Expected effect of the lung Composite Allocation Score system on US lung transplantation

Efforts are underway to transition the current lung allocation system to a continuous distribution framework whereby multiple factors are simultaneously combined into a Composite Allocation Score (CAS) to prioritize candidates for lung transplant. The purpose of this study was to compare discrete CAS scenarios with the current concentric circle-based allocation system to assess their potential effects on the US lung transplantation system using the Scientific Registry of Transplant Recipients' thoracic simulated allocation model. Six alternative CAS scenarios were compared over 10 simulation runs using data from individuals on the lung transplant waiting list from January 1, 2018, through December 31, 2019. Outcome measures were transplant rate, count, waitlist deaths, posttransplant deaths within 2 years, donor-to-recipient distance, and percentage of organs predicted to have flown. Across scenarios, waitlist deaths decreased by 36% to 47%, with larger decreases in deaths at lower placement efficiency weight and higher weighting of the waitlist outcomes. When waitlist outcomes were equally weighted to posttransplant outcomes, more transplants occurred in individuals with the highest expected posttransplant survival. All CAS scenarios led to improved overall measures of equity compared with the current Lung Allocation Score system, including reduced waitlist deaths, and resulted in similar posttransplant survival.

Can school-based distribution be used to maintain coverage of long-lasting insecticide treated bed nets: evidence from a large scale programme in southern Tanzania?

Many sub-Saharan African countries have achieved substantial gains in insecticide treated bednet coverage since 2005. The Tanzania National Malaria Control Programme identified school-based net distribution as one potential 'keep-up' strategy for the purpose of maintaining long-lasting insecticidal net (LLIN) coverage after a nationwide mass campaign in 2011. The School Net Programme (SNP) was implemented in three regions of southern Tanzania and distributed one LLIN to each enrolled child attending schools in primary grades (standards) 1, 3, 5 and 7, and secondary grades (forms) 2 and 4 in 2013 and again with slightly modified eligibility criteria in 2014 and 2015. Household surveys in the programme area as well as in a control area were conducted after each of the SNP distributions to measure ownership and use of long-lasting insecticide treated nets. Ownership of at least one LLIN after the first distribution was 76.1% (95% CI 70.8-80.7) in the intervention area and 78.6% (95% CI 74.4-82.3) in the control area. After the second distribution, ownership of at least one LLIN had dropped significantly in the control area to 65.4% (95% CI 59.5-71.0) in 2015 (P < 0.001), while coverage in the intervention area was maintained at 79.3% (95% CI 75.4 × 82.6). Ownership of at least one LLIN in intervention area remained stable following the second round of net distribution. During the same period LLIN ownership, especially of enough nets to ensure all household member access, fell significantly in the control area. These results demonstrate that the SNP may be sufficient to maintain stable LLIN coverage following a mass distribution of LLINs.

Logistical burden of offers and allocation inefficiency in circle-based liver allocation

Recent changes to liver allocation replaced donor service areas with circles as the geographic unit of allocation. Circle-based allocation might increase the number of transplantation centers and candidates required to place a liver, thereby increasing the logistical burden of making and responding to offers on organ procurement organizations and transplantation centers. Circle-based allocation might also increase distribution time and cold ischemia time (CIT), particularly in densely populated areas of the country, thereby decreasing allocation efficiency. Using Scientific Registry of Transplant Recipient data from 2019 to 2021, we evaluated the number of transplantation centers and candidates required to place livers in the precircles and postcircles eras, nationally and by donor region. Compared with the precircles era, livers were offered to more candidates (5 vs. 9; p  < 0.001) and centers (3 vs. 5; p  < 0.001) before being accepted; more centers were involved in the match run by offer number 50 (9 vs. 14; p  < 0.001); CIT increased by 0.2 h (5.9 h vs. 6.1 h; p  < 0.001); and distribution time increased by 2.0 h (30.6 h vs. 32.6 h; p  < 0.001). Increased burden varied geographically by donor region; livers recovered in Region 9 were offered to many more candidates (4 vs. 12; p  < 0.001) and centers (3 vs. 8; p  < 0.001) before being accepted, resulting in the largest increase in CIT (5.4 h vs. 6.0 h; p  < 0.001). Circle-based allocation is associated with increased logistical burdens that are geographically heterogeneous. Continuous distribution systems will have to be carefully designed to avoid exacerbating this problem.

Evaluation of Long-Lasting Insecticidal Net Distribution Through Schools in Southern Tanzania

Universal coverage with effective vector control remains the mainstay of malaria vector control in sub-Saharan Africa Tanzania has utilized a number of mechanisms for the maintenance of long-lasting insecticidal net (LLIN) coverage over time. Schools have been identified as one potential channel for continuous distribution of LLIN. This research aims to evaluate an annual school based LLIN distribution programme in Tanzania which began in 2013, called the School Net Programme (SNP). Following each of the first four rounds of SNP distribution, a household survey was conducted in intervention and comparison districts in Southern and Lake zones of Tanzania (N = 5083 households). Measures of ownership, access, and use were compared between intervention and comparison districts. Determinants of reach were assessed in intervention districts. Population access to an LLIN increased from 63.1% (95% CI: 58.8, 67.5) to 76.5% (95% CI: 72.9, 80.0) in the intervention districts between the first and last surveys. Access also rose in the comparison districts from 51.4% (95% CI: 46.9, 55.9) to 79.8% (95% CI: 77.3, 82.0) following a mass distribution and implementation of school-based distribution during the study period. LLIN use increased in intervention districts from 44.9% (95% CI: 40.5, 49.3) to 65.6% (95% CI: 59.4, 71.8) and from 57.2% (95% CI: 49.7, 64.7) to 77.4% (95% CI: 69.3, 85.5) specifically amongst primary school-aged children. Households reached by the SNP were wealthier households with children enrolled in school. The SNP in Tanzania was able to maintain population level LLIN ownership, use, and access in the absence of a mass distribution. The SNP successfully reached households which housed school-aged children. Alternative delivery strategies may need to be considered to reach households without children enrolled in school which experienced less benefit from the program.

Impact of the lung allocation system score modification by blood type on US lung transplant candidates

The lung continuous distribution system was modified on September 27, 2023, with the goal of increasing transplant access for blood type O candidates after an error was discovered in the simulation used to support the development of the initial allocation policy. This retrospective observational study compares national waitlist outcomes (transplant rate, waitlist mortality) under continuous distribution before (March 10, 2023, through September 26, 2023; premodification) and after (September 27, 2023, through April 14, 2024; postmodification) the blood type score modification. We fit adjusted Poisson regression models of the transplant rate and mortality rate. The transplant rate was lowest for type O candidates in both eras, but significantly increased after the score modification, from a premodification adjusted rate ratio (95% CI) of 0.40 (0.36, 0.45) to postmodification 0.52 (0.45, 0.59), relative to premodification type A candidates. The adjusted mortality incidence (95% CI) decreased in type O candidates from 3.6% (3.0%, 4.3%) premodification to 3.2% (2.6%, 3.8%) postmodification. In an exploratory analysis, we estimated there would have been the same number of waitlist deaths (approximately 105) if the modified score had been adopted at the start of continuous distribution; however, transplants would have shifted toward type O candidates (57.8 [95% CI: 35.1, 80.9] additional transplants) and deaths would have shifted away from type O candidates (4.6 [95% CI: 2.7, 6.8] fewer deaths).

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.

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.

Empirical Squared Hellinger Distance Estimator and Generalizations to a Family of α-Divergence Estimators

We present an empirical estimator for the squared Hellinger distance between two continuous distributions, which almost surely converges. We show that the divergence estimation problem can be solved directly using the empirical CDF and does not need the intermediate step of estimating the densities. We illustrate the proposed estimator on several one-dimensional probability distributions. Finally, we extend the estimator to a family of estimators for the family of α-divergences, which almost surely converge as well, and discuss the uniqueness of this result. We demonstrate applications of the proposed Hellinger affinity estimators to approximately bounding the Neyman-Pearson regions.

Going the distance: Geographic effects of the lung transplant composite allocation score

Background

In March 2023, the lung allocation policy underwent major changes from a tiered structure to a composite allocation score (CAS). The goal was to improve allocation equity for disadvantaged groups by deprioritizing transplant distance. This study examines the effects of CAS on geographic trends and transportation efficiency.

Methods

A retrospective cohort study was conducted using the United Network for Organ Sharing database, queried for adult lung transplants from September 1, 2022 to September 1, 2023. Outcomes were nautical distance of transplant, ischemic time, and flight required for transport (estimated as distance >100 miles). Perioperative complications and early survival were analyzed, with propensity matching to account for baseline differences.

Results

A total of 1,394 pre-CAS and 1,197 post-CAS patients were included in the study cohort. Post-CAS recipients were less likely to be ABO type O (39.2% vs 47.3%, p < 0.001) and were less likely to be an identical ABO match (82.7% vs 91.0%, p < 0.001). The CAS cohort traveled significantly further (354.0 miles [interquartile range (IQR): 139-657] vs 195.0 miles [IQR: 78-388], p < 0.001). CAS patients had longer ischemic times (6.8 hours [IQR: 5.3-8.9] vs 6.0 hours [IQR: 4.8-7.5], p < 0.001), and CAS procurements were more likely to require a flight for transport (n = 934, 78.0% vs n = 991, 71.1%, p < 0.001). However, waitlist time was shorter (28 days [IQR = 9-83] vs 33 days [IQR = 11-109]) as was the length of stay (24.21 ± 17.84 days vs 31.44 ± 30.19 days, p < 0.001) for CAS recipients, which remained true after propensity matching.

Conclusions

The lung CAS policy change was intended to eliminate geographic boundaries for disadvantaged patients but has expanded transplant distances, with an expected increase in ischemic time and need for flights which affect transplant economics. Although efforts were made to improve transplant availability for the disadvantaged ABO type O group, early assessment of the recipient cohort showed that recipients with blood group type O were actually less likely to be transplanted under the new policy. Positive effects include an overall decrease in waitlist time, but further investigation is warranted to evaluate the effectiveness, equity, and economic sustainability of the new policy.