By Ramsey Hachem, M.D.
Washington University School of Medicine Barnes-Jewish Hospital
Division of Pulmonary and Critical Care
June, 2008
In May 2005, the lung allocation system changed in the United States. Previously, lung organs were allocated based primarily on the length of time waiting for a transplant. Under this first come first served algorithm, all potential candidates had the same urgency status on the waiting list and there was no mechanism to account for an individual candidate’s clinical deterioration. However, as early as the 1990s, it was recognized that some patients, especially those with pulmonary fibrosis, often had a more aggressive course and deteriorated more rapidly while waiting for a transplant. So, in 1995 those with pulmonary fibrosis were awarded an additional ninety days on the waiting list when they were listed. Nonetheless, as the total number of patients waiting increased incrementally, waiting times rapidly increased and the additional ninety days awarded to those with pulmonary fibrosis became less meaningful. Indeed, more than 40% of patients transplanted in 2002 had waited greater than two years. However, somewhat surprisingly, mortality on the waiting list decreased consistently between 1993 and 2002 despite the rapid increase in waiting time. The reasons for this decrease in waitlist mortality are uncertain. While improvements in medical therapy for patients with end-stage lung disease probably contributed to this, it is likely that patients were being listed earlier in their disease course in anticipation of a long waiting time. These healthier candidates may have “diluted” the death rate on the waiting list. Nevertheless, despite the improvement in the overall mortality on the waiting list, there was a significant disparity between the different diagnostic groups. Categorically, patients with pulmonary fibrosis consistently had the highest death rate followed by those with cystic fibrosis and those with pulmonary hypertension, while those with emphysema had the lowest death rate on the waiting list. Thus, it seemed that the old allocation system favored those who could survive long enough to have a transplant rather than those most in need of a transplant.
In 1998, the Department of Health and Human Services mandated a review of all organ allocation systems to ensure allocation to those with the highest medical urgency while maximizing utility by avoiding futile transplantation. The United Network for Organ Sharing (UNOS) thoracic organ committee established a subcommittee to propose a new lung allocation system to comply with this mandate. The lung allocation subcommittee identified the following goals for the new allocation system: reduce the number of deaths on the waiting list, allocate organs based on medical urgency, and minimize the effects of waiting time. The subcommittee identified predictors of death on the waiting list and predictors of survival after transplantation. Based on these, statistical models were constructed to estimate the expected number of days that an individual candidate may live on the waiting list (waitlist urgency measure) and the expected number of days lived in the first year after transplantation (transplant survival measure). The transplant benefit measure was defined as the difference between the transplant survival measure and the waitlist urgency measure. The allocation score was then defined as the difference between the transplant benefit and the waitlist urgency measure. The waitlist urgency measure is subtracted twice from the transplant survival measure to emphasize the importance of reducing deaths on the waiting list in the new allocation system.
Since it was implemented, the new lung allocation system has had a significant impact on lung transplantation. First, waiting time has decreased substantially. The median waiting time for patients listed in 2000 was over 1500 days, but this was 130 days for those listed in 2006. It should be noted however that the median waiting time consistently decreased between 2000 and 2004 before the new allocation system was implemented; indeed, the median waiting time in 2004 was approximately 800 days. Nevertheless, there was a considerable decrease in waiting time between 2004 and 2006. In addition, deaths on the waiting list decreased further from approximately 130 deaths per 1000 patient-years in 2004 to 97 deaths per 1000 patient-years in 2006. Furthermore, the annual number of transplants in 2006 was at record of 1401. While the Organ Donation Breakthrough Collaborative contributed to this, it is likely that the greater efficiency of the new allocation system also played an important role. Not surprisingly, the new allocation system has had a significant impact on the distribution of diagnoses undergoing transplantation. Among those transplanted in 2004, 40% had emphysema, 17% had cystic fibrosis, and 22% had pulmonary fibrosis. In contrast, 30% of transplant recipients had emphysema in 2007, 13% had cystic fibrosis, and 31% had pulmonary fibrosis. Importantly, survival after transplantation has not changed significantly since the new allocation system was implemented. According to UNOS data, the two-year survival of 5500 patients transplanted between 2000 and 2005 was 72% and the two-year survival of 2300 patients transplanted since the new system was implemented was 70%.
Clearly, the new allocation system has reduced waiting time and deaths on the waiting list, while the annual number of transplants continues to increase and survival after transplantation has not changed. It is hopeful, that future refinements in the allocation system will continue to reduce mortality on the waiting list.