Variation in MOT Allocation and Ethical Implications
The most common MOT is SPK (>19 000 performed in the United States from 1988 to 2013), followed by SLK (>5000 performed). Most pancreas allografts are allocated through the kidney–pancreas waiting list, determined primarily by waiting time, with extra priority for sensitization. When one of the required organs is a liver, heart or lung, the MOT candidate is registered on the individual waiting list for the primary organ. If the candidate is allocated the primary organ, the additional required organ from the same donor is sequestered by that candidate. In these diverse clinical scenarios, MOT candidates commonly receive higher priority for the nonprimary organ (usually a kidney) than all other patients waiting for a single transplant with the same organ. These allocation systems raise serious equity concerns, particularly when those patients "passed over" for the nonprimary organ have strong ethical claims such as pediatric status or long waiting time.
Pancreas transplantation improves quality of life and reduces the risk of complications for candidates with Type 1 diabetes and severe hypoglycemic episodes. In SPK transplantation, the pancreas improves kidney allograft survival (vs. kidney transplant alone), and possibly improves patient survival. SPK candidates receive higher priority for the transplant than kidney-alone candidates with similar or greater waiting time. This allocation priority is available even to some SPK candidates with Type 2 diabetes, for whom the benefits of pancreas transplant are uncertain.
Liver and heart allocation are based on the "sickest first" principle. Liver transplant candidates are classified as status 1A or 1B based on medical urgency, then ranked according to their Model for End-Stage Liver Disease (MELD) score, which reflects mortality risk on the waiting list. For heart transplantation, candidates are classified as status 1A or 1B based on ventilator needs, circulatory status, inotropic support and life expectancy without transplant. For both liver and heart allocation, waiting time is considered within categories of similar medical urgency. Lung transplant allocation strives to maximize overall utility by integrating estimates of pre- and posttransplant survival. Lung candidates receive a Lung Allocation Score; higher scores indicate greater expected survival benefit from transplantation and lead to higher priority.
In contrast, kidney allocation has historically been based primarily on waiting times, while also giving priority to zero-antigen mismatches and subgroups such as sensitized patients. Notably, unlike patients with liver or lung failure, patients with end-stage renal disease (ESRD) can often survive for prolonged periods without transplantation because of chronic dialysis. Consequently, kidney-alone candidates may be perceived as having less medical urgency than subjects with primary heart, liver or lung failure and some degree of renal insufficiency. In MOT allocation, the kidney is always relegated to nonprimary organ status. In the year 2011, 548 kidneys (5% of all transplanted deceased donor kidneys) were allocated to MOT recipients. As shown in Figure 1, these kidneys are among the highest quality organs procured. Meanwhile, the percentage of adult patients receiving a deceased donor kidney transplant within 3 years has been declining since 1991. Among kidney-alone candidates, the percentage of individuals who will receive a kidney transplant by 3 years varies from 17% to 18% for those with blood type O or B to 45% for those with blood type AB.
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Figure 1.
Distribution of kidney donor profile index (KDPI) scores among recipients of kidney-alone and multi-organ transplant recipients. Figure generated using Organ Procurement and Transplantation Network data and represents a cohort of adult (≥18 years) solid organ transplant recipients who received a deceased donor kidney transplant from February 2002 to April 2013. p < 0.001 for each comparison of kidney-alone recipients to each other multi-organ transplant group.
Heart–lung combinations and intestine–liver combinations fall under different allocation provisions. For heart–lung, when a candidate is eligible for a heart, the lung from the same donor is allocated to that candidate before lung-only candidates. If the candidate is in greater need of the lung (vs. heart), then the heart is allocated to the heart–lung candidate from the same donor only if no isolated heart candidate with medical urgency (1A status) is eligible to receive the heart. In intestine–liver allocation, the liver is first offered via the liver match run and then may be offered to the combined liver–intestine candidates in the intestine match run.
Thus, MOT allocation varies substantially by organ combination. Each allocation strategy makes different accommodations to equity and utility. This heterogeneity shows that an ethics analysis of MOT practices must address the existing allocation systems and the clinical nuances of specific organ combinations.