Proposed Section Structure
This article is organized into the following sections to match the clinical and policy importance of the study: Highlights; Background and policy context; Study design and methods; Key findings; Clinical and policy interpretation; Strengths and limitations; Implications for practice and future policy; Funding and trial registration; References.
Highlights
First, geographic variation in access to donor lungs persists despite the shift from fixed distance boundaries to the Composite Allocation Score, or CAS, continuous distribution framework. Second, listing center location remained independently associated with distance-adjusted donor availability even after accounting for candidate biology and center size. Third, the May 7, 2026 CAS amendment, which increases the weight assigned to proximity, is projected to reduce effective donor availability overall and widen geographic disparities. Fourth, candidates with biological barriers to matching, particularly those with blood type O, appear more vulnerable to de-prioritization when proximity receives greater weight.
Background and Policy Context
Equity in organ allocation has long been a central goal of U.S. transplantation policy. In lung transplantation, the balance is especially delicate because donor lungs are highly perishable, transport logistics matter, and waitlist mortality can be substantial for medically urgent candidates. Historically, U.S. lung allocation relied on frameworks that used geographic boundaries or distance-based thresholds to influence organ offers. Those approaches were widely criticized because arbitrary geographic cut points could create unequal access for otherwise similar patients listed at different centers.
In March 2023, lung allocation transitioned to a continuous distribution model using the Composite Allocation Score. CAS was designed to rank candidates using several domains rather than rigid local-regional-national boundaries. These domains include medical urgency, expected posttransplant survival, biological compatibility, and geographic efficiency, the latter incorporating continuous donor-candidate distance rather than hard thresholds. In principle, this model should reduce abrupt inequities introduced by categorical geography.
Even so, geography may continue to matter in more subtle ways. Some centers are situated in regions with denser donor supply, different patterns of biologic compatibility, or more favorable transport networks. If so, candidates listed at different centers may still experience materially different access to donor lungs despite living under the same national allocation framework. That concern becomes more pressing in light of the Organ Procurement and Transplantation Network policy amendment approved on May 7, 2026, which increases the influence of donor-candidate proximity in CAS ranking.
The study by Valapour and colleagues directly addresses this issue by examining whether listing center location is associated with effective donor availability under the current system and by simulating how the amended policy may alter candidate prioritization.
Study Design and Methods
Design and Data Source
This was a retrospective cohort study using Scientific Registry of Transplant Recipients, or SRTR, data. SRTR is the principal national data resource for studying U.S. transplant candidates, donors, and recipients and is widely used for both policy evaluation and center-level performance analyses.
Study Population
The analysis included adult lung transplant candidates, recipients, and donors from September 27, 2023, through December 1, 2024. A total of 3,917 adult candidates from 61 transplant centers were studied. This time window is important because it reflects real-world practice after implementation of CAS continuous distribution, allowing assessment of whether the new framework achieved its geographic equity goals in practice.
Primary Outcome
The primary metric was distance-adjusted donor availability. This is a conceptually strong outcome because it moves beyond simple donor counts. The investigators defined it as the number of biologically compatible donors recovered within 30 days of listing, weighted by the CAS geographic efficiency function. In other words, donors counted more heavily if their distance from the candidate translated into more favorable geographic efficiency within the allocation algorithm.
This approach is clinically relevant. A donor is not equally “available” to every candidate. Availability depends on biologic match characteristics, the timing of recovery, and the degree to which transport distance affects candidate ranking. By integrating these features, the authors aimed to estimate practical access rather than theoretical donor supply.
Analytic Strategy
Spatial regression models were used to evaluate geographic variation across transplant centers. The models adjusted for center size and candidate biological characteristics, helping distinguish whether observed differences were due to listing geography itself rather than simply case mix. The authors also performed counterfactual analyses to simulate allocation under the amended CAS policy and match-run analyses to assess how actual donor offers during the study period would reorder candidates under the newer proximity weighting.
These methods are well chosen for a policy study. Spatial modeling captures the possibility that nearby centers may share similar access environments, while counterfactual allocation and match-run analyses translate abstract scoring changes into clinically interpretable effects on ranking.
Key Findings
Substantial Between-Center Variation in Effective Donor Access
The headline result is that center-level distance-adjusted donor availability varied substantially across the United States. The median center-level value was 1.90 donors per candidate-day, with an interquartile range of 1.75 to 2.14. That may sound like a narrow numerical spread at first glance, but in the context of organ allocation even modest differences in effective donor supply can translate into meaningful changes in waiting time, probability of offer acceptance, and risk of clinical deterioration while awaiting transplant.
Importantly, the investigators report that this variation was not fully explained by biologic factors or center size. After adjustment, listing center location remained significantly associated with donor availability. This is arguably the study’s most policy-relevant finding. It suggests that continuous distribution attenuated, but did not eliminate, the structural influence of geography on access.
Why Geography Still Matters Under a Continuous Model
The persistence of geographic variation under CAS likely reflects several overlapping mechanisms. Donor density is uneven across the country. Organ recovery practices and local procurement patterns vary. Candidate pools differ in blood type distribution and sensitization burden. Air transport availability, ischemic time considerations, and center-level offer acceptance behavior may also contribute. A continuous distance function is more refined than a geographic boundary, but it does not neutralize these real-world asymmetries.
This is an important conceptual point for clinicians and policymakers: replacing a categorical geography rule with a continuous geography rule does not mean geography disappears from the system. It simply means geography acts more smoothly.
The 2026 CAS Amendment May Reduce Effective Availability
The study’s counterfactual analyses project that increasing the influence of proximity in the amended CAS policy would reduce effective donor availability overall. From a policy perspective, this is notable because the amendment may have been motivated by logistical efficiency or concerns about ischemic time. Yet the analysis suggests that the tradeoff may include less equitable donor access.
Although the abstract does not provide a center-by-center quantitative estimate of the reduction, the directional finding is consistent and clinically intuitive: if proximity is weighted more heavily, candidates listed in donor-rich or geographically advantaged areas will move up relative to otherwise comparable candidates located farther away. That narrows the practical donor pool for some patients and centers.
Geographic Disparities Are Projected to Worsen
Beyond reducing overall effective availability, the amended policy is predicted to increase geographic disparities in donor access. This result deserves emphasis because it addresses not only efficiency but fairness. Allocation systems must inevitably incorporate some transport-related considerations, but a policy that systematically amplifies center-location advantages raises concerns about distributive justice, particularly when the disadvantaged group consists of medically similar candidates who simply happen to be listed in less favorable locations.
Candidate Reordering Was Frequent in Match-Run Analysis
The match-run analysis makes the policy implications concrete. Among candidates ranked within the top 10 under the prior CAS policy, 23.9% would fall outside the top 10 under the amended policy. That is a large degree of rank instability. In organ allocation, top-10 status matters because it often determines whether a candidate is realistically in contention for an offer. A nearly one-in-four displacement rate indicates that the scoring change is not a subtle administrative adjustment; it can meaningfully alter who receives access to a donor organ.
Biologically Harder-to-Match Candidates May Be Disproportionately Harmed
Candidates with biological barriers to donor matching, including those with blood type O, experienced greater de-prioritization under the amended policy. This is clinically and ethically significant. Blood type O candidates already face well-recognized constraints in many organ allocation contexts because they can only receive type O organs, whereas type O donor organs are in demand across recipient groups depending on organ-specific rules. If proximity weighting further disadvantages these candidates, the policy could compound biologic inequity with geographic inequity.
The same logic may extend, though the abstract does not specify details, to other harder-to-match subgroups such as highly sensitized candidates or those with rarer size or compatibility profiles. In continuous distribution systems, biologic disadvantage and geographic disadvantage can interact rather than act independently.
Clinical and Policy Interpretation
This study has immediate relevance for transplant physicians, surgeons, administrators, and policymakers. For clinicians counseling patients, the findings reinforce that listing location may still influence practical access to donor lungs even after the move to CAS. That does not mean center choice should be driven only by geography; center expertise, candidacy criteria, perioperative outcomes, and support systems remain critical. However, the data suggest that listing strategy and access discussions may warrant greater transparency.
For policy leaders, the study is a reminder that allocation reform should be evaluated using realized access metrics, not just theoretical fairness built into the scoring formula. A continuous distribution model may look more equitable on paper than a boundary-based model, but if effective donor availability still varies by listing center, additional calibration may be needed. The findings also caution against assuming that stronger proximity weighting is a neutral operational refinement. It may shift the equity-efficiency balance in ways that disadvantage certain regions and biologically constrained candidates.
There is also a broader systems implication. If proximity gains too much importance, centers in geographically favorable areas may become relatively more attractive to patients able to travel or dual-list, while disadvantaged populations with fewer resources may remain tied to centers with lower effective donor access. That could intensify socioeconomic inequities layered on top of geographic ones, although this specific study was not designed to test that hypothesis.
Strengths and Limitations
Strengths
The study has several notable strengths. It uses a national registry with near-comprehensive capture of U.S. lung transplant activity. The outcome metric, distance-adjusted donor availability, is more clinically meaningful than raw donor counts. The combination of spatial regression, counterfactual simulation, and match-run analysis provides both etiologic insight and practical policy relevance. Most importantly, the study addresses a live allocation question rather than a historical one, making the results timely for decision-makers.
Limitations
As with any retrospective registry-based analysis, the study is observational and cannot fully disentangle all mechanisms underlying geographic differences. Some contributors to donor access, such as center-specific offer acceptance practices, transport capacity, donor management quality, and local operational behavior, may be imperfectly measured in SRTR data. The study period, while appropriate for early post-CAS evaluation, is relatively short and may not capture longer-term adaptation by centers. In addition, the abstract focuses on donor availability and candidate ranking rather than hard clinical endpoints such as waitlist mortality, transplant rate, or posttransplant survival under the amended policy.
Another important limitation is that counterfactual analyses, while highly informative, remain simulations. They estimate what would likely happen under the amended scoring rules using observed donor events and candidate characteristics, but real-world implementation could trigger behavioral changes by centers and organ procurement organizations. Those responses may either magnify or attenuate the projected disparities.
Implications for Practice and Future Research
Several practical questions follow from this work. First, future evaluations should test whether center-level variation in distance-adjusted donor availability translates into differences in waitlist mortality, time to transplant, and posttransplant outcomes. Second, policymakers may need to reconsider how much weight geographic efficiency should carry relative to urgency and biology, especially for candidates who are already hard to match. Third, subgroup analyses are needed to identify whether certain diagnoses, sensitization levels, body size categories, or rural populations are especially affected.
There may also be a role for policy safeguards. For example, if stronger proximity weighting is retained for logistical reasons, the system might need compensatory adjustments that protect candidates facing biologic barriers. Another possibility is more explicit monitoring of geographic equity metrics after policy implementation, with pre-specified triggers for recalibration. In allocation science, iterative adjustment is often preferable to assuming a single scoring formula will solve all tensions permanently.
For transplant programs, these findings support continued transparency with patients about how allocation works in practice. Programs may also wish to examine whether local offer acceptance and donor utilization patterns can mitigate some access disadvantages without compromising outcomes.
Conclusion
The central message of this study is clear: geography still matters in U.S. lung transplantation, even under continuous distribution. Listing center location remained independently associated with effective donor availability after CAS implementation, indicating that policy modernization has not fully neutralized place-based disparities. The May 2026 amendment that increases proximity weighting is likely to worsen those disparities and may particularly disadvantage biologically harder-to-match candidates such as those with blood type O.
For clinicians, the study sharpens discussions about access, listing strategy, and the lived consequences of allocation policy. For policymakers, it is a strong warning that efficiency-oriented changes can unintentionally compromise equity. The challenge ahead is not simply to make geography continuous, but to ensure that geography does not remain destiny.
Funding and ClinicalTrials.gov
The abstract provided does not report specific funding information. No ClinicalTrials.gov registration applies to this retrospective SRTR registry analysis.
References
1. Valapour M, Gunsalus PR, Rose J, Lehr CJ, Baker SL, Dalton JE. Geography as a Determinant of Lung Transplant Access in the United States. Chest. 2026-05-22. PMID: 42176849.
2. Egan TM, Edwards LB. Effect of the lung allocation score on lung transplantation in the United States. J Heart Lung Transplant. 2016;35(4):433-439.
3. Organ Procurement and Transplantation Network. Continuous Distribution of Lungs. OPTN policy and public materials. Available through the OPTN policy resource center.
4. Scientific Registry of Transplant Recipients. SRTR risk adjustment and data source documentation. Available through the SRTR official reports and methodology resources.
5. Valapour M, Lehr CJ, Skeans MA, et al. OPTN/SRTR 2023 Annual Data Report: Lung. Am J Transplant or successor annual reporting platform, official SRTR/OPTN publication series.