Correspondence
Volume of Procedures at Transplantation Centers and Mortality after Liver Transplantation
N Engl J Med 2000; 342:1527-1528May 18, 2000
- Article
To the Editor:
Edwards et al.1 (Dec. 30 issue) compared the one-year survival at centers performing 20 or fewer liver transplantations per year with survival at centers performing more than 20 transplantations per year and found excess mortality in the former group.
The authors seek to determine a turning point for experience. Unfortunately, they did not choose a priori categories. They justified their choice of the number 20 by appealing to their Figure 1. They state that “mortality rates stabilized at centers that performed more than 20 transplantations per year and increased inversely with transplantation volumes of less than 20 per year.” However, this choice appears to have been arbitrary. To our eyes, in fact, the graph may very well “stabilize” beyond 20, perhaps at 30, transplantations per year, with the result that those sites with mortality rates above 30 percent would be included. The authors offer no statistical definition of “stability.”
Moreover, Figure 1 simply reflects the greater statistical variability of small samples than of large ones; the choice of cutoff point should not be based on it at all. For example, all the excess mortality may be attributable to centers performing 1 to 10 (or 1 to 15) liver transplantations, with those performing 11 to 20 (or 16 to 20) at no disadvantage. A statistical analysis that carefully compared proper subgroups, which Edwards et al. do not report, would be required for the accurate identification of a turning point for experience.
Hunsicker, one of the authors of the study by Edwards et al., was reported in the New York Times to have said that medical research shows that patients fare best in centers that perform at least 20 liver transplantations per year.2 Hunsicker has used the number 20 to argue that there should be one liver transplantation program in Iowa, not two, because the number of usable donated organs is about 40 per year. Hunsicker was quoted as saying, “So if we had two programs and we split evenly, then we would both be at the very lowest end of the numbers necessary to maintain competence.”2 Thus, we are concerned that the number 20 may have been chosen for political, not scientific, reasons.
2 ReferencesMitchell P. Laks, M.D., Ph.D.
Tamara Cohen, B.A.
Rochelle Hack, M.A.
Woodhull Medical and Mental Health Center, Brooklyn, NY 112061
Edwards EB ,Roberts JP ,McBride MA ,Schulak JA ,Hunsicker LG . The effect of the volume of procedures at transplantation centers on mortality after liver transplantation. N Engl J Med 1999;341:2049-2053
Full Text | Web of Science | Medline2
Stolberg SG. Iowa turf war over transplants mirrors feuds across the nation. New York Times. December 23, 1999:A1, A18.
To the Editor:
The use of survival data from transplantations performed more than five years ago is misleading, since one-year survival among patients improved steadily throughout the 1990s. The one-year mortality rates quoted by Edwards et al. are unacceptable by today's standards. The 20 percent mortality rate for “high-volume” centers is well above the national average reported more recently by the United Network for Organ Sharing (UNOS).1 According to recent UNOS data, one-year survival rates do not differ significantly between centers with different volumes of transplantation procedures. One-year survival at “low-volume” centers (with “low volume” arbitrarily defined as 23 or fewer liver transplantations per year) is an excellent 85.8 percent.1 This does not differ from the overall one-year survival of 86.9 percent and surpasses the 74.1 percent reported by Edwards et al.
It is unclear why Edwards et al. chose the arbitrary cutoff point of 20 transplantations per year for low-volume centers, since the mortality rates (shown in Figure 1 of their article) appear to be relatively stable at centers performing approximately 15 transplantations per year. Changing the definition of low volume from 20 to 15 would probably lead to the same conclusions yet have a substantially different effect on how many liver-transplantation centers would be viewed as low volume and less competent. In addition, the unacceptably high one-year mortality rates at centers performing fewer than 10 transplantations per year may have skewed the data, resulting in the apparent finding of lower survival at low-volume centers.
We take exception to the inference that at low-volume centers the lower mortality rates “may reflect statistical variation.” We have performed approximately 19 liver transplantations per year during the past five years. Our one-year survival rate of 94 percent (exceeding the expected survival rate of 82 percent) is included in recently reported UNOS data.1 We agree that this information must be made readily available to the public. However, we disagree with the classification of liver-transplantation centers as low-volume or high-volume and the implication that low-volume centers offer inferior transplantation services to their patients.
1 ReferencesDonald J. Hillebrand, M.D.
Waldo Concepcion, M.D.
Loma Linda University Medical Center Transplantation Institute, Loma Linda, CA 923541
United Network for Organ Sharing (UNOS) Web site. (See: http://www.unos.org/frame_Default.asp?Category/anrpt.)
To the Editor:
Can Edwards et al. explain more accurately the criteria used in their initial analysis to set the cutoff point at 20 patients per year? In addition, the performance of the multivariate models used was not specified; that is, what proportion of the variation in the outcome was explained by the independent variables? Moreover, the use of better clinical covariates — for instance, the severity of disease and coexisting illnesses — could have improved the case-mix adjustment.
Edwards et al. also acknowledge that some unmeasured factors may have influenced the relation between volume and outcome. The total number of solid-organ transplantations — not just liver transplantations — done at the hospitals may be an important covariate. Similarly, the volume of liver surgery other than transplantations, the amount and quality of intensive care resources, and the availability of outstanding services for the treatment of infectious diseases and outstanding hematology and interventional radiology services may also be important determinants of the rates of death and complications.
Vincent Bettschart, M.D.
Bernard Burnand, M.D.
François Mosimann, M.D.
University Hospital of Lausanne, CH-1011 Lausanne, SwitzerlandAuthor/Editor Response
The authors reply:
To the Editor: In an analysis in which a covariate (such as sample size) is related to an outcome (such as patient survival) in a nonlinear way, there is always an element of judgment about which model to use. The least subjective approach is to use a nonparametric method to graph the relation and to divide the centers into two groups according to the apparent inflection point. Those centers with volumes closest to the cutoff point will be less divergent from the reference group.
Laks et al. suggest that the less favorable outcomes in the smaller centers might be the result of greater variability in the outcome because of smaller numbers. This would be true for the outcome at a single center. But the outcomes at the smaller centers, as a class, are less favorable than those in the reference group. The total number of patients at the smaller centers was sufficient for this conclusion to be very robust, as confirmed by the statistical tests of significance that we used. Laks et al. also ask whether political issues in Iowa affected the choice of cutoff point. The initial analyses for this study were done before the issue in Iowa arose, and it therefore had no influence on these analyses.
Hillebrand and Concepcion note that UNOS data on more recent liver transplantations did not show a significant effect of volume. However, that analysis lacks risk adjustment and does not account for center affiliation. Our analysis of data from 1987 through 1991 and 1992 through 1994 demonstrates that the effect of volume has been present during different eras and is unlikely to disappear suddenly.
In response to Bettschart et al.: our multivariate model, which included only donor and recipient factors, accounted for about 40 percent of the variability in center-specific mortality rates. Effects related to the transplantation center, of which the volume is a component, may account for a significant portion of the remaining 60 percent. However, Cox and Wermuth cautioned that the interpretation of R2 is misleading in linear regressions with binary responses because “low values are inevitable even if an important relation is present.”1
Clearly, many transplantation-center factors that might contribute to variability in outcomes were not included in this model because the data were unavailable in the scientific registry. We agree that having more clinical detail permits better discrimination, but the message is clear: a lower volume of procedures at transplantation centers is associated with higher mortality after liver transplantation.
1 ReferencesErick B. Edwards, Ph.D.
United Network for Organ Sharing, Richmond, VA 23225John P. Roberts, M.D.
University of California, San Francisco, San Francisco, CA 94143Lawrence G. Hunsicker, M.D.
University of Iowa College of Medicine, Iowa City, IA 522421
Cox DR ,Wermuth N . A comment on the coefficient of determination for binary responses. Am Stat 1992;46:1-4
CrossRef | Web of Science
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