Geographic disparities in lung transplantation in the United States before and after the November 2017 allocation change

Abstract

BACKGROUND:

The primary lung allocation unit was expanded from the donation service area to a 250-mile radius in 2017. Prior to the change, geographic disparities in donor lung availability impacted waitlist outcomes. We sought to determine if the new allocation system improved these disparities.

METHODS:

We conducted a retrospective cohort study comparing the 2-year period before and after the change. Donor lung availability was defined as the ratio of donor lungs to waitlist candidates in the primary allocation unit. Transplant centers were divided into quartiles by donor lung availability. Multivariable competing risk models were used to determine the association between lung availability and waitlist outcomes. Multivariable Cox proportional hazards models compared post-transplant survival.

RESULTS:

Prior to the allocation change, the unadjusted transplant rate at centers in the lowest and highest quartiles was 132 and 607 transplants per 100 waitlist years. Candidates in the lowest quartile of donor lung availability had a 61% adjusted lower transplantation rate compared to candidates in highest quartile (sub-hazard ratio [sHR]: 0.39, 95% confidence interval [CI]: 0.34-0.44). After the allocation change, the disparity decreased resulting in an unadjusted transplant rate of 141 and 309 among centers in the lowest and highest quartiles. Candidates in the lowest quartile had a 38% adjusted lower transplantation rate compared to those in the highest (sHR: 0.62, 95% CI: 0.57-0.68). There was no significant difference in 1-year post-transplant survival.

CONCLUSIONS:

Although the expansion of the primary allocation unit improved disparities in waitlist outcomes without any change in post-transplant survival, there still remain significant differences due to geography.

The U.S. Department of Health and Human Services issued the “Final Rule” in 1999 as a guideline for organ allocation. The “Final Rule” mandated that organ transplants are allocated primarily based on medical urgency with broad geographic sharing while trying to avoid futility and preserving efficiency.¹ After years of work, debate and planning, the United Network of Organ Sharing (UNOS) created the Lung Allocation Score (LAS) as the primary determinant of lung allocation, which accounts for both waitlist urgency and post-transplant survival.² Although the LAS-based system was very successful in diverting donor lungs for transplantation based primarily on medical urgency rather than waiting time, it failed to address the geographic disparities in organ availability.³

Prior to November 24, 2017, donor lungs were first offered to candidates listed at transplant centers within the local donation service area (DSA) before being offered to those outside of the local DSA.⁴ Unfortunately, the borders of a DSA are arbitrary and are not based on population density, size, or donor supply,⁵ resulting in disparate waiting list outcomes based on geography.^5-8 Ultimately, a waitlisted lung transplant candidate on mechanical support in our institution in New York City filed a lawsuit against the U.S. Department of Health and Human Services arguing that organ allocation based on geography was not consistent with the Final Rule.⁹

This lawsuit was filed on November 19, 2017; an emergency hearing was held on the following day, and the donor lung allocation system was changed dramatically on November 24, 2017.^4,9 The primary lung allocation unit was expanded from the DSA to a 250 nautical mile radius around the donor hospital. Using Thoracic Simulation Allocation Modelling to model broader geographic sharing, Mooney et al predicted that this change would result in a reduction in waitlist mortality and a rise in acuity at the time of transplant.¹⁰

In our earlier analysis of the DSA-based allocation system, we coined the term “local donor lung availability” defined as the number of donor lungs per wait list candidates, essentially quantifying supply and demand in a specific geographic primary allocation unit. We demonstrated that local lung availability varied significantly across the country and that low local lung availability was associated with both lower transplant rates and increased risk of death on the waiting list.⁵ We hypothesized that compared to the pre-allocation change era, donor lung availability in the post-allocation change era would (1) vary less by geography, and (2) be associated with decreased geographic disparities in lung transplantation and waiting list mortality.

Materials and methods

Study design

In this retrospective cohort study, all patients actively listed for lung transplant in the United States between November 24, 2015 and November 24, 2019 were included. Patients were excluded if they were less than 12 years of age, listed for multiorgan transplant, listed at more than one transplant center, or listed at transplant centers that were not active during the study period (Figure S1). Data were provided by UNOS, and this study was approved by the Columbia University Institutional Review Board.

Patients were divided into 2 cohorts based on when they were active on the waitlist relative to November 24, 2017: the preallocation change cohort and the postallocation change cohort. The primary predictor variable was donor lung availability which was defined as the number of donor lungs per person on waitlist. This was calculated by dividing the average number of annual donor lungs obtained for transplant in the primary allocation unit (DSA or 250 nautical miles from the transplant center) by the average waiting list size in that allocation unit. The waitlist size of the primary allocation unit was calculated at the time of each donor lung obtained for transplant. Prior to the allocation change, the waitlist size and consequently the donor lung availability would be the same for all transplant centers in the same DSA. However, after the allocation change, the waitlist size, donor lung supply and consequently the donor lung availability would vary for nearly all transplant centers due to different 250 nautical mile radiuses (S1). Transplant centers were divided into donor lung availability quartiles (DLA quartiles).

The primary outcome was rate of lung transplantation. The pre-allocation change cohort included all patients who were active on the waitlist from November 24, 2015 until November 24, 2017. Waitlist outcomes were censored on November 24, 2017 for this cohort. Patients who were not transplanted or delisted for death/clinical deterioration were censored on November 24, 2017 for this cohort. The postallocation change cohort included all patients who were active on the waitlist from November 25, 2017 until November 24, 2019, with follow-up through March 1, 2020. In both cohorts, the LAS was recorded at listing or at the start of the study period. All waitlisted patients were also censored at the time of delisting for reasons other than transplant, death/delisting for clinical deterioration.

The secondary outcomes were death/delisting for clinical deterioration, waitlist outcomes by LAS subgroups, changes in transplantation rate at transplant centers that moved between donor lung availability quartiles after the allocation changes, LAS at transplant and 1-year post-transplant survival. For post-transplant survival, only transplants prior to January 1, 2019 were included in the analysis to allow at least 1-year follow-up for the analysis.

Statistical analysis

The chi-squared test and Kruskal-Wallis test were used to compare baseline characteristics. Multivariable competing risk regression models were used to determine the relationship between donor lung availability and waitlist outcomes¹¹ and a multivariable Cox-proportional hazards model was used to compare post-transplant survival.

We used a directed acyclic graph to select the covariates that closed backdoor paths and to select the precision variables^9,12,13 (Figure S2). Center organ acceptance practices were estimated using the most recently published data from the Scientific Registry of Transplant Recipients (SRTR).¹⁴ In the primary model we adjusted for the LAS at listing (or LAS at study start), center organ acceptance, blood type, requirement for prospective crossmatch, requirement for double lung transplant, age, ethnicity/race, height and diagnosis (Figure S2).

For the primary model, there was no missing data for the primary outcome or any covariates. We performed sensitivity analyses to corroborate our findings (Table S1). These included an analysis with only patients newly activated on the transplant list during each study period, an analysis with different censoring for the post allocation cohort and an analysis using a mixed effects Poisson regression model.⁷

For our secondary outcomes, we used an adjusted competing risk fine and gray model to compare the transplantation rate before and after the allocation change for patients that were listed at centers that moved to a higher or lower donor lung availability quartile. The primary predictor variable was the allocation change and the adjusted covariates were the same as the primary model. We assumed that organ acceptance practices did not change during the study periods.

All statistical analyses were performed using Stata version 15.1 (StataCorp, College Station, TX) using stcreg, stcox, and mepoisson commands.¹⁵

Results

There were 6524 lung transplant candidates waitlisted in the pre-allocation change cohort and 6847 lung transplant candidates waitlisted in the postallocation change cohort. Age, mechanical support, body mass index, height, and blood type were similar at listing. There was a small increase in Black and Hispanic transplant recipients after the allocation change with a corresponding decrease in white transplant recipients. The median LAS at transplant rose modestly from the pre- to the postallocation change period while the 75th percentile LAS at transplant rose more significantly from 51.3 to 57.6. Among transplanted patients, diagnosis group A (obstructive lung disease) was less frequent as an indication for transplant in the post allocation cohort while diagnosis group D (interstitial lung disease) was more frequent. As expected, the median waitlist size of the primary allocation unit was considerably larger in the postallocation change cohort: 192 (interquartile range [IQR]: 122-370) compared with 61 (IQR: 28-88). The donor lung availability was only slightly higher in the post- allocation change cohort: 133 (IQR: 114-194) compared with 112 (IQR: 74-165) lungs per 100 waitlist candidates (Table 1).

Table 1.

Baseline Characteristics

	Preallocation change November 24, 2015- November 24, 2017	Postallocation change November 25, 2017- November 25, 2019	p-value
Total wait list candidates	6524	6847
Transplants	4435	5138
LAS at listing (median, IQR)a	36.9 (33.5-43.5)	37.7 (33.9-45.2)	<0.001
LAS at transplant (median, IQR)b	40.3 (35.0-51.3)	41.8 (35.5-57.6)	<0.001
Diagnosis group for transplanted recipientsb, % (n)			<0.001
Group A (obstructive lung disease)	29.7% (1940)	26.7% (1828)
Group B (pulmonary vascular disease)	4.8% (315)	5.2% (357)
Group C (cystic fibrosis)	9.9% (648)	8.4% (574)
Group D (interstitial lung disease)	55.5% (3621)	59.7% (4088)
Age (median, IQR)	60 (51-65)	61 (52-66)	<0.001
Female, % (n)	47.0% (3065)	45.6% (3124)	0.12
Ethnicity/race, % (n)			0.003
White	78.6% (5125)	76.4% (5231)
Black	9.9% (648)	10.4% (712)
Hispanic	8.1% (527)	9.8% (671)
Other	3.4% (224)	3.4% (233)
Mechanical support at listing, ECMO or mechanical ventilator, % (n)	4.1% (270)	4.3% (296)	0.60
Double lungs required, % (n)	56.6% (3695)	59.1% (4044)	0.005
Blood type, % (n)			0.017
0	46.0% (3000)	48.2% (3302)
A	39.0% (2546)	36.6% (2507)
B	11.5% (749)	11.3% (771)
AB	3.5% (229)	3.9% (267)
Height (cm), median (IQR)	167.6 (160.0-175.3)	168.0 (160.0– 175.3)	0.34
Body mass index at listing, median (IQR)	25.9 (21.9-29.2)	26.1 (22.3-29.3)	0.025
Mean PA pressure, median (IQR)	25 (20-31)	25 (20-31)	0.37
LAS groupa, % (n)			<0.001
LAS < 35	37.5% (2446)	33.6% (2301)
LAS 35-45	40.5% (2640)	41.1% (2817)
LAS 45-55	10.2% (665)	10.3% (707)
LAS ≥ 55	11.8% (773)	14.9% (1022)
Prospective cross-match required, % (n)	7.3% (473)	7.0% (476)	0.50
Waitlist size of primary allocation unit, median (IQR)	61 (28-88)	192 (122-370)	<0.001
Donor Lung availability (donor lungs per 100 persons on waitlist), median (IQR)	112 (74-165)	133 (114-194)	<0.001

Abbreviations: IQR, interquartile range; n, number; LAS, lung allocation score; ECMO, extracorporeal membrane oxygenation.

^aRepresents LAS at listing or if candidate already on the list, the LAS on start date was used (November 24, 2015 for the preallocation change cohort or November 25, 2017 for the postallocation change cohort).

^bOnly includes transplanted patients.

Candidates at centers in the lowest lung availability quartile had significantly higher LAS at transplant in both the pre- and postallocation change cohorts compared with candidates in the highest lung availability quartile (Tables S2 and S3). Waitlist candidates were otherwise similar across DLA quartiles, except more candidates required a double lung transplant and a prospective crossmatch at transplant centers in the highest lung availability quartile (Tables S2 and S3).

The disparity in donor lung availability between the highest and lowest DLA quartiles decreased in the post-allocation change period, and the distribution of “organ rich” and “organ poor” transplant programs changed in the United States (Figures 1 and 2). In the preallocation change cohort, there was nearly a 7-fold difference in median donor lung availability between the lowest and highest DLA quartiles (72 compared with 484 donor lungs per 100 waitlist candidates) (Table 2). In the postallocation change cohort, there was only a 2.5-fold difference in median donor lung availability between the lowest and highest DLA quartiles (104 vs 242 lungs per 100 waitlist candidates) (Table 2).

Figure 1.

The donor lung availability is calculated at the level of the transplant center (donor lungs in the primary allocation unit per waitlist candidates in the primary allocation unit). The mean donor lung availability of all transplant centers within each respective donation service area is reported before and after the November 2017 allocation change. Of note, before the allocation change all transplant centers in a DSA have the same donor lung availability.

Figure 2.

Monthly donor lung availability at transplant centers by DLA quartile. Transplant centers are divided into quartiles by their donor lung availability before and after the allocation change. The monthly donor lung availability is calculated at each transplant center: the number of donor lungs each month from the primary allocation unit divided by the waitlist size for the primary allocation unit on the first of the month. The monthly DLA is then calculated as the average monthly donor lung availability of all transplants centers in that quartile. The primary allocation unit was the DSA until November 2017 and after it became a 250 nautical mile radius from the transplant center. Dashed arrowed line indicates date of allocation change. DLA, donor lung availability.

Table 2.

Waitlist Outcomes Before and After the Allocation Change

	Preallocation change				Postallocation change
Outcome	Lowest quartile (fewest donor lungs)	Second quartile	Third quartile	Highest quartile (most donor lungs)	Lowest quartile (fewest donor lungs)	Second quartile	Third quartile	Highest quartile (most donor lungs)
Total waiting list candidates	2770	1846	1273	635	2459	1729	1310	1349
Donor lung availability, (donor lungs per 100 waitlist years)	72 (61-80)	137 (112-145)	213 (165-322)	484 (390-527)	104 (67-115)	132 (127-138)	191 (168-194)	242 (223-297)
Transplant ratea, (95% CI)	132 (126-139)	161 (152-170)	262 (246-279)	607 (557-657)	141 (134-147)	171 (162-181)	270 (254-287)	309 (291-327)
Transplantation
No. transplanted	1675	1218	978	564	1703	1258	1058	1119
Unadjusted, sHR (95% CI)	0.36 (0.32-0.39), p < 0.001	0.41 (0.37-0.46), p < 0.001	0.59 (0.53-0.66), p < 0.001	1 (ref)	0.59 (0.55-0.63), p < 0.001	0.70 (0.64-0.76), p < 0.001	0.94 (0.86-1.03), p = 0.172	1 (ref)
Adjustedb sHR, (95% CI)	0.39 (0.34-0.44), p < 0.001	0.47 (0.42-0.54), p < 0.001	0.63 (0.55-0.72), p < 0.001	1	0.62 (0.57-0.68), p < 0.001	0.72 (0.66-0.78), p < 0.001	1.06 (0.96-1.17), p = 0.274	1
Death or removal for clinical deterioration
No. died/removed	294	173	92	26	264	173	98	93
Unadjusted sHR (95% CI)	2.69 (1.79-4.03), p < 0.001	2.33 (1.54-3.54), p < 0.001	1.79 (1.15-2.77), p = 0.009	1	1.57 (1.23-1.98), p < 0.001	1.47 (1.14-1.89), p = 0.003	1.09 (0.82-1.46), p = 0.537	1
Adjustedb sHR, (95% CI)	2.20 (1.38-3.50), p = 0.001	2.23 (1.41-3.55), p = 0.001	1.71 (1.06-2.76), p = 0.027	1	1.29 (1.01-1.66), p = 0.041	1.27 (0.98-1.65), p = 0.070	0.96 (0.71-1.28), p = 0.766	1

Abbreviations: sHR, sub-hazard ratio; CI, confidence Interval.

^aUnadjusted, Transplants per 100 waitlist years.

^bAdjusted for lung allocation score, center acceptance rate, recipient age, prospective cross match, listing strategy (double lung only), blood type, diagnosis, height and age.

Waitlist outcomes

The disparity in waitlist outcomes between centers in high and low DLA quartiles diminished after the allocation change. Before the allocation change, there was a 4.6-fold difference in unadjusted transplantation rate between centers in the lowest and highest DLA quartiles; 132 and 607 transplants per 100 waitlist years, respectively. After the allocation change, this decreased to a 2.2-fold difference; 141 and 309 transplants per 100 waitlist years, respectively.

Before the allocation change, candidates listed at transplant centers in the lowest lung availability quartile had a 61% adjusted lower rate of transplantation (sub-hazard ratio [sHR]: 0.39, 95% confidence interval [CI]: 0.34-0.44, p < 0.001) and a 120% adjusted increased risk of death/delisting for clinical deterioration (sHR: 2.20, 95% CI: 1.38-3.50, p = 0.001) compared with the candidates listed at centers in the highest DLA quartile in both (Table 2, Figure 3a, b). After the allocation change these differences in waitlist outcomes decreased, and candidates in the lowest DLA quartile had a 38% adjusted lower rate of transplantation (sHR: 0.62, 95% CI: 0.57-0.68, p < 0.001) and a 29% adjusted increased risk of death/delisting for clinical deterioration (sHR: 1.29, 95% CI: 1.01-1.66, p = 0.041) compared with the candidates listed at centers in the highest DLA quartile (Table 2, Figure 3c, d).

Figure 3.

Cumulative incidence of transplant (a and c) and cumulative incidence of death or delisting for clinical deterioration (b and d) on the waitlist before and after the allocation change in November 2017. Adjusted for lung allocation score, center acceptance rate, recipient age, prospective crossmatch, listing strategy (double lung only), blood type, diagnosis, height, and age.

In the preallocation change cohort, the impact of a center moving from the lowest to highest DLA quartile on a candidate’s transplantation rate was comparable to a 20-point increase in the LAS at the time of listing, a change from lowest quartile of LAS at listing (median LAS 32.7, IQR: 32.1-33.1) to the highest quartile of LAS at listing (median 53.7, IQR: 47.0-76.7, adjusted for covariates) (Tables S4 and S5). In the post allocation change cohort, the impact of a center moving from the lowest to highest DLA quartile on a candidate’s transplantation rate was comparable to a 7.5 point increase in LAS at the time of listing, an increase from the lowest quartile of LAS at listing to the third quartile of LAS at listing (median 40.5, IQR: 39.0-42.4; Tables S4 and S5).

Transplant center changes in transplant rate

There were 14 (23%) lung transplant centers in the preallocation change cohort that moved from a higher to a lower DLA quartile after the allocation change, with a median decrease of 41% in donor lung availability. At these transplants centers, the allocation change was associated with an 11% reduction in transplantation rate (HR 0.89, 95% CI: 0.82-0.97, p = 0.007).

There were 21 (34%) lung transplant centers in the preallocation change cohort that moved from a lower to a higher DLA quartile after the allocation change, with a median increase of 70% in donor lung availability. At these transplant centers, the allocation change was associated with a 25% increase in transplantation rate (HR 1.25, 95% CI: 1.16-1.35, p < 0.001).

LAS subgroups

Before the allocation change, the lowest DLA quartile was associated with reduced transplantation rates compared with the highest DLA quartile across all LAS subgroups. However, waitlist candidates with LAS < 35 were the most affected by donor lung availability. In this subgroup prior to the allocation change, candidates in the lowest DLA quartile had a 79% reduced transplantation rate and a 4.6-fold increased risk of death/delisting for clinical deterioration compared to the highest DLA quartile. After the allocation change, the disparity in waitlist outcomes decreased across LAS subgroups. For waitlist candidates with LAS < 35, candidates in the lowest DLA quartile had a 52% reduced transplantation rate and 2-fold increased risk of death/delisting for clinical deterioration after the allocation change (Figure 4, Table S6). Additionally, after the allocation change waitlist candidates with a very high LAS (LAS > 55), had no significant difference in transplantation rate based on donor lung availability (Figure 4, Table S6).

Figure 4.

Subgroup analysis: transplantation rate and risk of dcath/delisting for clinical deterioration by LAS subgroup before and after the allocation change. After the allocation change, low LAS patients had the greatest reduction in disparate waitlist outcomes due to donor lung availability.

Post-transplant survival

Postlung transplant survival at 1-year was not significantly different between the pre- and postallocation change cohorts (HR 0.98, 95% CI: 0.84-1.14, p = 0.758). There were no significant differences in 1-year post-transplant survival across DLA quartiles in the pre or the postallocation change cohorts (Table S7).

Discussion

Prior to the geographic allocation change on November 24, 2017, there were large differences in donor lung availability among transplant centers that resulted in disparate waitlist outcomes.^5,7 The expansion of the primary allocation unit from the DSA to a 250 nautical mile radius from the donor hospital was intended to improve access to donor lungs and correct these disparities. In this study, we demonstrate that the allocation change resulted in a reduction in the geographic differences in donor lung availability and reduced disparity in waitlist outcomes between the organ poor and organ rich lung transplant centers. Nevertheless, there still remains notable difference in donor lung availability and waitlist outcomes among lung transplant centers due to geography.

Donor lung availability captures the geographic differences in supply/demand of donor lungs and waitlist candidates. In this study, the difference in median donor lung availability between the lowest and highest quartiles decreased considerably from the pre- to the postallocation change cohort, from nearly a 7-fold difference to a 2.5-fold difference. Perhaps not surprisingly, many centers moved between quartiles after the allocation change helping to decrease the disparities in donor lung availability. Waitlist candidates benefited when their listing transplant center had an increase in donor lung availability.

Interestingly, in the preallocation change cohort, center listing behavior differed according to the relative availability of donor organs. Waitlist candidates listed at centers in areas with high organ availability were often listed for double lung transplant only or prospective crossmatch. Prospective crossmatch and double lung only listing (rather than single, double or either listing) is a more restrictive listing strategy that increases the wait time for lungs and decreases the transplantation rate.¹² It is possible that transplant centers in organ rich DSAs did not have the stressor of prolonged wait times. While transplant centers in organ poor DSAs had a more liberal listing approach to reduce the risk of poor waitlist outcomes. Although there remains considerable uncertainty and debate regarding optimal transplant strategies for highly sensitized patients and single vs double lung transplantation in COPD and ILD, donor lung availability should not be the primary driver for differences between transplant centers.^12,16

One fear with the expansion of the allocation unit is a subsequent increase in transplants for critically ill patients with extremely high LAS. After the allocation change, lungs are shared over a broader area with a large number of waitlist candidates in the primary allocation zone. This allows extremely high LAS patients to “pull” more lungs, which could potentially result in a decrease in post-transplant survival.^10,17 However, in our cohort, there was no change in the number of patients on ECMO or mechanical ventilation at the time of transplant, which was consistent with work done by others.¹⁸ Additionally, the majority of lung transplants still occurred for patients with LAS < 50, and we did not observe a significant change in 1-year post-transplant survival following the change in allocation. The modest rise in LAS at transplant may have contributed to a decrease in the proportion of COPD patients as transplant recipients after the allocation change. A rise in LAS at transplant and an increase in transplants for ILD has been a trend since the implementation of the LAS in 2005; however, the impact of broader geographic sharing on disease specific indications for transplant should continue to be monitored.¹⁹

Unsurprisingly, the LAS at transplant rose after the allocation change which could be disadvantageous to patients with low LAS. However, the patients who had the most disparate waitlist outcomes due to geography were candidates with a low LAS at listing, and this disparity improved after the allocation change. This is consistent with our prior work where we reported that differences in donor lung availability were associated with more disparate waitlist outcomes in low LAS patients.⁵ Broader geographic sharing of donor lungs leveled the playing field for candidates with low LAS as there were less stark differences in donor lung availability between transplant centers. This has important implications as patients will be less inclined to seek dual listing if organ availability is similar at many institutions.

There is a risk that broader sharing may reduce pressure on low production DSAs/OPOs to improve donation rates. However, penalizing certain patients for the poor performance of their local OPO/DSA will only exacerbate current health disparities associated with wealth as dual listing tends to benefit people of greater means who have the resources to travel and seek listing at multiple centers.

Although the allocation change on November 24, 2017 came with little planning, it reduced disparities in donor lung availability and waitlist outcomes among transplant centers. Since the change, UNOS has been preparing to overhaul the allocation of donor organs in the United States, starting first with donor lung allocation. In December 2018, the Organ Procurement and Transplantation Network (OPTN) Board of Directors approved the continuous distribution framework for all allocation systems.²⁰

The continuous distribution framework avoids the problem with an allocation “cliff” where a waitlist candidate may be included in the primary allocation unit at 249 miles but a more suitable candidate would not be included at 251 nautical miles.^21,22 The OPTN Thoracic Organ Transplantation Committee proposed using a point-based system composed of medical urgency, post-transplant survival, candidate biology, patient access and placement efficiency to generate a composite score.²² Fundamentally, this allocation system appears fair and logical; however, it is critical that the OPTN and UNOS recognize the persistence of geographic differences in donor lung availability. Overemphasizing placement efficiency at the expense of medical urgency and patient access could increase the geographic disparities compared with the current allocation system. Broader geographic sharing does have the potential to harm small transplant programs due to increased competition for lungs and potentially less resources in smaller programs. As the continuous distribution framework is developed, UNOS and the OPTN must consider the impact on small transplant programs, specifically those programs in rural areas that offer lung transplantation to people living over broad geographic areas. Interestingly, after the allocation change, there was a modest increase in Black and Hispanic lung transplant recipients with a corresponding decrease in white recipients which may reflect improved equity and access to transplantation. However, this requires further exploration and should be closely monitored after the change to continuous distribution system.

This study has several limitations. First, all data collected were reported by clinical personnel to UNOS and are therefore subject to missingness and inaccuracy. Our study excludes pediatric candidates younger than 12 as donor lungs are allocated differently in this age group. The primary predictor variable, donor lung availability, is affected by differences in waitlist size due to center listing behavior and by differences in donor lung supply related to center behavior and risk tolerance for donor lung acceptance. We attempted to account for these issues using a multivariable model adjusting for important waitlist characteristics and adjusting for center acceptance practices using SRTR offer acceptance ratio. We also did not account for donor lung offers beyond the primary allocation unit. We adjusted for prospective crossmatch requirement rather than calculated panel reactive antibody (cPRA) as this was not available in the standard UNOS file. Our primary predictor variable calculated total donor lung availability rather than blood-type specific donor lung availability, however, we included blood type as a covariate in our model. Finally, due to relatively short post-transplant follow-up, we are unable to evaluate long-term post-transplant outcomes or quality of life differences after lung transplant.

In summary, prior to the expansion of geographic lung allocation in November 2017, there were large disparities in donor lung availability across the United States that were associated with disparate waitlist outcomes. These differences in donor lung availability and associated disparate waitlist outcomes decreased after the expansion of the primary allocation unit to 250 nautical miles. However, there still remains important difference in donor lung availability due to patients’ geographic location that remain associated with waitlist outcomes. As the OPTN and UNOS consider the weighting of medical urgency, patient access and placement efficiency in the development of a continuous distribution algorithm, the impact of differences in supply and demand of organs across the country should be carefully considered.

Abbreviations:

UNOS

United Network of Organ Sharing

DSA

Donor Service Area

LAS

Lung Allocation Score

SRTR

Scientific Registry of Transplant Recipients

IQR

Interquartile range

sHR

sub-Hazard Ratio

OPTN

Organ Procurement and Transplantation Network

DLA quartile

Donor Lung Availability Quartile