Continuous distribution as an organ allocation framework

Association of the initial implementation of continuous distribution allocation policy with outcomes for lung transplant candidates by blood type

On March 9, 2023, allocation of donor lungs in the United States changed to continuous distribution (CD). Initial implementation of policy was flawed due to a programming error affecting priority of candidates by blood type. Although this issue has been identified and addressed, the impact of initial policy implementation on blood type-O candidates has not been rigorously evaluated. We used data from the Scientific Registry of Transplant Recipients to evaluate candidate (n = 4738) and recipient (n = 4437) outcomes following CD policy implementation. Using cumulative incidence plots accounting for competing risks and multivariable Cox models, incidence of transplantation prepolicy was similar by blood type (8-month incidence 81.0% vs 80.5% for blood types-A, B, or AB and blood type-O, respectively, P = .57). Following CD policy, blood type-O candidates had lower incidence of transplantation relative to other blood types (8-month incidences 86.3% vs 92.1%, respectively, P < .001), with a significantly lower adjusted hazard ratio for transplantation (0.67; 95% confidence interval, 0.54-0.82). Blood type-O candidates were more likely Hispanic and 'Other' race and had higher rates of waitlist removal or death following CD (6.0% vs 2.9%, P = .003). Among transplants performed prior to CD, 48% were blood type-O recipients compared to 40% post-CD, representing 138 fewer blood type-O transplants than expected. Type-O blood candidates had significant decline in prognosis relative to other blood groups following initial implementation of CD policy.

The liver allocation landscape: MELD 3.0 and continuous distribution

PURPOSE OF REVIEW

This review highlights recent advancements in liver organ allocation, specifically the transition to MELD 3.0 and the potential introduction of continuous distribution. These developments are timely, as they address the increasing need for a more efficient, equitable, and personalized system for prioritizing liver transplant candidates.

RECENT FINDINGS

The review covers two key innovations: MELD 3.0: A refined version of the original MELD score, designed to improve the prioritization process by incorporating additional factors that offer a more accurate and urgent measure of transplant need. This approach aims to better assess the severity of liver disease and the need for transplantation. Continuous distribution: A dynamic approach that shifts away from the static allocation model. It integrates multiple donor and recipient variables - such as geographic location, organ quality, and recipient condition - into a continuous, flexible allocation process. This framework seeks to make more nuanced decisions based on a broader set of factors that reflect transplant suitability.

SUMMARY

These innovations aim to enhance fairness and patient outcomes by refining candidate prioritization and reducing disparities in access to transplants. However, implementing these systems presents challenges, such as technical complexities and regional differences in access. Ongoing evaluation is necessary to ensure their effectiveness and equitable implementation across diverse patient populations.

Automated abdominal CT imaging biomarkers and clinical frailty measures associated with postoperative deceased-donor liver transplant outcomes

OBJECTIVE

To quantify the potential of fully automated CT-based body composition metrics and clinical frailty data in predicting liver transplant recipient postoperative outcomes.

METHODS

AI-enabled body composition tools were applied to pre-transplant abdominal CT scans in a retrospective cohort of first-time deceased-donor liver transplant recipients. Clinical frailty data (Fried frailty score) was obtained from an established transplant database. Age- and sex-corrected hazard ratios (HRs) were analyzed according to highest-risk quartiles compared with the other three quartiles combined. Area under the receiver operating characteristic curve (ROC AUC) analysis in univariate and multivariate scenarios was also performed.

RESULTS

598 liver transplant recipients (median age, 56 years [IQR, 49-61]; 383 men/215 women) were included from 2005 to 2021. Mean clinical follow-up interval after transplant was 8.6 ± 4.5 years, with 224 deaths (mean interval, 5.3 ± 3.9 years post-transplant) and 246 graft failures (mean interval, 4.7 ± 4.0 years post-transplant) observed. Univariate HRs for post-transplant survival included 1.53 (95% CI, 1.14-2.06) for muscle attenuation, 1.66 (95% Cl, 1.24-2.22) for aortic Agatston score, 1.35 (1.02-1.80) for SAT area, and 1.82 (1.35-2.46) for liver volume. For those meeting the frailty criteria, HR was 2.14 (1.08-4.22). Multivariate 10-year AUC for predicting mortality was 0.675 using liver volume, aortic Agatston score, and muscle attenuation. 10-year univariate AUC for clinical frailty assessment was 0.601 but increased to 0.878 when combined with CT measures.

CONCLUSION

Automated CT measurements of muscle density (myosteatosis), aortic calcification, subcutaneous fat, and liver volume are predictive of mortality in liver transplant recipients. Frailty was likewise predictive. Combining CT and clinical frailty assessment was complementary.

KEY POINTS

Question What is the prognostic value of pre-transplant CT-based body composition measures for deceased-donor liver transplant outcomes, and how do they correlate with frailty assessment? Findings Increased post-transplant mortality was associated with pre-transplant increased liver volume, increased abdominal aortic Agatston score, decreased skeletal muscle attenuation, and decreased subcutaneous adipose tissue area. Clinical relevance Pre-transplant AI-enabled body composition measures have predictive value for post-transplant survival, offering a novel and objective diagnostic tool to identify high-risk transplant recipients that are complementary to clinical assessments.

National Organ Procurement and Transplant Network Heart Allocation Policy: 6 Years Later

In 2014, the Organ Procurement and Transplant Network began reappraisal of the United States heart transplant allocation policy. Driven by ongoing discordance between organ supply and demand, high waitlist mortality, and increasing exception requests, the Thoracic Committee radically redesigned the priority scheme and drafted a 6-tiered algorithm, included durable device complications into policy, expanded broader sharing, and increased the number of mandatory listing variables to develop a future heart allocation score. This became the 2018 New Heart Allocation Policy. Changes in allocation priority have resulted in a significant increase in the use of temporary mechanical circulatory support in waitlisted candidates with a concomitant decrease in the number of patients bridged to transplanted with durable left ventricular assist device support. The number of exception requests continues to increase, particularly for patients listed status 2 and for multiorgan transplants. Importantly, fewer patients are being delisted for clinical improvement, suggesting missed opportunities for recovery. The current review will critically evaluate the 2018 heart allocation policy 6 years later, briefly focusing on the history of heart allocation in the United States, the current and evolving algorithms for candidate prioritization including continuous distribution, the impact of technology and innovation on transplant rates and future policy development, and the ongoing regulatory oversight and governance changes in the Organ Procurement and Transplant Network.

Using Unsupervised Clustering to Characterize Phenotypes Among Older Kidney Transplant Recipients: A Cohort Study

Background

Older kidney transplant recipients have inferior outcomes compared to younger recipients, and this risk may be compounded by donor characteristics.

Objective

We applied an unsupervised machine learning clustering approach to group older recipients into similar phenotypes. We evaluated the association between each cluster and graft failure, and the impact of donor quality on outcomes.

Design

This is a nationally representative retrospective cohort study.

Setting and Patients

Kidney transplant recipients aged ≥65 years identified from the Scientific Registry of Transplant Recipients (2000-2017).

Measurements and Methods

We used unsupervised clustering to generate phenotypes using 16 recipient factors. Donor quality was evaluated using 2 approaches, including the Kidney Donor Risk Index (KDRI). All-cause graft failure was analyzed using multivariable Cox regression.

Results

Overall, 16 364 patients (mean age 69 years; 38% female) were separated into 3 clusters. Cluster 1 recipients were exclusively female; cluster 2 recipients were exclusively males without diabetes; and cluster 3 recipients were males with a higher burden of comorbidities. Compared to cluster 2, the risk of graft failure was higher for cluster 3 recipients (adjusted hazard ratio [aHR] = 1.25, 95% confidence interval [CI] = 1.19-1.32). Cluster 3 recipients of a lower quality (KDRI ≥1.45) kidney had the highest risk of graft failure (aHR = 1.74, 95% CI = 1.61-1.87) relative to cluster 2 recipients of a higher quality kidney.

Limitations

This study did not include an external validation cohort. The findings should be interpreted as exploratory and should not be used to inform individual risk prediction nor be applied to recipients <65 years of age.

Conclusions

In a national cohort of older kidney transplant recipients, unsupervised clustering generated 3 clinically distinct recipient phenotypes. These phenotypes may aid in complementing allocation decisions, providing prognostic information, and optimizing post-transplant care for older recipients.

Reduced short-term survival following liver transplant in patients with acute-on-chronic liver failure: Reevaluating OPTN data

BACKGROUND

Prior studies show severe acute-on-chronic liver failure (ACLF) at liver transplantation (LT) negatively impacts short-term, but not long-term, post-LT outcomes. However, not accounting for ACLF's time-varying effect on the waitlist may underappreciate its dynamic nature. Moreover, excluding those who died or dropped off the waitlist raises concerns about selection bias.

METHODS

This US nationwide retrospective cohort study estimated the effect of severe ACLF (grade 3) (ACLF-3) on post-LT outcomes, including adult, first-time deceased donor LT candidates listed from June 2013 to May 2023. A marginal structural model (MSM) to address selection bias and time-varying exposure (ACLF-3) was applied, with extended Cox proportional hazard models using a Heaviside step function to assess the hazard of death after LT.

RESULTS

Among 31,267 eligible candidates for LT (baseline cohort), 11.3% (n = 3518) had ACLF-3 at listing; 13.6% (n = 4243) died or dropped out while on the LT waitlist. Of the 27,024 patients who received LT (transplanted cohort), 12.3% (n = 3333) had ACLF-3 at LT. ACLF-3 at LT (but not at waitlisting) was associated with a higher hazard of death, with the hazard ratio of 1.80 (95% CI: 1.09-2.97) within 1 year after LT but not thereafter. This marginal structural model effect size was 9% higher than conventional multivariable Cox proportional hazard models. Sensitivity analyses corroborated these findings.

CONCLUSIONS

Compared to previous studies, ACLF-3 at LT in our marginal structural model was associated with a discernible increase in short-term mortality after transplant, presumably due to our addressing of selection bias, while long-term survival was similar to those without severe ACLF at LT. However, potential vulnerability to posttransplant complications warrants further investigation.

Targeted Broader Sharing for Liver Continuous Distribution

BACKGROUND

In recent years, changes to US organ allocation have aimed to improve equity and accessibility across regions. The Organ Procurement and Transplantation Network plans to adopt continuous liver distribution, prioritizing candidates based on a weighted composite allocation score (CAS) incorporating proximity, ABO types, medical urgency, and pediatric priority. The Liver Committee has requested research on CAS variations that account for geographical heterogenicity.

METHODS

We describe a method for designing a geographically heterogeneous CAS with targeted broader sharing (CAS-TBS) to balance the highly variable geographic distributions of liver transplant listings and liver donations. CAS-TBS assigns each donor hospital to either broader sharing or nearby sharing, adjusting donor-candidate distance allocation points accordingly.

RESULTS

We found that to reduce geographic disparity in the median Model for End-stage Liver Disease at transplant (MMaT), >75% of livers recovered in regions 2 and 10 should be distributed with broader sharing, whereas 95% of livers recovered in regions 5 and 1 should be distributed with nearby sharing. In a 3-y simulation of liver allocation, CAS-TBS decreased MMaT by 2.1 points in high-MMaT areas such as region 5 while increasing MMaT only by 0.65 points in low-MMaT areas such as region 3. CAS-TBS significantly decreased median transport distance from 202 to 167 nautical miles under acuity circles and decreased waitlist deaths.

CONCLUSIONS

Our CAS-TBS design methodology could be applied to design geographically heterogeneous allocation scores that reflect transplant community values and priorities within the continuous distribution project of the Organ Procurement and Transplantation Network. In our simulations, the incremental benefit of CAS-TBS over CAS was modest.

Artificial Intelligence in Nephrology: Clinical Applications and Challenges

Artificial intelligence (AI) is increasingly used in many medical specialties. However, nephrology has lagged in adopting and incorporating machine learning techniques. Nephrology is well positioned to capitalize on the benefits of AI. The abundance of structured clinical data, combined with the mathematical nature of this specialty, makes it an attractive option for AI applications. AI can also play a significant role in addressing health inequities, especially in organ transplantation. It has also been used to detect rare diseases such as Fabry disease early. This review article aims to increase awareness on the basic concepts in machine learning and discuss AI applications in nephrology. It also addresses the challenges in integrating AI into clinical practice and the need for creating an AI-competent nephrology workforce. Even though AI will not replace nephrologists, those who are able to incorporate AI into their practice effectively will undoubtedly provide better care to their patients. The integration of AI technology is no longer just an option but a necessity for staying ahead in the field of nephrology. Finally, AI can contribute as a force multiplier in transitioning to a value-based care model.

Inconsistent values and algorithmic fairness: a review of organ allocation priority systems in the United States

BACKGROUND

The Organ Procurement and Transplant Network (OPTN) Final Rule guides national organ transplantation policies, mandating equitable organ allocation and organ-specific priority stratification systems. Current allocation scores rely on mortality predictions.

METHODS

We examined the alignment between the ethical priorities across organ prioritization systems and the statistical design of the risk models in question. We searched PubMed for literature on organ allocation history, policy, and ethics in the United States.

RESULTS

We identified 127 relevant articles, covering kidney (19), liver (60), lung (24), and heart transplants (23), and transplant accessibility (1). Current risk scores emphasize model performance and overlook ethical concerns in variable selection. The inclusion of race, sex, and geographical limits as categorical variables lacks biological basis; therefore, blurring the line between evidence-based models and discrimination. Comprehensive ethical and equity evaluation of risk scores is lacking, with only limited discussion of the algorithmic fairness of the Model for End-Stage Liver Disease (MELD) and the Kidney Donor Risk Index (KDRI) in some literature. We uncovered the inconsistent ethical standards underlying organ allocation scores in the United States. Specifically, we highlighted the exception points in MELD, the inclusion of race in KDRI, the geographical limit in the Lung Allocation Score, and the inadequacy of risk stratification in the Heart Tier system, creating obstacles for medically underserved populations.

CONCLUSIONS

We encourage efforts to address statistical and ethical concerns in organ allocation models and urge standardization and transparency in policy development to ensure fairness, equitability, and evidence-based risk predictions.

Common definitions and variables are needed for the United States to join the conversation on acute-on-chronic liver failure

Acute-on-chronic liver failure (ACLF) is a variably defined syndrome characterized by acute decompensation of cirrhosis with organ failures. At least 13 different definitions and diagnostic criteria for ACLF have been proposed, and there is increasing recognition that patients with ACLF may face disadvantages in the current United States liver allocation system. There is a need, therefore, for more standardized data collection and consensus to improve study design and outcome assessment in ACLF. In this article, we discuss the current landscape of transplantation for patients with ACLF, strategies to optimize organ utility, and data opportunities based on emerging technologies to facilitate improved data collection.

Calibration of Priority Points for Sensitization Status of Kidney Transplant Candidates in the United States

Key Points

There are multiple factors associated with high sensitization levels among kidney transplant candidates, which differ by candidate sex. Since the initiation of the kidney allocation system, candidates with higher sensitization have higher rates of deceased donor transplantation. Priority points assigned to candidates associated with sensitization have led to inequities in access to deceased donor transplantation.

Background

A primary change to the national organ allocation system in 2014 for deceased donor kidney offers was to weight candidate priority on the basis of sensitization (i.e ., calculated panel reactive antibody percentage [cPRA%]) using a sliding scale. Increased priority for sensitized patients could improve equity in access to transplantation for disadvantaged candidates. We sought to evaluate the effect of these weights using a contemporary cohort of adult US kidney transplant candidates.

Methods

We used the national Scientific Registry of Transplant Recipients to evaluate factors associated with sensitization using multivariable logistic models and rates of deceased donor transplantation using cumulative incidence models accounting for competing risks and multivariable Cox models.

Results

We examined 270,912 adult candidates placed on the waiting list between January 2016 and September 2023. Six-year cumulative incidence of deceased donor transplantation for candidates with cPRA%=80–85 and 90–95 was 48% and 53%, respectively, as compared with 37% for candidates with cPRA%=0–20. In multivariable models, candidates with high cPRA% had the highest adjusted hazards for deceased donor transplantation. There was significant effect modification such that the association of high cPRA% with adjusted rates of deceased donor transplantation varied by region of the country, sex, race and ethnicity, prior dialysis time, and blood type.

Conclusions

The results indicate that the weighting algorithm for highly sensitized candidates may overinflate the need for prioritization and lead to higher rates of transplantation. Findings suggest recalibration of priority weights for allocation is needed to facilitate overall equity in access to transplantation for prospective kidney transplant candidates. However, priority points should also account for subgroups of candidates who are disadvantaged for access to donor offers.

Utilization of older deceased donors for pediatric liver transplant may negatively impact long-term survival

BACKGROUND

Multiple adult studies have investigated the role of older donors (ODs) in expanding the donor pool. However, the impact of donor age on pediatric liver transplantation (LT) has not been fully elucidated.

METHODS

UNOS database was used to identify pediatric (≤18 years) LTs performed in the United States during 2002-22. Donors ≥40 years at donation were classified as older donors (ODs). Propensity analysis was performed with 1:1 matching for potentially confounding variables.

RESULTS

A total of 10,024 pediatric liver transplantation (PLT) patients met inclusion criteria; 669 received liver grafts from ODs. Candidates receiving OD liver grafts were more likely to be transplanted for acute liver failure, have higher Model End-Stage Liver Disease/Pediatric End-Stage Liver Disease (MELD/PELD) scores at LT, listed as Status 1/1A at LT, and be in the intensive care unit (ICU) at time of LT (all p < 0.001). Kaplan-Meier (KM) analyses showed that recipients of OD grafts had worse patient and graft survival (p < 0.001) compared to recipients of younger donor (YD) grafts. KM analyses performed on candidates matched for acuity at LT revealed inferior patient and graft survival in recipients of deceased donor grafts (p < 0.001), but not living donor grafts (p > 0.1) from ODs. Cox regression analysis demonstrated that living donor LT, diagnosis of biliary atresia and first liver transplant were favorable predictors of recipient outcomes, whereas ICU stay before LT and transplantation during 2002-12 were unfavorable.

CONCLUSION

Livers from ODs were used for candidates with higher acuity. Pediatric recipients of livers from ODs had worse outcome compared to YDs; however, living donor LT from ODs had the least negative impact on recipient outcomes.

Development and Validation of a Risk Score Predicting Death Without Transplant in Adult Heart Transplant Candidates

Importance

The US heart allocation system prioritizes medically urgent candidates with a high risk of dying without transplant. The current therapy-based 6-status system is susceptible to manipulation and has limited rank ordering ability.

Objective

To develop and validate a candidate risk score that incorporates current clinical, laboratory, and hemodynamic data.

Design, Setting, and Participants

A registry-based observational study of adult heart transplant candidates (aged ≥18 years) from the US heart allocation system listed between January 1, 2019, and December 31, 2022, split by center into training (70%) and test (30%) datasets. Adult candidates were listed between January 1, 2019, and December 31, 2022.

Main Outcomes and Measures

A US candidate risk score (US-CRS) model was developed by adding a predefined set of predictors to the current French Candidate Risk Score (French-CRS) model. Sensitivity analyses were performed, which included intra-aortic balloon pumps (IABP) and percutaneous ventricular assist devices (VAD) in the definition of short-term mechanical circulatory support (MCS) for the US-CRS. Performance of the US-CRS model, French-CRS model, and 6-status model in the test dataset was evaluated by time-dependent area under the receiver operating characteristic curve (AUC) for death without transplant within 6 weeks and overall survival concordance (c-index) with integrated AUC.

Results

A total of 16 905 adult heart transplant candidates were listed (mean [SD] age, 53 [13] years; 73% male; 58% White); 796 patients (4.7%) died without a transplant. The final US-CRS contained time-varying short-term MCS (ventricular assist-extracorporeal membrane oxygenation or temporary surgical VAD), the log of bilirubin, estimated glomerular filtration rate, the log of B-type natriuretic peptide, albumin, sodium, and durable left ventricular assist device. In the test dataset, the AUC for death within 6 weeks of listing for the US-CRS model was 0.79 (95% CI, 0.75-0.83), for the French-CRS model was 0.72 (95% CI, 0.67-0.76), and 6-status model was 0.68 (95% CI, 0.62-0.73). Overall c-index for the US-CRS model was 0.76 (95% CI, 0.73-0.80), for the French-CRS model was 0.69 (95% CI, 0.65-0.73), and 6-status model was 0.67 (95% CI, 0.63-0.71). Classifying IABP and percutaneous VAD as short-term MCS reduced the effect size by 54%.

Conclusions and Relevance

In this registry-based study of US heart transplant candidates, a continuous multivariable allocation score outperformed the 6-status system in rank ordering heart transplant candidates by medical urgency and may be useful for the medical urgency component of heart allocation.

Patient years lost due to cytomegalovirus serostatus mismatching in the scientific registry of transplant recipients

Background

The cytomegalovirus (CMV) mismatch rate in deceased donor kidney transplant (DDKT) recipients in the US remains above 40%. Since CMV mismatching is common in DDKT recipients, the cumulative effects may be significant in the context of overall patient and graft survival. Our primary objective was to describe the short- and long-term risks associated with high-risk CMV donor positive/recipient negative (D+/R-) mismatching among DDKT recipients with the explicit goal of deriving a mathematical mismatching penalty.

Methods

We conducted a retrospective, secondary analysis of the Scientific Registry of Transplant Recipients (SRTR) database using donor-matched DDKT recipient pairs (N=105,608) transplanted between 2011-2022. All-cause mortality and graft failure hazard ratios were calculated from one year to ten years post-DDKT. All-cause graft failure included death events. Survival curves were calculated using the Kaplan-Meier estimation at 10 years post-DDKT and extrapolated to 20 years to provide the average graft days lost (aGDL) and average patient days lost (aPDL) due to CMV D+/R- serostatus mismatching. We also performed an age-based stratification analysis to compare the relative risk of CMV D+ mismatching by age.

Results

Among 31,518 CMV D+/R- recipients, at 1 year post-DDKT, the relative risk of death increased by 29% (p<0.001), and graft failure increased by 17% (p<0.001) as compared to matched CMV D+/R+ group (N=31,518). Age stratification demonstrated a significant increase in the risk associated with CMV mismatching in patients 40 years of age and greater. The aGDL per patient due to mismatching was 125 days and the aPDL per patient was 100 days.

Conclusion

The risks of CMV D+/R- mismatching are seen both at 1 year post-DDKT period and accumulated throughout the lifespan of the patient, with the average CMV D+/R- recipient losing more than three months of post-DDKT survival time. CMV D+/R- mismatching poses a more significant risk and a greater health burden than previously reported, thus obviating the need for better preventive strategies including CMV serodirected organ allocation to prolong lifespans and graft survival in high-risk patients.

Reassessing the survival benefit of deceased donor liver transplantation: retrospective cohort study

INTRODUCTION

Currently in the United States, deceased donor liver transplant (DDLT) allocation priority is based on the model for end-stage liver disease including sodium (MELD-Na) score. The United Network for organ sharing's 'Share-15' policy states that candidates with MELD-Na scores of 15 or greater have priority to receive local organ offers compared to candidates with lower MELD-Na scores. Since the inception of this policy, major changes in the primary etiologies of end-stage liver disease have occurred and previous assumptions need to be recalibrated.

METHODS

The authors retrospectively analyzed the Scientific Registry of Transplant Recipients database between 2012 and 2021 to determine life years saved by DDLT at each interval of MELD-Na score and the time-to-equal risk and time-to-equal survival versus remaining on the waitlist. The authors stratified our analysis by MELD exception points, primary disease etiology, and MELD score.

RESULTS

On aggregate, compared to remaining on the waitlist, a significant 1-year survival advantage of DDLT at MELD-Na scores as low as 12 was found. The median life years saved at this score after a liver transplant was estimated to be greater than 9 years. While the total life years saved were comparable across all MELD-Na scores, the time-to-equal risk and time-to-equal survival decreased exponentially as MELD-Na scores increased.

CONCLUSION

Herein, the authors challenge the perception as to the timing of DDLT and when that benefit occurs. The national liver allocation policy is transitioning to a continuous distribution framework and these data will be instrumental to defining the attributes of the continuos allocation score.

Removing geographic boundaries from liver allocation: A method for designing continuous distribution scores

BACKGROUND

The Organ Procurement and Transplantation Network (OPTN) is eliminating geographic boundaries in liver allocation, in favor of continuous distribution. Continuous distribution allocates organs via a composite allocation score (CAS): a weighted sum of attributes like medical urgency, candidate biology, and placement efficiency. The opportunity this change represents, to include new variables and features for prioritizing candidates, will require lengthy and contentious discussions to establish community consensus. Continuous distribution could instead be implemented rapidly by computationally translating the allocation priorities for pediatric, status 1, and O/B blood type liver candidates that are presently implemented via geographic boundaries into points and weights in a CAS.

METHODS

Using simulation with optimization, we designed a CAS that is minimally disruptive to existing prioritizations, and that eliminates geographic boundaries and minimizes waitlist deaths without harming vulnerable populations.

RESULTS

Compared with Acuity Circles (AC) in a 3-year simulation, our optimized CAS decreased deaths from 7771.2 to 7678.8 while decreasing average (272.66 NM vs. 264.30 NM) and median (201.14 NM vs. 186.49 NM) travel distances. Our CAS increased travel only for high MELD and status 1 candidates (423.24 NM vs. 298.74 NM), and reduced travel for other candidates (198.98 NM vs. 250.09 NM); overall travel burden decreased.

CONCLUSION

Our CAS reduced waitlist deaths by sending livers for high-MELD and status 1 candidates farther, while keeping livers for lower MELD candidates nearby. This advanced computational method can be applied again after wider discussions of adding new priorities conclude; our method designs score weightings to achieve any specified feasible allocation outcomes.

Ethics of allocation of donor organs

PURPOSE OF REVIEW

There is no widely accepted single ethical principle for the fair allocation of scarce donor organs for transplantation. Although most allocation systems use combinations of allocation principles, there is a particular tension between 'prioritizing the worst-off' and 'maximizing total benefits'. It is often suggested that empirical research on public preferences should help solve the dilemma between equity and efficiency in allocation policy-making.

RECENT FINDINGS

This review shows that the evidence on public preferences for allocation principles is limited, and that the normative role of public preferences in donor organ allocation policy making is unclear. The review seeks to clarify the ethical dilemma to the transplant community, and draws attention to recent attempts at balancing and rank-ordering of allocation principles.

SUMMARY

This review suggests that policy makers should make explicit the relative weights attributed to equity and efficiency considerations in allocation policies, and monitor the effects of policy changes on important ethics outcomes, including equitable access among patient groups. Also, it draws attention to wider justice issues associated not with the distribution of donor organs among patients on waiting lists, but with barriers in referral for transplant evaluation and disparities among patient groups in access to waiting lists.

A Conceptual Model for Sources of Differential Selection in Lung Transplant Allocation

Rationale: In the United States, donor lungs are allocated to transplant candidates on the basis of lung allocation scores (LAS). However, additional factors beyond the LAS can impact who is transplanted, including listing and donor-organ acceptance practices. These factors can result in differential selection, undermining the objectivity of lung allocation. Yet their impact on the lung transplant pathway has been underexplored. Objectives: We sought to systematically examine sources of differential selection in lung transplantation via qualitative methods. Methods: We conducted semistructured qualitative interviews with lung transplant surgeons and pulmonologists in the United States between June 2019 and June 2020 to understand clinician perspectives on differential selection in lung transplantation and the LAS. Results: A total of 51 respondents (30 surgeons and 21 pulmonologists) identified many sources of differential selection arising throughout the pathway from referral to transplantation. We synthesized these sources into a conceptual model with five themes: 1) transplant center's degree of risk tolerance and accountability; 2) successfulness and fairness of the LAS; 3) donor-organ availability and regional competition; 4) patient health versus program health; and 5) access to care versus responsible stewardship of organs. Conclusions: Our conceptual model demonstrates how differential selection can arise throughout lung transplantation and facilitates the further study of such selection. As new organ allocation models are developed, differential selection should be considered carefully to ensure that these models are more equitable.

Assessment of donor quality and risk of graft failure after liver transplantation: The ID2 EAL score

Accurate assessment of donor quality at the time of organ offer for liver transplantation candidates may be inadequately captured by the donor risk index (DRI). We sought to develop and validate a novel objective and simple model to assess donor risk using donor level variables available at the time of organ offer. We utilized national data from candidates undergoing primary LT (2013-2019) and assessed the prediction of graft failure 1 year after LT. The final components were donor Insulin-dependent diabetes mellitus, Donor type (DCD or DBD), cause of Death = CVA, serum creatinine, Age, height, and weight (length). The ID² EAL score had better discrimination than DRI using bootstrap corrected concordant index over time, especially in the current era. We explored donor-recipient matching. Relative risk of graft failure ranged from 1.15 to 3.5 based on relevant donor-recipient matching by the ID² EAL score. As an example, for certain recipients, a young DCD donor offer was preferable to an older DBD with relevant comorbidities. The ID² EAL score may serve as an important tool for patient discussion about donor risk and decisions regarding offer acceptance. In addition, the score may be preferable to succinctly capture donor risk in future organ allocation that considers continuous distribution (www.iddealscore.com).

Clinical impact of a modified lung allocation score that mitigates selection bias

BACKGROUND

The Lung Allocation Score (LAS) is used in the U.S. to prioritize lung transplant candidates. Selection bias, induced by dependent censoring of waitlisted candidates and prediction of posttransplant survival among surviving, transplanted patients only, is only partially addressed by the LAS. Recently, a modified LAS (mLAS) was designed to mitigate such bias. Here, we estimate the clinical impact of replacing the LAS with the mLAS.

METHODS

We considered lung transplant candidates waitlisted during 2016 and 2017. LAS and mLAS scores were computed for each registrant at each observed organ offer date; individuals were ranked accordingly. Patient characteristics associated with better priority under the mLAS were investigated via logistic regression and generalized linear mixed models. We also determined whether differences in rank were explained more by changes in predicted pre- or posttransplant survival. Simulations examined how 1-year waitlist, posttransplant, and overall survival might change under the mLAS.

RESULTS

Diagnosis group, 6-minute walk distance, continuous mechanical ventilation, functional status, and age demonstrated the highest impact on differential allocation. Differences in rank were explained more by changes in predicted pretransplant survival than changes in predicted posttransplant survival, suggesting that selection bias has more impact on estimates of waitlist urgency. Simulations suggest that for every 1000 waitlisted individuals, 12.8 (interquartile range: 5.2-24.3) fewer waitlist deaths per year would occur under the mLAS, without compromising posttransplant and overall survival.

CONCLUSIONS

Implementing a mLAS that mitigates selection bias into clinical practice can lead to important differences in allocation and possibly modest improvement in waitlist survival.

Dynamic impact of liver allocation policy change on donor utilization

Liver allocation policy was changed to reduce variance in median MELD scores at transplant (MMaT) in February 2020. "Acuity circles" replaced local allocation. Understanding the impact of policy change on donor utilization is important. Ideal (I), standard (S), and non-ideal (NI) donors were defined. NI donors include older, higher BMI donors with elevated transaminases or bilirubin, history of hepatitis B or C, and all DCD donors. Utilization of I, S, and NI donors was established before and after allocation change and compared between low MELD (LM) centers (MMaT ≤ 28 before allocation change) and high MELD (HM) centers (MMaT > 28). Following reallocation, transplant volume increased nationally (67 transplants/center/year pre, 74 post, p .0006) and increased for both HM and LM centers. LM centers significantly increased use of NI donors and HM centers significantly increased use of I and S donors. Centers further stratify based on donor utilization phenotype. A subset of centers increased transplant volume despite rising MMaT by broadening organ acceptance criteria, increasing use of all donor types including DCD donors (98% increase), increasing living donation, and transplanting more frequently for alcohol associated liver disease. Variance in donor utilization can undermine intended effects of allocation policy change.

"Beyond MELD" - Emerging strategies and technologies for improving mortality prediction, organ allocation and outcomes in liver transplantation

In this review article, we discuss the model for end-stage liver disease (MELD) score and its dual purpose in general and transplant hepatology. As the landscape of liver disease and transplantation has evolved considerably since the advent of the MELD score, we summarise emerging concepts, methodologies, and technologies that may improve mortality prognostication in the future. Finally, we explore how these novel concepts and technologies may be incorporated into clinical practice.

Evolving Characteristics of Heart Transplantation Donors and Recipients: JACC Focus Seminar

Although the burden of end-stage heart failure continues to increase, the number of available organs for heart transplantation (HT) remains inadequate. The HT community has been challenged to find ways to expand the number of donor hearts available. Recent advances include use of hearts from donors infected with hepatitis C virus as well as other previously underutilized donors, including those with left ventricular dysfunction, of older age, and with a history of cocaine use. Concurrently, emerging trends in HT surgery include donation after circulatory death, ex vivo normothermic heart perfusion, and controlled hypothermic preservation, which may enable procurement of organs from farther distances and prevent early allograft dysfunction. Contemporary HT recipients have also evolved in light of the 2018 revision to the U.S. heart allocation policy. This focus seminar discusses recent trends in donor and recipient phenotypes and management strategies for successful HT, as well as evolving areas and future directions.

Mitigating selection bias in organ allocation models

BACKGROUND

The lung allocation system in the U.S. prioritizes lung transplant candidates based on estimated pre- and post-transplant survival via the Lung Allocation Scores (LAS). However, these models do not account for selection bias, which results from individuals being removed from the waitlist due to receipt of transplant, as well as transplanted individuals necessarily having survived long enough to receive a transplant. Such selection biases lead to inaccurate predictions.

METHODS

We used a weighted estimation strategy to account for selection bias in the pre- and post-transplant models used to calculate the LAS. We then created a modified LAS using these weights, and compared its performance to that of the existing LAS via time-dependent receiver operating characteristic (ROC) curves, calibration curves, and Bland-Altman plots.

RESULTS

The modified LAS exhibited better discrimination and calibration than the existing LAS, and led to changes in patient prioritization.

CONCLUSIONS

Our approach to addressing selection bias is intuitive and can be applied to any organ allocation system that prioritizes patients based on estimated pre- and post-transplant survival. This work is especially relevant to current efforts to ensure more equitable distribution of organs.

Evaluation and selection of the liver transplant candidate: updates on a dynamic and evolving process

PURPOSE OF REVIEW

Although conceptually unchanged, the evaluation and selection of the liver transplant candidate has seen significant recent advances. Expanding criteria for transplant candidacy, improved diagnostics for risk stratification and advances in prognostic models have paralleled recent changes in allocation and distribution that require us to revisit core concepts of candidate evaluation and selection while recognizing its now dynamic and continuous nature.

RECENT FINDINGS

The liver transplant evaluation revolves around three interrelated themes: candidate selection, donor selection and transplant outcome. Introduction of dynamic frailty indices, bariatric surgery at the time of liver transplant in obese patients and improved therapies and prognostic tools for hepatobiliary malignancy have transformed candidate selection. Advances in hypothermic organ preservation have improved outcomes in marginal donor organs. Combined with expansion of hepatitis C virus positive and split donor organs, donor selection has become an integral part of candidate evaluation. In addition, with liver transplant for acute alcohol-related hepatitis now widely performed and increasing recognition of acute-on-chronic liver failure, selection of critically ill patients is refining tools to balance futility versus utility.

SUMMARY

Advances in liver transplant candidate evaluation continue to transform the evaluation process and require continued incorporation into our clinical practice amidst a dynamic backdrop of demographic and policy changes.