Subgroup mortality probability models: Are they necessary for specialized intensive care units?

OBJECTIVES:To examine the sensitivity of the performance of the latest Mortality Probability Model at intensive care unit admission (MPM0-III) to case-mix variations and to determine how specialized models for these subgroups would affect intensive care unit performance assessment. MPM0-III is an important benchmarking tool for intensive care units in Project IMPACT. Overall, MPM0-III has excellent discrimination and calibration but its performance varies on six common patient subsets. DESIGN:A total of 124,171 patients in six subgroups (complex cardiovascular, trauma, elective surgery, medical, neurosurgery, and emergency surgery) were divided randomly into development (60%) and validation (40%) groups. A logistic regression model was developed to predict hospital mortality for each subgroup, using MPM0-III variables. Model performance was evaluated on the validation sets, using Hosmer-Lemeshow and receiver operating characteristic statistics. Intensive care unit standardized mortality ratios, using the subgroup models and MPM0-III, were compared. A sensitivity analysis was used to identify the occurrence of each subgroup associated with degraded MPM0-III performance. SETTING:One hundred thirty-five intensive care units at 98 hospitals participating in Project IMPACT between 2001 and 2004. ICUs with