The value of the PRISM scores in comparing pediatric liver transplant outcomes

Confronted with the challenges of the insufficient hepatic allograft supply and the mandate to use a continuous scale of liver disease severity (1), the transplant community has joined with United Network of Organ Sharing (UNOS) to develop measures for prioritizing organ allocation (2–4). Presently, UNOS relies on Model for End-Stage Liver Disease (MELD) and Pediatric End-Stage Liver Disease (PELD) scores for liver graft allocation as a consequence of a careful, retrospective statistical outcome analysis of pretransplant mortality data. The article by Carroll et al. (5) published in the August Issue of Pediatric Transplantation focuses on identification methods for outcome prediction following liver transplantation. They address the question of predicting the best utilization of the liver graft, a limited and precious resource. If one follows the outcome parameter of a scoring system with the chief objective to predict mortality caused by liver disease without liver replacement then the current MELD and PELD scales fit that model construct. The initial scoring method was the Childs–Turcotte–Pugh (CTP) score (6) which was designed to evaluate the risk of portosystemic surgery based on liver disease severity. Unfortunately, while UNOS used this system for liver graft allocation, the CTP system had limitations as it lacked a continuous scale, incorporated subjectivity and laboratory variability. MELD proved more useful in predicting survival after a transjugular intrahepatic portosystemic shunt (TIPS) based on a scale using serum creatinine, total serum bilirubin, and the INR for prothrombin time to assess the degree of end-stage liver disease in adults (7). However, adjustment of other variables such as liver disease type or clinical factors such as ascites and encephalopathy did not improve predictability of outcome (4). PELD followed the principles of MELD but revised the calculation of score by deleting creatinine and adding age less than one and growth failure to fit the retrospective analysis of over 1000 children in the Studies of Pediatric Liver Transplantation (SPLIT). Again the endpoint of death within 3 months, as in MELD, was the primary outcome endpoint. Further prospective investigations will assist in the validation of these scoring systems. Intensive care specialists and others have also shared the interest in the value of outcome measuring models. One such system is APACHE (8) or Acute Physiology and Chronic Health Evaluation, which was developed to predict patient survival in the adult intensive care units (ICU). Pediatric intensivists modified APACHE for children using six physical parameters and eight laboratory values routinely measured in the pediatric intensive care (PICU) setting (9). Numerous studies have utilized this system called PRISM or Pediatric Risk of Mortality for predicting outcomes and permitting data comparison between centers. In 1996, Pollack et al. (10) updated PRISM to PRISM III in a prospective analysis of 11 165 admissions and 543 deaths from 32 pediatric intensive care units. PRISM III contained 17 physiologic values subdivided into 26 ranges (and proved better than the original PRISM model at predicting mortality). As in MELD and PELD, PRISM III does not use clinical subjective assessment predictors and compares well in predicting outcome (11). Unlike MELD and PELD, PRISM III utilizes parameters from multiple organ systems and emphasizes the importance of progressive multi-organ system failure. Although it would be convenient to allocate hepatic allografts based on the parameters of liver function, it would be unrealistic to assume that only these parameters would influence the outcome of the transplant. Intercurrent respiratory failure, renal failure, cardiac dysfunction, and other organ involvement greatly decrease a patient’s ability to survive a transplant. Pediatr Transplantation 2004: 8: 208–209