Data on all patients who were registered on the waiting list in the United States were obtained from the OPTN. These data were available from the Standard Transplant Analysis and Research file as of May 1, 2007. The predictor variables in this analysis consisted of the MELD score and the serum sodium concentration at the time of registration on the waiting list. The data file also included the outcome of waiting (i.e., transplantation, death, or withdrawal from the list for other reasons), which was used to define the outcome variable in the analysis — namely, death before transplantation and within 90 days after registration.
Since our data spanned 2 calendar years, we adopted a strategy to build a prediction model based on data from 2005 and to validate the model using the 2006 data. Since we planned to assign a prognostic score based on the MELD score and the serum sodium concentration to the current and future registrants on the waiting list, it was important that a model derived from one calendar year remain valid when it was applied to another year. Our target population was all adults with cirrhotic liver disease in the United States who were listed for a first liver transplantation. Therefore, among all registrants on the waiting list, we excluded those who were younger than 18 years of age, those with liver disease other than cirrhosis (e.g., acute liver failure or hepatocellular carcinoma), and those listed for repeat liver transplantation, as well as patients for whom complete laboratory data were not available within 5 days after they had been placed on the waiting list. In addition, since we were interested in the effects of hyponatremia, a small proportion of patients with hypernatremia (se- rum sodium concentration >150 mmol per liter) (24 of 6793 patients, or 0.4%) were excluded from the model development. However, patients with hypernatremia were included in the model validation in order to ensure that the new score could be applied globally. In candidates listed at more than one center concurrently, we identified the first date of registration on the waiting list, using a de-identified unique candidate code.
Since our data spanned 2 calendar years, we adopted a strategy to build a prediction model based on data from 2005 and to validate the model using the 2006 data. Since we planned to assign a prognostic score based on the MELD score and the serum sodium concentration to the current and future registrants on the waiting list, it was important that a model derived from one calendar year remain valid when it was applied to another year. Our target population was all adults with cirrhotic liver disease in the United States who were listed for a first liver transplantation. Therefore, among all registrants on the waiting list, we excluded those who were younger than 18 years of age, those with liver disease other than cirrhosis (e.g., acute liver failure or hepatocellular carcinoma), and those listed for repeat liver transplantation, as well as patients for whom complete laboratory data were not available within 5 days after they had been placed on the waiting list. In addition, since we were interested in the effects of hyponatremia, a small proportion of patients with hypernatremia (se- rum sodium concentration >150 mmol per liter) (24 of 6793 patients, or 0.4%) were excluded from the model development. However, patients with hypernatremia were included in the model validation in order to ensure that the new score could be applied globally. In candidates listed at more than one center concurrently, we identified the first date of registration on the waiting list, using a de-identified unique candidate code.
