In 1984, the United States Congress passed the National Organ Transplant Act that called for a national network to coordinate the procurement and allocation of organs for transplantation and for the collection of clinical data related to transplantation. This network was named the Organ Procurement and Transplantation Network (OPTN). The United Network of Organ Sharing (UNOS) evolved from the South-Eastern Organ Procurement Foundation which was an association of transplantation professionals. UNOS was awarded the initial contract to serve as the OPTN in 1986 and has done so since then.
Organ allocation policies have evolved following the introduction of a number of landmark policy changes:
In 2000, the Department of Health and Human Services (HHS) implemented the ‘Final Rule’ as a framework for the operation of the OPTN. The Final Rule broadly consists of:
- Use of sound medical practices
- Best use of donor organs
- Minimization of waste
In 2004, the Kidney Committee of the OPTN was charged by the government to develop an allocation policy based on the Final Rule. This led to the Kidney Allocation System (KAS) that was implemented in late 2014.
The KAS aimed to fulfill the goals of the Final Rule. The estimated post-transplant survival (EPTS) score was developed to identify patients who might benefit most from donor kidneys predicted to have the best graft survival times. This score, ranging from 0% for patients expected to survive the longest to 100% for patients with the shortest life expectancy post-transplant, is based on 4 factors:
- Candidates’ time on dialysis
- Current diagnosis of diabetes mellitus
- Prior solid organ transplantation
- Candidates’ age
The Kidney Donor Profile Index (KDPI) is a score based on 10 donor and other factors that ranges from 0% to 100%. A KDPI score of 0% describes a donor kidney that is expected to have better graft survival than all other kidneys transplanted in the previous year. Longevity matching, one of the main intended goals of the KAS, aims to transplant patients with EPTS scores <20% with donor kidneys with KDPI scores <20%. Another aim of the new KAS was to increase access for those on the wait list for many years due to sensitization (i.e., patients with high panel of reactive antibodies, PRA). Points are now allocated in a graded fashion for increasing panel-reactive antibodies (PRA) scores (up to 202 points for patients with a PRA of 100%). A change to the wait list start time to the first dialysis or GFR below 20 ml/min was also introduced to make the system more equitable. This change in wait list start time aimed to improve the outcomes of patients who were referred to transplantation late, an issue that tended to occur more frequently in minority populations.
The current KAS has now been in operation for over 4 years and many are investigating whether the intended goals of the KAS are being realized. The current study by Sethi et al recently published in AJKD examines the impact of the KAS on utility and equity of deceased organ donor allocation in the USA. To do this, the authors focused on how the calculated PRA (cPRA) and KDPI scores impact patient and graft survival. They focused on high cPRA kidney transplant recipients who have benefitted from the new KAS.
The publication studied adults transplanted between 2009, when cPRA scores were introduced, to 2017. This included a study population of 84,451 deceased donor kidney recipients. The study population was split into 5 groups based on cPRA, 0%, 1% to 79%, 80% to 89%, 90% to 98%, and 99% to 100%. Posttransplant patient survival and graft survival were the outcomes of interest. Time-to-event analyses were performed for recipients categorized using cPRA scores, KDPI (≤20%, 21%-85%, and >85%), and the combination of cPRA and KDPI categories. Cox proportional hazards were used and fitted to adjust for usual factors associated with death and graft loss.The authors found that there was an increase in the use of low KDPI (<20%) kidneys transplanted into patients with cPRA 99% – 100% without a proportional increase in high KDPI kidneys transplanted in the same group (5.2% to 15.8% of all low KDPI kidneys transplanted pre-KAS and in 2017, respectively). This increased use of low KDPI kidneys was not explained by priority allocation to low EPTS patients with cPRA 99% -100%. In 2017, patients with EPTS scores <20% made up only 22.7% of recipients with cPRA 99% – 100% transplanted with low KDPI kidneys (<20%).
Unsurprisingly, patient survival stratified by the 3 KDPI categories decreased as the KDPI rose, ranging from 71% for those with KDPI score > 85% compared to 89% for KDPI scores < 20%:
The cumulative incidence of graft failure over 5 years segregated by KDPI category is shown in Figure 5 below. Within the 3 KDPI categories, there was little difference in the cumulative incidence of graft loss by cPRA category.
After adjusting for confounders, cPRA levels above 80% were associated with increased risk of graft loss and patient survival compared to a cPRA of 0%.
The main findings from this study were that the association of KDPI score with patient and graft survival is similar irrespective of degree of recipient allosensitization. Therefore, there is no relative benefit of low KDPI kidneys compared to high KDPI kidneys in highly sensitized recipients. Sethi et al explore possible reasons why there has been an increase in low KDPI kidneys transplantated into highly sensitized recipients. As there is now increased priority given to highly sensitized patients, there are more kidneys available to these patients. Thus, some centers are perhaps more selective when accepting kidneys for such patients. The unintended consequence of such practices is the reduced availability of low KDPI kidneys for patients with low EPTS scores (an intended consequence of the new KAS). There are many potential ways to mitigate this unintended consequence. One suggested by the authors is to move local candidates with EPTS scores ≤ 20% and pediatric candidates so that they are ranked above regional or national candidates with cPRA levels of 99% to 100%. This might increase the use of higher KDPI kidneys nationally without restricting low KDPI kidneys for local highly sensitized recipients.
This paper highlights a very important issue in kidney donor allocation. Best use of donor kidneys is a tenant of the Final Rule. Additionally, the allocation system in the US is likely to change again in the near future as donor service areas will be defined by circular regions of a certain nautical mile radius. This may further upset the dynamics of kidney donor allocation and come with other unintended consequences which will need to be explored by studies like these.
Title: Allocation of the Highest Quality Kidneys and Transplant Outcomes Under the New Kidney Allocation System
Authors: S. Sethi, R. Najjar, A. Peng, J. Mirocha, A. Vo, S. Bunnapradist, S.C. Jordan, and E. Huang