis of medical urgency for national allocation, although such candidates may be prioritized locally if there is only one kidney transplant center. (See 'Medical urgency' above.)
ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges William M Bennett, MD, who contributed to an earlier version of this topic review.
