Elsevier

Transplantation Proceedings

Volume 39, Issue 10, December 2007, Pages 3169-3174
Transplantation Proceedings

Liver transplantation
Candidate
Overview of the MELD Score and the UNOS Adult Liver Allocation System

https://doi.org/10.1016/j.transproceed.2007.04.025Get rights and content

Abstract

On February 27, 2002, the United Network for Organ Sharing (UNOS) introduced a new allocation policy for cadaveric liver transplants, based on the Model for End-Stage Liver Disease (MELD) score. This new policy stratifies the patients based on their risk of death while on the waiting list. We analyzed the background and main features of this new allocation policy to evaluate the effects on waiting list dynamics as well as the accuracy of MELD as a predictor of pretransplantation mortality and posttransplantation outcome. MELD has proved to be accurate as a predictor of waiting list mortality, but seems to be less accurate to predict posttransplantation outcome. Immediate effects of the new policy were a reduction in the waiting list, while organs were primarily directed to sicker patients with reduced waiting times. There was a statistically but not significantly reduced number of patients removed from the list due to death or severity of sickness. The balance between medical urgency and transplant benefit is still to be defined as is the relationship between pretransplantation criteria and posttransplantation outcomes, and the way this relationship should be included in the allocation policy.

Section snippets

History of the Model for End-Stage Liver Disease

Over time, increasing evidence suggested that a prioritization system should allocate organs based on the need for a transplant, rather than on the waiting time. In 1999, a Liver Disease Severity Score (LDSS) Committee was formed within the Organ Procurement and Transplantation Network (OPTN), an institution administered by the United Network for Organ Sharing (UNOS). This committee was assigned the task to define a model that would accurately predict mortality on the waiting list. The Model

What is Meld?

The MELD score was first reported in 20002 to be a predictor of mortality risk among patients undergoing TIPS. It is an equation using 3 clinical laboratory measurements that were observed to have statistical impact on patient mortality risk (Table 1). The MELD score is currently considered to be the most accurate predictor of 90-day mortality risk for patients on the liver transplant waiting list. Before being validated and introduced in practice, the score was found to accurately predict

UNOS Territory

UNOS is a private, nonprofit organization that was contracted by the US Government to regulate and manage the organ allocation system in the United States. The territory of the United States is divided into 11 UNOS regions. Within each region, there are several Organ Procurement Agencies (OPO). Each OPO serves several transplant centers within their territory. The OPO centralizes the patients from the waiting lists of all centers within its territory and grants them priority by MELD score, so

How Does the Allocation System Work?

Patients are prioritized on the waiting list within each blood type group by descending MELD score. This implies that the organs are basically allocated to the waitlisted patients with the highest mortality risk at the given time. Within each MELD score, livers are first allocated to blood type-identical patients. To prevent inequitable distribution of organs, blood type O livers may only be attributed to blood type O recipients. The system allows patients with special situations (like very

Too Healthy Versus Too Sick

One of the problems that has been raised is the balance between allocating the organs to the sickest patients (with the drawback of poorer posttransplantation outcome) versus allocating them to patients who would benefit most from transplantation by having the best survival rates. The interest of a liver transplant candidate is best served when the expected survival is higher than that without transplantation. It has been shown that among patients with MELD scores under 15, the mortality risk

Meld Exceptions

It is recognized that there are categories of patients whose MELD scores do not reflect the true urgency of their need for transplantation. Several categories of patients qualify under current regulations for various degrees of upgrading of their MELD scores to values that would allow them to undergo transplantation within a predictable time frame. Examples of exceptions include patients with hepatocellular carcinoma (HCC); biliary strictures; recurrent biliary sepsis; refractory upper

Transplantation of Patients with HCC

Prioritization of patients with HCC is one of the most debated topics. Under the previous organ allocation system, Yao et al10 found a 25% yearly dropout rate from the waiting list due to progressive disease. Still, putting these patients at advantage may diminish the chances of otherwise sicker candidates to receive an organ in time. Currently, T2 tumors (namely, 1 nodule 2.0 to 5.0 cm: 2 or 3 nodules, all <3.0 cm as defined by the American Liver Tumor Study Group Modified

Meld and Retransplantation

Retransplanted patients tend to have a poorer outcome than patients undergoing a first liver transplantation. Data reported by Edwards and Harper in a study based on OPTN data13 showed that the MELD scores of retransplanted patients were higher than those of first-transplant patients. Also, in patients with MELD scores higher than 20, the relative risk of death on the waiting list was higher for relisted patients at similar MELD scores. Similar data were reported by other authors.14 These

Improving the Predictive Accuracy of the MELD Score

Several factors have been studied as potential elements to improve the estimation of pretransplantation mortality. There are data to suggest that the change in MELD or PELD scores over short periods may correlate with higher mortality risk on the waiting list.15, 16 Ascites, encephalopathy, and variceal bleeding do not add significant improvement to the statistical performance of MELD. Serum sodium seems to be a reliable surrogate for ascites. In one single-center study, hyponatremia correlated

Transplant Statistics in the MELD Era

After the implementation of MELD as the basis of organ allocation, a large amount of statistical work has been done to follow the results of this change on organ distribution.

Future Trends

Even if the new organ allocation policy is considered to be an improvement in comparison to the waiting time-based prioritization, it is still far from perfect. There are still many areas of concern and many discrepancies. Inter- and intraregional differences in number and size of transplant centers, as well as in the efficiency of OPOs, make a difference in the patients’ chances of receiving a transplant and in the average transplantation MELD scores.6, 18 Thus, patients with equal MELD scores

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