Lung allocation in the United States has changed remarkably over the last two decades. Historically, lung allocation was based on wait time, and donor organs were offered first within donation service areas (DSAs)—geographic regions that evolved through local relationships in the early years of organ transplantation. Importantly, these were not designed to optimize organ allocation or distribution.
In 2000, the US Department of Health and Human Services issued the Final Rule requiring that the Organ Procurement and Transplantation Network (OPTN) develop policies for equitable allocation of deceased donor organs based on sound medical judgement in order to achieve the best use of donated organs, avoid futile transplants, promote patient access to transplants, and promote the efficient management of organ placement.1 Specifically, the Final Rule directs that allocation shall not be based on a candidate’s place of residence or place of listing.
In 2005, donor lung allocation in the United States moved from a wait time-based system to an urgency-weighted lung allocation score (LAS). The LAS was based on an estimate of transplant benefit calculated using a waitlist urgency measure and a posttransplant survival measure.2 The LAS led to an increase in number of lung transplants and a marked decrease in waitlist mortality.3 Further, the LAS era led to an increase in proportion of lung transplant recipients over the age of 65 years, with fibrotic lung disease becoming the most frequent indication for transplant.
Prioritizing patients who are sicker led to unintended consequences, including an increase in transplant hospitalization costs and the use of posttransplant tracheostomy and extracorporeal membrane oxygenation.4 Organ offers remained restricted to the DSAs, and geographical variation in donor lung availability and access to transplant persisted. In November 2017, after a lawsuit challenged the use of DSAs to distribute lungs in the United States, OPTN revised its lung distribution policy, replacing DSAs with 250-mile-radius circles extending from the donor hospital.5
Following this, the OPTN Board of Directors created an Ad Hoc Committee on Geography, which established guiding principles for handling geographic distribution of organs for transplant.6 The committee recommended organ distribution without geographic boundaries.7 This led to the development of a continuous distribution framework.
The lung was the first organ to transition to a continuous distribution system.8 The composite allocation score (CAS) has been used to prioritize lung transplant candidates since March 9, 2023.9 The CAS was designed to broaden geographic distribution of donor lungs by removing rigid geographic boundaries and to simultaneously account for multiple candidate attributes in the distribution of donor lungs.
The CAS comprises five components: medical urgency (predicted one-year waitlist survival); posttransplant outcomes (five-year posttransplant survival); candidate biology (blood type, human leukocyte antigen sensitization, candidate height); patient access (prior living donation, pediatric age group); and placement efficiency (travel, proximity). Community involvement was an integral part of the development of the CAS at each stage. Unlike the LAS—which was calculated for each candidate—the CAS is calculated for each organ offer.
The CAS has had a transformative effect on lung transplant clinical practice. In 2024—the first full year of implementation of the CAS—lung transplantation in the United States reached record high levels, with 3,404 lung transplants performed.10 Also in 2024, we observed 3,780 new adult candidates added to the waitlist (highest recorded), the highest deceased donor lung transplant rate for adult candidates (351 transplants per 100 patient-years, a more than threefold increase from 2013), and a marked decrease in waitlist times.
More than two-thirds of candidates on the waitlist received lung transplant within three months of listing. Pretransplant mortality rate for patients who are the most sick is decreasing (135 deaths per 100 patient-years in 2024 from 214 deaths per 100 patient-years in 2015), despite their increasing numbers (26% of candidates on the waitlist in 2024 compared with 17% a decade ago). Geographic distances between donor and recipient hospitals have grown markedly, with more than half of all lung transplants involving travel distances of 500 miles or more. Modest improvements have been noted in one-year posttransplant survival (90.6% for 2023). Long-term outcomes from the CAS era are awaited.
The advent of the CAS has altered the factors associated with lung transplant in the United States—younger age is associated with higher likelihood of transplant, and sex-based disparities have diminished.11 Under the CAS, blood type O, shorter stature, and diagnosis groups B (pulmonary hypertension) and D (obstructive lung disease) were associated with lower likelihood of transplant.
As the most substantial change in lung allocation since the LAS, the CAS has resulted in several intended improvements, but some limitations and additional opportunities for further refinement of allocation policy continue to be identified.
References
1. US Department of Health and Human Services. Organ Procurement and Transplantation Network; final rule. 42 CFR §121.8(a)(1),(2),(5),(8);§121.8(b)(2)-(3). https://www.ecfr.gov/current/title-42/chapter-I/subchapter-K/part-121/section-121.8
2. Egan TM, Murray S, Bustami RT, et al. Development of the new lung allocation system in the United States. Am J Transplant. 2006;6(5 pt 2):1212-1227. doi:10.1111/j.1600-6143.2006.01276.x
3. 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. doi:10.1016/j.healun.2016.01.010
4. Maxwell BG, Mooney JJ, Lee PHU, et al. Increased resource use in lung transplant admissions in the lung allocation score era. Am J Respir Crit Care Med. 2015;191(3):302-308. doi:10.1164/rccm.201408-1562OC
5. Egan TM. From 6 years to 5 days for organ allocation policy change. J Heart Lung Transplant. 2018;37(5):675-677. doi:10.1016/j.healun.2017.12.010
6. OPTN/UNOS Ad Hoc Committee on Geography. Geographic organ distribution principles and models recommendations report. Organ Procurement and Transplantation Network; 2018.
7. Snyder JJ, Salkowski N, Wey A, Pyke J, Israni AK, Kasiske BL. Organ distribution without geographic boundaries: a possible framework for organ allocation. Am J Transplant. 2018;18(11):2635-2640. doi:10.1111/ajt.15115
8. OPTN Thoracic Organ Transplantation Committee. Continuous distribution of lungs: concept paper. Organ Procurement and Transplantation Network; 2019.
9. OPTN Lung Transplantation Committee. Establish continuous distribution of lungs. Organ Procurement and Transplantation Network; 2021.
10. Valapour M, Masotti M, Schladt DP, et al. OPTN/SRTR 2024 annual data report: lung. Am J Transplant. 2026;26(8S1):S474-S550. doi:10.1016/j.ajt.2026.05.724
11. Mupfudze TG, Weiss S, Hawkins CJ, et al. Likelihood of lung transplantation before and after introduction of the lung composite allocation score. J Heart Lung Transplant. Published online April 15, 2026. doi:10.1016/j.healun.2026.04.011
