Beyond the numbers: Rational and ethical application of outcome models for organ allocation in liver transplantation

Applying predictive models to heath policy—particularly when allocating scarce life-saving resources— must be understood in the context of clinical practice patterns and ethical principles. Unlike most medical and surgical interventions in the developed world, the availability of liver transplantation (LTx) is limited by the supply of scarce donor organs rather than monetary resources or the number of healthcare providers. Fair and just utilization of liver grafts requires that policymakers understand how to balance the needs and interests of each of the stakeholders. For an individual and for society, the utility of LTx is largely dominated by the individual’s risk of death without LTx. Accordingly, the liver allocation system in the United States is based on a patient’s pre-LTx Model for End-Stage Liver Disease (MELD) score, which accurately ranks the risk of death without LTx in the vast majority of patients awaiting LTx. Since the implementation of MELD-based allocation in 2002, there are fewer patients on the waiting list, fewer deaths on the waiting list, more patients undergoing LTx, and despite taking sicker patients to transplant (median MELD score 18.4 in pre-MELD era vs. 24 in MELD era), the survival rates are unchanged (92.7% and 94% at 3 months after LTx). Although predicted survival after LTx guides bedside decision-making regarding whether or not to perform LTx when an organ is offered to a patient, in the United States predicted survival after LTx does not dictate organ allocation. Patients that are too sick or too well are made “inactive” or removed from the waitlist based on the clinical assessment of the transplant team. Rational application of predictive models of mortality after LTx can aid in such assessments and show promise as tools for further optimization of liver allocation, particularly models incorporating both donor and recipient characteristics. The articles by Luca et al. and Dawwas et al. in this issue of Liver Transplantation highlight the ongoing commitment of the liver transplant community to evidence-based improvements in liver allocation. Although the two articles approach the issue from opposite sides of the utility fulcrum (urgency for LTx vs. outcome after LTx) both evaluate the increasing recognized surrogate for severity of chronic liver disease— serum sodium concentration [Na]. Hyponatremia occurs commonly in patients with cirrhosis and is a strong independent predictor of LTx waitlist mortality. A recent prospective survey of 997 patients with cirrhosis (53%were hospitalized) reports the prevalence of [Na] 135, 130, 125, and 120 mmol/L at 49%, 22%, 5.7%, and 1.2%, respectively. As [Na], like the parameters of the MELD score, is a readily available, objective, and reproducible mortality predictor in endstage liver disease, it has been considered a strong candidate for addition to a liver allocation model. Although hyponatremia can identify LTx candidates with higher risk for waitlist mortality and therefore higher urgency for transplantation, pretransplantation hyponatremia has been associated with worse posttransplantation outcomes, including higher rates of