Allocation Out of Sequence in Liver Transplantation Introduces Systematic Disparities in Access

New research is validating concerns about the fairness and transparency of allocation out of sequence (AOOS) in the context of liver transplantation, highlighting systematic disparities that favor a small subset of centers and negatively impact other centers that are bypassed as a result.1

The data were presented at the American Association for the Study of Liver Diseases (AASLD) The Liver Meeting 2025 by Sidian Lin, a PhD student at Harvard University and visiting researcher at Beth Israel Deaconess Medical Center, and raise questions about equity and transparency in allocation in light of national patterns of AOOS bypass.1

The Organ Procurement and Transplantation Network (OPTN) defines AOOS as when an organ is offered or accepted or transplanted into a transplant candidate or potential transplant recipient that deviates from the match sequence and is not consistent with OPTN policy. In 2024, 19% of organ allocations were AOOS.2

Concerns about AOOS include potential violation of federal laws and policies, including the National Organ Transplant Act, the Final Rule, and OPTN policies and bylaws; unfair bypass of patients seeking a life-saving organ transplant; and damaged public trust in the integrity, transparency, and fairness of the organ procurement and transplant system.2

“AOOS has recently drawn scrutiny from HRSA and OPTN due to concerns of fairness, transparency, and rising non-use,” Lin and colleagues wrote.1 “Despite this, little is known about national patterns of AOOS bypass, which centers are ‘winners’ versus ‘losers,’ and how flows of organs between centers shape utilization.”

To assess this gap in research, investigators analyzed the Standard Transplant and Recipient datafile* for all liver offer-level data from 2022-2024. AOOS donors were identified using OPTN bypass codes with text mining for “expedited placement/open offer”.1

For each donor, investigators defined a bypass flow as an instance where one center was bypassed and another ultimately accepted the organ. Flows were aggregated to quantify frequency, directionality, and geography. OPO-level AOOS rates were computed, and centers were categorized as net beneficiaries or net bypassed based on standardized net AOOS gain.1

During the 3-year study period, a total of 3644 donors were identified as AOOS, generating > 1,500,000 bypass codes and 7000 distinct bypass flows. AOOS occurred in 10 of the 11 OPTN regions, confirming it is a national rather than a regional phenomenon.1

Investigators noted a small number of centers were disproportionately advantaged, with 11 centers accounting for more than half of all bypass inflows – and these centers were distinct for having significantly higher volume. By contrast, some high-volume centers were among the top bypassed centers, including 7 of the 11 recipient centers.1

At the OPO level, initiation of AOOS varied widely (3.5%–40.2%), with 15 among 57 OPOs responsible for half of all AOOS cases. Geographic mapping revealed regional asymmetries in bypass flows, with certain centers consistently receiving organs across long distances.1

“These findings echo HRSA concerns that AOOS may undermine equity and transparency in allocation,” investigators concluded.1 “Future work should assess whether AOOS improves organ utilization, and develop policy solutions that preserve rescue of hard-to-place organs while ensuring fairness across centers.”

References

1. Lin S, Lee D, Saghafian S. National patterns of Allocation Out Of Sequence (AOOS) in liver transplantation. Presented at the American Association for the Study of Liver Diseases (AASLD) The Liver Meeting 2025. Washington, DC. November 7-11, 2025.

2. OPTN. Allocation Out of OPTN Sequence (AOOS). Accessed November 8, 2025. https://optn.transplant.hrsa.gov/policies-bylaws/a-closer-look/allocation-out-of-optn-sequence-aoos/