Continuous distribution and the Composite Allocation Score

Continuous distribution is a framework for allocating deceased-donor organs in the United States that replaces hard geographic boundaries and discrete priority tiers with a single, points-based Composite Allocation Score (CAS) calculated for each candidate against each donor organ.[1][2] Instead of sorting candidates into a fixed sequence of categories (for example, "local, then regional, then national" within urgency brackets), continuous distribution combines all of the relevant medical, biological, and logistical factors into one weighted numeric score and ranks candidates continuously from highest to lowest, with no factor acting as an absolute, all-or-nothing cutoff.[1][2]

Continuous distribution was implemented first for the lung, effective March 9, 2023, making the lung the first organ in the U.S. system to use a Composite Allocation Score.[2][3] As of mid-2026, lung remains the only organ live on continuous distribution; kidney, pancreas, liver, intestine, and heart frameworks are at various stages of development and public comment.[1] This article describes the framework, the lung CAS as the live example, and the policy rationale; the broader history of allocation geography is in Geographic distribution and acuity circles, and the mechanics of producing a ranked list are in The match run.

Why continuous distribution

Continuous distribution is the OPTN's response to long-standing concerns about both geography and rigid tiers in allocation.[1][2] Under the older systems, candidates were ranked within hard geographic units (the donation service area (DSA) and OPTN region) and within discrete classification brackets; a candidate just across an arbitrary boundary, or just below a categorical threshold, could be passed over even when only marginally different from a candidate who received priority.[1][2] The federal OPTN Final Rule (42 CFR §121.8) requires that allocation be based on sound medical judgment, that it not be based on a candidate's place of residence or place of listing except as medically necessary, and that organs be distributed over "as broad a geographic area as feasible."[4] Continuous distribution operationalizes those requirements by treating distance as one weighted factor among many rather than as a boundary that defines who is even considered, and by removing the cliffs created by discrete tiers.[1][2]

The framework is organ-by-organ: the OPTN develops a separate continuous-distribution policy for each organ through its committees, simulation modeling, community "values prioritization" exercises, and public comment, then forwards it to the Board of Directors.[1] This is why continuous distribution arrives as a sequence of dated, organ-specific events rather than as a single system-wide switch.[1][3]

How a Composite Allocation Score works

In continuous distribution, each candidate receives points across several attributes grouped into goal categories.[1][2] Each attribute is weighted, meaning it has a maximum point value reflecting how much the transplant community decided it should contribute to the total; the points are summed into one composite score, and the match run ranks candidates by that score.[1][2] The general goal categories are:[1][2]

• Medical urgency (how soon the candidate is likely to die without a transplant);
• Post-transplant outcome / survival (how well the candidate is expected to do after transplant);
• Candidate biology (factors that make a candidate harder to match, such as blood type, body size/height, and immune sensitization measured by calculated panel-reactive antibody, cPRA);
• Patient access (equity factors such as pediatric status and prior living-donor status); and
• Placement efficiency (logistical factors, principally proximity / distance between the donor hospital and the candidate's transplant hospital, and travel efficiency).

No single attribute is an absolute gate; a candidate who scores low on one attribute can still rank highly if the weighted sum of the others is high.[1][2]

The lung Composite Allocation Score (live example)

The lung CAS, effective March 9, 2023, replaced the Lung Allocation Score (LAS) and dissolved the prior tier-and-geography classification.[2][3] All lung candidates (including, for the first time, those younger than 12, who previously used separate priority rankings) receive a lung CAS.[2][3] Per OPTN/UNOS, the lung CAS allocates points roughly as follows:[2]

• up to 25 points for medical urgency (expected waitlist outcome without a transplant);
• up to 25 points for post-transplant survival (likelihood of surviving at least five years after transplant);
• up to 15 points for candidates who are hard to match for offers (biological access);
• up to 5 points each for blood type (harder-to-match types receive more), cPRA (sensitization), and height/size factors; and
• a patient-access / efficiency component that includes priority for candidates listed before age 18 and for prior living organ donors, plus proximity (travel efficiency) points.

The specific maximum point values and weights were derived through community input, OPTN Lung and Thoracic Committee deliberations, simulation modeling, and mathematical optimization, and are set out in the OPTN's published guide to calculating the lung CAS.[1][2] The policy was projected to reduce waitlist deaths and direct more transplants to the most medically urgent candidates while still accounting for organ quality and placement efficiency.[2][3]

Status of continuous distribution for other organs

As of mid-2026, only lung allocation uses continuous distribution.[1] The OPTN has continuous-distribution projects under development for the other organs, each at a different stage:[1]

• Kidney and pancreas are in active development and public comment, building on the 2021 removal of the DSA and region from those systems.[1]
• Liver and intestine continuous distribution is under development, following the 2020 move to acuity circles.[1]
• Heart continuous distribution has been in development since 2022, with a community values-prioritization exercise completed in 2024 and the Heart Committee still refining attribute weights and rating scales as of 2025; it had not been implemented as of mid-2026.[1]

Because timing depends on policy development and Board approval, the per-organ rollout status should be treated as a moving target and re-confirmed against the OPTN's continuous-distribution pages.[1]

See also

• OPTN allocation policy
• The match run
• Lung Allocation Score (LAS)
• Geographic distribution and acuity circles
• Heart allocation status system
• MELD / MELD-Na / MELD 3.0 (and PELD)

References

• HRSA / OPTN. Continuous Distribution. https://www.hrsa.gov/optn/policies-bylaws/policy-issues/continuous-distribution
• UNOS / OPTN. New lung allocation policy in effect. https://unos.org/news/new-lung-allocation-policy-in-effect/
• HRSA / OPTN. New lung allocation policy in effect. https://www.hrsa.gov/optn/news-events/news/new-lung-allocation-policy-effect
• 42 CFR §121.8, Allocation of organs (OPTN Final Rule). Legal Information Institute, Cornell Law School. https://www.law.cornell.edu/cfr/text/42/121.8