Perspectives on donor lung allocation from both sides of the Atlantic: The United States

Abstract

Donor lung allocation in the United States focuses on decreasing waitlist mortality and improving recipient outcomes. The implementation of allocation policy to match deceased donor lungs to waitlisted patients occurs through a unique partnership between government and private organizations, namely the Organ Procurement and Transplantation Network under the Department of Health and Human Services and the United Network for Organ Sharing. In 2005, the donor lung allocation algorithm shifted towards the prioritization of medical urgency of waitlisted patients instead of time accrued on the waitlist. This led to the Lung Allocation Score, which weighs over a dozen clinical variables to predict a one-year estimate of survival benefit, and is used to prioritize waitlisted patients. In 2017 the use of local allocation boundaries was eliminated in favor of a 250 nautical mile radius from the donor hospital as the first unit of distance used in allocation. The next upcoming iteration of donor allocation policy is expected to use a continuous distribution algorithm where all geographic boundaries are eliminated. There are additional opportunities to improve donor lung allocation, such as for patients with high antibody titers with access to a limited number of donors..

Introduction

Donor lung allocation in the United States (U.S.) aims to efficiently distribute the limited supply of donor lungs by prioritizing patients based on a combination of predicted one year waiting list mortality and one year survival benefit. To accomplish this, government and private organizations collaborate to provide the funding, infrastructure and research expertise for donor lung allocation and for the development and implementation of donor lung allocation policy.

Administrative organization

The U.S. Department of Health and Human Services (DHHS) is led by a cabinet level secretary who must first be appointed by the U.S. President and then confirmed by the U.S. Congress. The DHHS oversees well-known federal agencies such as the National Institutes of Health (NIH), the Centers for Disease Control (CDC), and the Health Resources and Services Administration (HRSA). It is HRSA who awards the contract for the Organ Procurement and Transplantation Network (OPTN), and since 1986 this contract has been held by the United Network for Organ Sharing (UNOS) as part of a unique public-private partnership. UNOS is a non-profit private organization which oversees the development and implementation of U.S. allocation policy. In order to do this, UNOS coordinates the efforts of transplant hospitals and Organ Procurement Organizations (OPOs). Each of the 58 OPOs in the U.S. are independent, non-profit organizations with their own administrative leadership and clinical teams. They work to identify potential organ donors, clinically manage organ donors in preparation for procurement, and help match donors with waitlisted patients.¹

As an example of how this collaboration works: a team from a local OPO will obtain consent from the patient’s family for organ donation and lead the donor evaluation, which includes a medical and social history, collection of testing already performed as well as additional relevant testing (e.g. bronchoscopy). The OPO then uploads the donor information onto a UNOS run website which identifies potential waitlist matches based on several variables which include ABO blood type, thoracic size, medical urgency of the waitlisted patient, and geographic distance between the donor and transplant hospitals. The donor and waitlisted patient might be at the same hospital, or separated by hundreds of miles, but the national algorithm administered by UNOS to match donors and waitlisted patients is uniformly applied throughout the U.S.

There have been two key shifts in U.S. donor lung allocation policy:

1. Implementing the Lung Allocation Score (LAS) in 2005 which prioritized medical urgency over waitlist time.

2. Implementing a broader geographic sharing policy in 2017 which increased access for high urgency waitlisted patients to donor lungs beyond the traditional local allocation boundaries.

The Lung Allocation Score (LAS)

Prior to 2005, donor lung allocation was based on time accumulated on the waitlist. More time equated to higher priority for donor lungs. This led to an average wait time of two years and an unacceptably high waitlist mortality rate among patients at risk for rapid clinical deterioration who were unlikely to accrue enough time on the waitlist². In 2000, the DHHS published the “Final Rule”, a directive to OPTN/UNOS to re-examine organ distribution policies and direct organs to those most in need³. The OPTN Thoracic Committee, which includes administrators, physicians, surgeons, and UNOS staff, in coordination with statisticians from the Scientific Registry of Transplant Recipients (SRTR) undertook years of research and discussion which culminated in the Lung Allocation Score (LAS) policy. Implementation of the LAS in May 2005 fundamentally changed U.S. donor lung allocation by prioritizing patient medical urgency, which is based on both the predicted 1 year waiting list mortality and 1 year survival benefit ⁴.

To calculate a patient’s LAS, over a dozen clinical variables are uploaded such as the patient’s native lung disease, age, renal function, six minute walk test, and data to indicate if pulmonary hypertension is present. With this clinical information the LAS algorithm then weighs the risk of waitlist death at twice the weight given to the risk of post-transplant survival to generate a numeric score, which is then normalized to between 0 to 100; higher numbers represent a higher priority for transplant. The LAS can be updated at any time depending on changes in the patient’s clinical status. To illustrate how the LAS reflects medical urgency, take for example a 60 year old waitlisted patient with emphysema wearing 4 liters/minute of continuous oxygen with no evidence of secondary pulmonary hypertension, presence of mild hypercarbia, and who can carry out activities of daily living as an outpatient. This patient would have a LAS of ~35. A 60 year old waitlisted patient with pulmonary fibrosis with evidence of secondary pulmonary hypertension and requiring mechanical ventilatory support in an intensive care unit would have a LAS of ~80.⁵

After five years with the LAS the number of waitlist deaths decreased from 500 per year to 300 per year and the number of lung transplants doubled⁶. In addition, there was a shift in the distribution of recipient diagnoses, where fibrotic lung disease (commonly idiopathic pulmonary fibrosis) overtook obstructive disease (commonly emphysema) as the most frequent indication for lung transplant, and the number of patients age ≥65 years at time of transplant doubled. Subsequent registry studies have shown the LAS has effectively prioritized waitlisted patients who stand to gain a survival benefit from transplant, especially for those with pulmonary fibrosis or cystic fibrosis ^7–8. However, there have been unintended consequences. By prioritizing sicker patients, the costs related to the transplant hospitalization have increased 40%, and the use of post-transplant tracheostomy and extra corporeal membrane oxygenation (ECMO) have also increased ^9–10. There are also additional clinical variables which can be markers of medical urgency but are not part of the LAS algorithm. These variables include a persistent or recurring pneumothorax, history of hemoptysis, multi-drug resistant infections, or the use of ECMO as a bridge to transplant¹¹. Given the difficulty in objective assessment of these variables, a transplant hospital may submit an appeal to the Lung Review Board at UNOS if the LAS is felt to not accurately reflect the patient’s risk of waitlist death¹².

Broader geographic sharing

On November 20, 2017, a lawsuit was filed by a waitlisted lung patient against the DHHS in the U.S. District Court for Southern New York to eliminate the nationwide use of Donation Service Areas (DSAs)¹³. A DSA represents the geographic area overseen by an OPO with boundaries set in the early years of transplant out of consideration for donor and recipient hospital relationships (Figure 1). During donor allocation, DSA boundaries preferentially kept donor lungs in the local DSA with locally waitlisted patients before considering patients in other DSAs. This led to scenarios where donor lungs were allocated to low LAS patients (low urgency) within the DSA boundaries even when there was a patient with a higher LAS (high urgency) just outside the DSA boundaries ¹⁴, leading to the unintended consequence of an avoidable waitlist death. The plaintiff’s argument was that DSAs used arbitrary boundaries to prioritize organs for allocation and discontinuing their use could increase patient access to lung transplant. In response, OPTN/UNOS held emergency meetings, which resulted in the following policy change five days later on November 25, 2017: The boundaries of all 58 DSAs were no longer used in donor lung allocation, and donor lungs would now be first offered to transplant hospitals within a 250 nautical mile (nm) radius of the donor hospital (1nm = 1.15 statute mile).¹⁵

Figure 1.

Donation Service Areas (DSAs) of the United States. Each color represents a DSA which is a geographic region overseen by an Organ Procurement Organization. There are 58 DSAs. DSA borders and state borders do not necessarily correlate. Map published by the Association of Organ Procurement Organizations.

In the years leading up to this lawsuit there had been increasing evidence of a geographic disparity in lung allocation. Among studies using national registry data, Benvenuto et al. found that patients waitlisted in geographic areas in the lowest quartile of donor lung availability had an 84% increased risk for waitlist death compared to patients waitlisted in areas in the highest quartile of donor lung availability¹⁶. Kosztowski et al. showed that patients could double their chances for transplant by strategically moving from one DSA to another¹⁷, reinforcing the notion that where a patient listed for transplant was a major determinant of access to transplant.

A year after this change in allocation policy, OPTN/UNOS released a report summarizing nationwide trends¹⁸. There has been an increase in the mean LAS from 47.25 to 49.61, representing an increase in the severity of illness at the time of transplant; as well as an increase in the median distance between transplant and donor hospital from 114nm to 166nm. One of the consequences of broader sharing was the increased travel distance for organ procurement teams from the transplant hospital to donor hospital. Puri et al. examined the experience at Washington University in St. Louis, where the majority of organ procurements shifted to non-local donors, and this has both increased travel time and doubled their hospital’s costs¹⁹.

Current allocation and the future

With the implementation of the LAS in 2005 and broader geographic sharing in 2017, the current U.S. allocation algorithm for adult donors (≥18 years) is as follows: A donor lung is identified by a local OPO and the clinical information is uploaded to UNOS’ website where the organ offer is extended to transplant hospitals within a 250nm radius of the donor hospital. Each transplant hospital manages its own patient waitlist. The waitlists are screened for identical and compatible ABO blood types and thoracic size matching. Waitlisted patients who pass these initial screens are then prioritized based on the LAS, with higher scores receiving higher priority. If the transplant hospital declines the organ offer, the OPO then reaches out to the transplant hospital with a patient who has the next highest LAS. If there is no waitlist match within a 250nm radius, the next consideration is for transplant hospitals out to a radius of 500nm from the donor hospital, and the algorithm continues in 500nm increments until a match is identified, all waitlisted eligible recipients have been exhausted or the OPO withdraws the offer²⁰. Organ donors <18 years of age undergo separate allocation algorithms.

Heart, liver, and kidney transplantation each have their own unique allocation algorithms and have adopted some form of broader geographic sharing ^21–23. However, a relatively newer concept, “continuous distribution” has been introduced which the lung allocation system will be among the first to implement. Under continuous distribution, all geographic boundaries would be eliminated, including the recently established 250nm radius. A formula would generate a relative distribution score which weighs the medical urgency of a waitlist patient, the likelihood of graft survival, and proximity to the donor location.^24–26 This proposal underwent a nationwide public comment period which concluded in October, 2019 ²⁷.

As the allocation system evolves, there are other opportunities for the allocation system to address. These include: 1) increasing priority for patients with high antibody sensitization levels. Patients with high Panel Reactive Antibody (PRA) titers are associated with higher waitlist mortality^28–29 due to lower access to matching donors. 2) standardizing the evaluation of requests submitted to the Lung Review Board. The rate of requests, typically for a higher LAS, is associated with native lung disease³⁰ and dependent on the advocacy of a transplant program in submitting a request, 3) incorporating emerging technologies such as Ex Vivo Lung Perfusion (EVLP) which could increase the number of donor lungs and prolong ischemic times, further reducing the role of geography in allocation³¹, and 4) encouraging the utilization of donation after circulatory death (DCD) donors. DCD and donation after brain death donors have similar transplant outcomes, yet DCDs comprise only 2% of lung transplants in the U.S.³²

Summary

Major changes in U.S. allocation policy have included the introduction of the LAS in 2005 and the implementation of broader geographic sharing of donor lungs in 2017. Organ allocation continues to evolve, and the next step, that of continuous distribution, aims to completely eliminate the use of geographic boundaries. While there may be unintended consequences to any policy change, the intended consequence is to continue to reduce waitlist mortality and optimize patient outcomes regardless of geographic location.

Abbreviations:

DCD

Donation after Circulatory Death

DHHS

Department of Health and Human Services

DSA

Donation Service Area

ECMO

Extra Corporeal Membrane Oxygenation

EVLP

Ex Vivo Lung Perfusion

HRSA

Health Resources and Services Administration

LAS

Lung Allocation Score

NM

Nautical Miles

OPO

Organ Procurement Organization

OPTN

Organ Procurement and Transplantation Network

PRA

Panel Reactive Antibody

SRTR

Scientific Registry of Transplant Recipients

UNOS

United Network for Organ Sharing