Deceased Donor Organ Allocation

1. National Framework: OPOs and UNOS/OPTN Roles

2. Donor Organ Quality and Scoring Systems

• Kidney Donor Profile Index (KDPI): The most widely used donor quality metric is the KDPI for kidneys. KDPI is a percentile score (0–100%) that combines multiple donor factors (e.g. age, kidney function, medical comorbidities, cause of death) into a single number representing the expected risk of graft failure relative to other kidneys (). A lower KDPI indicates a kidney likely to function longer, whereas a higher KDPI (e.g. ≥85%) suggests shorter expected graft survival (). Kidneys with very high KDPI are considered “extended criteria” and may be allocated to candidates who are older or have lower expected post-transplant survival, to ensure organ utility is maximized. KDPI is used during allocation to match longevity of the organ with the recipient’s needs – for example, policy pairs the best-quality 20% of kidneys with patients who have the highest Estimated Post-Transplant Survival (EPTS) scores (top 20%) (). In practice, transplant teams use KDPI to decide whether to accept an organ; a high-KDPI kidney might be declined for a young patient but accepted for an older patient who can benefit from a shorter-term graft.
• Liver Donor Risk Index (DRI): For livers, a Donor Risk Index was formulated (Feng et al., 2006) to quantify the relative risk of graft failure based on donor factors such as age, cause of death (e.g. stroke), organ quality, donation after cardiac death (DCD), and partial grafts. The DRI provides a single risk score where >1.0 indicates higher risk than an “average” liver donor (). This index has given the transplant community a common language to describe liver donor quality and is used in research and center decision-making () (). For instance, an older donor liver with DCD status will have a higher DRI, signaling a higher chance of early failure. However, DRI is not explicitly used in the OPTN liver allocation algorithm (unlike KDPI for kidneys) (). Instead, centers consider DRI alongside recipient factors when deciding to accept an organ. The concept of extended criteria donors is also applied in liver allocation – organs from older donors, DCD donors, or those with certain medical history may be labeled as higher-risk; these organs are often offered to centers willing to accept more risk, sometimes after being turned down by others.
• Heart and Lung Donor Assessment: Unlike kidney and liver, there are currently no standardized, widely-used numeric donor quality indices for heart or lung donors (). Evaluation of heart and lung donor organs is more qualitative, based on clinical parameters and donor history. Important heart donor factors include donor age, heart function (ejection fraction, coronary anatomy), ischemic time, and any history of cardiac disease. Research efforts have proposed scores (e.g. a Heart Donor Risk Index or Heart Donor Score) incorporating factors like donor age, left ventricular function, and ischemia time, which correlate with post-transplant survival (). For example, one study identified longer cold ischemia, older donor age, donor-recipient size mismatch, and elevated donor serum creatinine/BUN as risk factors for heart transplant outcomes (). In practice, hearts from donors over a certain age or with suboptimal function might be considered “marginal” and are carefully assessed via echocardiography or even transplanted with caution. Similarly, lung donor quality is judged by factors such as the donor’s smoking history, chest X-ray, blood oxygenation (PaO₂/FiO₂ ratio), lung compliance, and presence of injury or infection. Extended criteria lung donors (for example, older than 55, heavy smoker, or some lung damage) may still be used due to organ scarcity but typically only for recipients in dire need. While no single lung donor index is universally used, transplant teams heavily weigh the donor’s oxygenation efficiency and lung imaging – a donor lung with poor oxygenation or infiltrates may be declined unless the recipient’s need is critical.
• Infectious and Safety Considerations: Another aspect of donor organ “quality” is the risk of disease transmission. Organs from donors with certain behavioral or medical risk factors (e.g. IV drug use, active viral infections) may be classified by CDC as increased risk donors. These organs can be lifesaving and are often of good functional quality, but they carry a small risk of transmitting infections (like HIV, hepatitis B or C). With modern screening and treatments (e.g. using hepatitis C positive donors for negative recipients and treating post-transplant), use of these organs has increased. While not a quality “score,” centers must balance the urgency of a recipient’s need against these potential risks when accepting such organs.

3. Allocation Process: Match Runs and Recipient Prioritization

• Medical Urgency: How sick the patient is or how immediate their need is. For example, liver candidates are prioritized by the MELD score (a continuous lab value score where higher = more urgent) or status 1 for fulminant failure, heart candidates by urgency status (1–6, with 1 being most critical), and lung candidates historically by the Lung Allocation Score (LAS). Urgency ensures the sickest patients (or those who would die soon without transplant) get priority.
• Waiting Time: To promote fairness, time accrued on the waiting list is factored in, especially when urgency differences are small. For kidney transplantation, waiting time (including time on dialysis even before listing) is a major component of priority (). Longer waiting time can elevate a candidate’s rank if other factors are equal. In the heart and lung systems, waiting time may be less emphasized compared to urgency status or LAS, but within each status tier, time waiting can act as a tiebreaker.
• Donor-Recipient Compatibility: This includes blood type compatibility (ABO matching) and body size matching. Organs are generally offered only to candidates with a compatible blood type (or acceptable incompatibility in special cases). Size matching is critical for thoracic organs – a donor heart or lung must fit in the recipient’s chest, so the system often matches donors and recipients of similar size (e.g., based on weight or height). HLA tissue matching is also considered for kidney allocation; while broad HLA matching is rare, kidney candidates who happen to match at key HLA loci with a donor can receive priority points because better HLA matches improve long-term graft survival (). Pediatric candidates get priority for some organ offers (for instance, pediatric hearts are often offered first to other children due to size and developmental considerations, and kidneys from young donors are preferentially offered to pediatric recipients) – this is both for size compatibility and ethical priority to children.
• Sensitization and Immune Compatibility: Highly sensitized patients (those with strong immunologic reactivity to most donors, often measured by Panel Reactive Antibody % or CPRA) have a harder time finding a compatible organ. The allocation system awards additional priority to highly sensitized candidates to improve their chances of an offer. In kidney allocation, for example, candidates with CPRA above 80% receive incremental allocation points on a sliding scale – up to a maximum boost equivalent to several years of waiting time for 100% CPRA (). This helps balance equity by ensuring patients who are difficult to match immunologically aren’t passed over simply due to their antibody profiles. Similar considerations apply in heart and lung allocation (sensitized heart candidates can be listed with priority in some cases, and lung allocation now factors sensitization into the score as well).
• Geography (Distance): The distance between the donor hospital and the candidate’s transplant center is considered because of the practical limits of organ preservation time and transport. In general, candidates closer to the donor are given some advantage in the match run, to minimize organ travel time. This was historically handled by geographic zones (local vs regional vs national offers). Modern policy is moving toward continuous distance-based scoring, but still, distance remains a key factor – a nearby candidate might outrank an identical candidate across the country, especially for hearts or lungs. We discuss geography in detail below, but in summary, each organ type has a threshold within which organs are first offered locally/regionally before broader distribution.

4. Cold Ischemia Time and Organ Viability

• Heart: Ideally transplanted within 4–6 hours of procurement (). Hearts are the most sensitive to ischemia; prolonged time on ice can lead to primary graft failure. Because of this, heart allocation is very time-and-distance sensitive – hearts are offered first to nearby status 1 patients to minimize transit time. In practice, many heart transplants are done within a few hundred miles of the donor. If a heart must travel longer distances, teams may use chartered flights to keep total ischemia time under the ~4-6 hour window. Ex vivo heart perfusion devices (e.g. “heart in a box” systems) are an emerging technology that can extend safe preservation time by continuously perfusing the heart with warm oxygenated blood, potentially allowing longer distance heart sharing in the future.
• Lung: 4–8 hours is the typical viability window for lungs (). Like hearts, lungs are also quite sensitive; about 6 hours is a commonly cited safe limit. Thus, lung offers also prioritize geographically closer recipients. The lung allocation system historically used a 500-mile zone for the highest priority offers (and now continuous scoring with distance) to reflect this. Techniques like cold static preservation with an inflation of the lungs, or ex vivo lung perfusion (EVLP) machines, can sometimes extend lung preservation by a few hours, which may help in long-distance sharing or when an organ needs re-evaluation.
• Liver: 8–12 hours of cold ischemia is usually safe for liver transplants (). Livers tolerate longer preservation than hearts or lungs, so they can be transported further. It’s not uncommon for a liver to be shipped across the country if needed (though shorter is always better). After about 12 hours, liver graft injury risk rises (leading to complications like initial poor function or non-function). Therefore, while location is considered for livers, the larger ischemic window gives OPOs flexibility to allocate a liver to a more distant high-priority patient if local candidates are low urgency. Advances like hypothermic machine perfusion or normothermic perfusion are increasingly used to improve organ viability and even assess marginal livers during transit, potentially pushing the boundaries of how long livers can be preserved.
• Kidney: 24–36 hours (or even more) is the general guideline for kidney preservation () (). Kidneys are relatively resilient to cold storage, especially with modern preservation solutions and pulsatile perfusion pumps. This long window allows kidneys to be shipped anywhere in the country if appropriate. Indeed, kidneys from one coast are sometimes offered to patients on the other coast if no closer matches accept them, thanks to the ability to preserve kidneys on ice overnight. However, longer cold times do correlate with higher risk of delayed graft function (DGF, where the kidney might not work immediately and the patient needs dialysis post-transplant) and somewhat shorter long-term graft survival (). To mitigate this, if a kidney has already been on ice for many hours, OPOs may give it priority to nearer patients or use a machine perfusion pump to improve its condition during transportation. Nonetheless, because dialysis can support kidney patients temporarily, allocation favors getting the kidney to the best-matched or longest-waiting patient even if that means a longer transport, as long as CIT remains in a reasonable range.

5. Organ-Specific Allocation Policies and Recipient Selection

5.1 Kidney Allocation

• Waiting Time: Candidates accrue points for time on the list. Notably, time spent on dialysis before listing is retroactively credited, so that two patients with identical medical profiles are prioritized by who has waited (or been on dialysis) longer. This ensures older waiting times translate to higher priority ().
• Donor-Recipient Matching: Kidneys are matched by blood type (some blood groups like O are universal donors to others, AB recipients can accept any type, etc.). HLA matching: if a candidate happens to be a zero HLA antigen mismatch with the donor (a rare perfect match), current policy gives that candidate priority because such matches have excellent outcomes. Historically, HLA-DR matching earned points in the allocation system (), and while the new framework is evolving, HLA match is still a favorable factor.
• Sensitization (CPRA): As mentioned, highly sensitized patients receive additional priority. In the UNOS Kidney Allocation System, CPRA is used to assign points in a sliding scale; for example, a CPRA of 100% yields the maximum bonus (~4 points) which is equivalent to several years of waiting time advantage (). This policy change dramatically improved access to transplants for patients who are hard to match immunologically by pushing them higher on compatible donor lists.
• Pediatrics: Children under 18 at listing get priority for kidneys from donors under 35 (to maximize graft longevity and benefit the young recipient). Pediatric candidates also had advantages in the scoring system to reduce waiting time, given the urgency to transplant children for normal development (dialysis is particularly harmful to children’s growth).
• Donor KDPI and EPTS: As discussed, kidneys with KDPI ≤20% (very high quality) are preferentially allocated to candidates with EPTS ≤20% (those expected to live the longest after transplant). Conversely, kidneys with KDPI ≥85% (lower quality) are offered through an “expedited placement” sequence often to centers that have opted-in for these organs, frequently to older or less risk-averse patients (). This matching of organ life expectancy with patient need is intended to ensure that the best organs go to those who could benefit the longest, while higher-risk organs still get used in appropriate recipients rather than being discarded.
• Geography: Until 2021, kidney allocation was constrained by Donation Service Areas (DSAs) and regions – kidneys were first offered locally (within the OPO’s DSA) then regionally, leading to geographic disparities. In March 2021, a new policy removed DSA and region from kidney allocation, moving to a 250 nautical mile (NM) radius based system (). Now, a kidney is first offered to candidates within 250 NM of the donor hospital, using the above priority points. Candidates inside that circle receive up to 2 proximity points (more points the closer they are) to boost local transplant opportunities (). If no suitable match is found in that circle, the circle can expand (or effectively, offers go beyond 250 NM). Additionally, there are some nationwide priorities that override distance: for instance, patients who are 100% sensitized have effectively nationwide priority, and if a kidney is highly difficult to place, UNOS can route it to any center that will accept. The switch to radius-based allocation was aimed at reducing geographic inequity while still accounting for practical CIT limits (). Early data suggest it has indeed broadened distribution – kidneys are traveling farther on average – with a modest improvement in equity, though differences in transplant rates by region still exist.

5.2 Liver Allocation

5.3 Heart Allocation

5.4 Lung Allocation

6. Key Allocation Challenges and Ongoing Innovations

• Geographic Disparities: One of the foremost challenges has been inequality in access based on geography. Historically, where a patient lived had a major impact on wait times and transplant rates () (). This was due to the use of local donation areas (DSAs) and regional units that sometimes kept organs in areas with lower demand while patients in other areas died waiting. The OPTN Final Rule (1998) requires that allocation shall not be based on a candidate’s place of residence/listing except as needed for medical efficiency (). This has driven the recent policy shifts (removing DSAs, using circles or continuous distance scoring) to broaden sharing. For example, eliminating DSA in thoracic allocation in 2017–2020 and introducing acuity circles for liver and kidney in 2020–2021 were aimed at reducing geographic inequity () (). These changes have improved things incrementally, but not eliminated disparity. Some regions still have fewer donors or higher demand, leading to longer waits (e.g. in parts of the Northeast for kidneys, or certain urban vs rural differences). Research indicates that broader sharing (even national sharing) can reduce waitlist deaths, but at the potential cost of more travel and logistical complexity () (). The transplant community continues to refine policies – e.g. deciding the optimal circle size or whether to use uniform national allocation with only distance scoring. Data-driven monitoring is ongoing to see if new policies are truly equalizing transplant access. A 2021 review noted that while lung, heart, liver and kidney policies have moved toward broader sharing, evidence of disparity reduction is still emerging () (). The consensus is that patients’ chance of transplant should rely on their medical need, not ZIP code – an ethic of distributive justice that underpins current reforms.
• Ethical Considerations: Organ allocation must balance competing ethical principles: utility (maximizing overall transplant success and graft survival) vs equity (ensuring fair access for all, including the most vulnerable). Policies like kidney KDPI/EPTS matching try to balance utility (don’t “waste” a low-KDPI kidney on a patient with very limited lifespan) with equity (still give everyone some chance). Prioritizing sicker patients (heart status 1, high MELD, high LAS) follows a “sickest-first” ethic to save lives, but if taken to extreme can risk poorer outcomes if organs are given to patients too sick to survive. Allocation for kidneys gives some weight to post-transplant survival (longevity matching) which sparked debate on age discrimination – the “fair innings” argument suggests younger patients should get some priority to have a chance at a full life () (), whereas older patients still deserve transplant but perhaps with different organ offers. Pediatric priority is an ethical choice almost universally accepted: children get priority for many organs because of both medical need and societal value of saving children. Another ethical challenge is multi-organ allocation: if one patient needs a multi-organ transplant (e.g. heart-kidney or liver-kidney), they may be prioritized to receive multiple organs over other patients who each need one of those organs. This can be life-saving for the multi-organ patient but denies two other patients an organ. Policies are evolving to fairly handle multi-organ listings (with “safety net” provisions, e.g. if you get a liver, you have some priority for a kidney if needed, but not automatically taking a kidney from the kidney-alone list unless criteria met). Transparency and public trust are crucial, so OPTN ensures policies go through public comment and ethical review by committees. The concept of justice also extends to ensuring access regardless of race, sex, socioeconomic status. There have been disparities (for instance, Black patients had longer dialysis times before transplant historically, or rural patients might have less access to transplant centers). Efforts like removing institutional biases, using race-neutral eGFR calculations for listing, and federally enforcing OPO performance standards (so that all regions procure as many organs as possible) are ethical and policy responses to these disparities.
• Donor Organ Utilization and Discards: Another challenge is that a significant fraction of recovered organs (especially kidneys) are not transplanted, often due to late declines or perceived quality issues. This represents lost opportunities. Allocation rules alone can’t solve this, but they try to ameliorate it by expediting placement of lower-quality organs and by providing better information. For example, UNOS developed an Organ Offer Explorer and predictive analytics tools to help centers visualize outcomes with certain organs () (). The goal is to encourage transplanting organs that are currently discarded but could benefit patients (like kidneys from older donors that might function for 5 years – not ideal, but better than dialysis for some). Policymakers discuss whether there should be consequences for repeatedly declining usable organs or whether allocation should force allocation of an organ to a candidate if medically suitable (currently, the final acceptance is always up to the transplant center/patient).
• Innovations in Matching Algorithms: The OPTN is actively modernizing allocation through better algorithms. The Continuous Distribution framework is a prime example of innovation () (). Instead of the old classification system (which had hard cutoffs and sequence-based rules) () (), continuous distribution uses a linear composite score to rank candidates by multiple factors at once. This is a more flexible and granular approach, treating allocation as an optimization problem. Lungs have led the way with a points-based CAS (); kidneys and pancreata are expected to adopt a similar points-based continuous model in the coming years, combining waiting time, CPRA, KDPI matching, distance, etc., into one formula. Research is underway to simulate different weighting of factors to produce the most equitable outcomes. Another algorithmic innovation is the use of machine learning and data science to improve organ matching efficiency. For instance, predictive models can identify which offers a given patient or center is likely to accept (). UNOS has implemented “offer filters” where centers can pre-specify donor criteria they would refuse, so those offers can be bypassed to save time. This reduces the slow trickle-down of offers and gets organs to an accepting center faster, thus reducing CIT and non-use. There’s also interest in integrating continuous distribution with real-time outcomes prediction – e.g. continuously updating a candidate’s priority based on changes in their condition, or predicted benefit from a particular organ offer.
• Data Transparency and Policy Changes: Allocation policies are continuously refined. Large-scale changes (like kidney 2021, liver 2020, heart 2018, lung 2023) are often followed by detailed monitoring reports from the Scientific Registry of Transplant Recipients (SRTR). These data help identify unintended consequences (for example, after heart policy change, was there a rise in post-transplant mortality? Or after kidney circles, did travel distance increase too much?). The transplant community uses this information to tweak policies. An example is the adjustment of exception scores or the recent removal of race from the eGFR calculation for listing (to not disadvantage Black patients). Furthermore, public policy and legal challenges sometimes arise – e.g. lawsuits have been filed by patients or regions unhappy with allocation rules (notably, a 2017 lawsuit led to removing DSA for lungs). As a result, the OPTN must balance stakeholder interests with medical fairness. The Final Rule guidance that distribution must be based on medical urgency and effectiveness, not arbitrary boundaries, provides a legal and ethical compass ().