External validation of a modified model of Acute Physiology and Chronic Health Evaluation (APACHE) II for orthotopic liver transplant patients

INTRODUCTION The purpose of the study was to validate the newly derived postoperative orthotopic liver transplantation (OLTX)-specific diagnostic weight for the Acute Physiology and Chronic Health Evaluation (APACHE) II mortality prediction system in independent databases. METHODS Medical records of 174 liver transplantation patients admitted postoperatively to the adult intensive care units at King Fahad National Guard Hospital and the University of Wisconsin were reviewed, and data on age, sex, the underlying liver disease, APACHE II scores and the hospital outcome were collected. Predicted mortality was calculated using: 1) the original APACHE II diagnostic weight of postoperative other gastrointestinal surgery and 2) the newly derived OLTX-specific diagnostic category weight. Standardized mortality ratio and 95% confidence intervals were calculated. Calibration was evaluated with the Hosmer-Lemeshow goodness-of-fit C-statistic. Discrimination was tested by 2 x 2 classification matrices and by computing the areas under the receiver operating characteristic curves. Patient characteristics and outcome data were compared between the two hospitals. RESULTS APACHE II significantly overestimated mortality when the original diagnostic weight was used, but provided a closer estimate of mortality with the OTLX-specific diagnostic weight. The C-statistic analysis showed better calibration for the new approach; discrimination was also improved. The performances of the prediction systems were similar in the two hospitals. The new model provided more accurate estimates of hospital mortality in each hospital. DISCUSSION APACHE II provided an accurate estimate of mortality in liver transplant patients when the OLTX-specific diagnostic weight was used. With the new model, APACHE II can be used as a valid mortality prediction system in this group of patients.