Lung Allocation Score (LAS)

The Lung Allocation Score (LAS) was the numeric urgency-and-benefit score used to prioritize candidates for deceased-donor lung transplantation in the United States from 2005 to 2023.[1][2] Implemented on May 4, 2005, the LAS replaced an allocation system based largely on accrued waiting time, and it ranked candidates aged 12 and older by a measure of net transplant benefit, balancing how sick a candidate was (waitlist urgency) against how well they were expected to do after transplant (post-transplant survival).[1][2][3] The LAS was a normalized score on a 0-to-100 scale; a higher score meant higher priority.[3][4]

The LAS was retired on March 9, 2023, when lung allocation moved to continuous distribution and the LAS was replaced by the lung Composite Allocation Score (CAS).[5][6] This article describes the LAS as a historical allocation tool; for the current system see Continuous distribution and the Composite Allocation Score, and for the surgical procedure see Lung transplantation.

Background: what the LAS replaced

Before the LAS, U.S. lung allocation under OPTN oversight prioritized candidates principally by time accrued on the waiting list, together with blood-type compatibility and geography.[1][2] Because waiting time, not medical urgency, drove priority, the system disadvantaged the sickest candidates, who could die before accumulating enough waiting time to receive an offer.[1][2] The LAS was developed by the OPTN Thoracic Organ Transplantation Committee to redirect lungs toward the candidates expected to benefit most, and its introduction sharply shortened the time the sickest candidates waited: in 2005, the year of implementation, a quarter of patients were transplanted within 54 days of listing, compared with 183 days the year before.[1]

What the LAS measured

The LAS was designed around the concept of net transplant benefit.[1][3] It combined two model-derived estimates:[1][3]

• a waitlist urgency measure, the expected number of days a candidate would live during an additional year on the waiting list without a transplant; and
• a post-transplant survival measure, the expected number of days a candidate would live during the first year after a transplant.

By weighing expected survival without a transplant against expected survival with one, the LAS prioritized candidates who were both urgently in need and likely to survive the operation, rather than prioritizing by waiting time alone.[1][3] The two estimates were combined and normalized to a continuous score from 0 to 100, calculated for all candidates aged 12 and older.[3][4]

Clinical variables

The LAS models drew on roughly thirty candidate and clinical parameters.[3] Variables used over the life of the score included:[3]

• diagnosis group (see below) and age, height, weight, and body mass index;
• forced vital capacity (FVC) (percent predicted);
• pulmonary arterial pressures (systolic and mean), pulmonary capillary wedge pressure, cardiac index, and central venous pressure;
• supplemental-oxygen requirement at rest;
• mechanical ventilation status and functional/assistance status;
• serum creatinine and carbon dioxide partial pressure (PCO2) (and its trend);
• six-minute-walk distance; and
• diabetes status.

A 2015 revision updated the model, adding or modifying variables (including the handling of elevated creatinine and bilirubin, central venous pressure when cardiac index was low, and six-minute-walk distance below a threshold) and discontinuing some prior ones.[3]

Diagnosis groups

Candidates were assigned to one of four diagnosis groups, which the score treated differently because the natural history of lung disease varies by category:[7]

• Group A, obstructive lung disease (for example, emphysema/COPD, including alpha-1 antitrypsin deficiency);
• Group B, pulmonary vascular disease (for example, idiopathic pulmonary arterial hypertension);
• Group C, cystic fibrosis and immunodeficiency disorders; and
• Group D, restrictive lung disease (for example, idiopathic pulmonary fibrosis).

Pediatric allocation

The LAS applied only to candidates aged 12 and older.[3][4] Candidates younger than 12 were not scored by the LAS; instead they were allocated through two separate priority (urgency) rankings.[5] When lung allocation moved to continuous distribution in 2023, the two pediatric priority rankings were folded into the lung CAS, so that for the first time a single composite scoring approach applied across all ages.[5]

Geographic changes during the LAS era

The LAS determined a candidate's priority, but the geographic unit over which lungs were offered changed during the LAS era.[6] On November 24, 2017, in response to litigation and an HHS directive about geographic inequity, the OPTN removed the donation service area (DSA) as the first unit of lung distribution and began offering adult-donor lungs first to candidates at transplant hospitals within 250 nautical miles of the donor hospital; 250 nautical miles was chosen to limit ischemic time and transport cost.[8] The progression from the DSA to a fixed distance, and then to the boundaryless continuous-distribution framework, is described in Geographic distribution and acuity circles.

Replacement by the Composite Allocation Score

After nearly eighteen years in use, the LAS was replaced on March 9, 2023 by the lung Composite Allocation Score (CAS) under continuous distribution.[5][6] The CAS preserves the LAS's core idea of balancing medical urgency against expected post-transplant survival, but it incorporates additional factors (biological access such as blood type and sensitization, patient-access factors such as pediatric and prior-living-donor status, and placement efficiency) into a single weighted score and removes the hard geographic boundary, so that distance becomes one weighted attribute rather than a cutoff.[5][6] The lung was the first organ to make this transition.[5][6]

See also

• Continuous distribution and the Composite Allocation Score
• Lung transplantation
• OPTN allocation policy
• The match run
• Geographic distribution and acuity circles

References

• The new allocation era and policy (lung). PMC8662501. https://pmc.ncbi.nlm.nih.gov/articles/PMC8662501/
• Disparities in lung transplantation before and after introduction of the Lung Allocation Score. PMC3714222. https://pmc.ncbi.nlm.nih.gov/articles/PMC3714222/
• Lung allocation (review of LAS variables and 2015 update). PMC5594149. https://pmc.ncbi.nlm.nih.gov/articles/PMC5594149/
• HRSA / OPTN. Lung allocation based on the Composite Allocation Score (CAS): questions and answers for patients and caregivers. https://www.hrsa.gov/optn/patients/resources/lung/lung-allocation-faqs
• UNOS / OPTN. New lung allocation policy in effect. https://unos.org/news/new-lung-allocation-policy-in-effect/
• HRSA / OPTN. Continuous Distribution. https://www.hrsa.gov/optn/policies-bylaws/policy-issues/continuous-distribution
• OPTN / SRTR Annual Data Report: Lung (diagnosis groups A-D). Am J Transplant. https://www.amjtransplant.org/article/S1600-6135(25)00031-0/fulltext
• HRSA / OPTN. Policy modification to lung distribution sequence (250 nautical miles, 2017). https://optn.transplant.hrsa.gov/news/policy-modification-to-lung-distribution-sequence/