Prospective Independent Validation of IMPACT Modeling as a Prognostic Tool in Severe Traumatic Brain Injury

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Abstract
Clinical trials in traumatic brain injury (TBI) have been fraught with failure due in part to heterogeneity in pathology and insensitive outcome measurements. The International Mission for Prognosis and Analysis of Clinical Trials in TBI (IMPACT) prognostic model has been purposed as a means of risk adjustment and outcome prediction for use in trial design and analysis. The purpose of this study was to evaluate the performance of the IMPACT model in predicting 6-month functional outcome and mortality using prospectively collected data at a large, Level 1 neurotrauma center. This population-based cohort study included all TBI patients ≥14 years of age admitted with a Glasgow Coma Scale (GCS) score of ≤8 (severe TBI) to the University of Pittsburgh Medical Center between July 1994 and May 2009. Clinical data were prospectively collected and linked to 6-month functional outcome (Glasgow Outcome Scale [GOS]) and mortality. The discriminatory power and calibration of the three iterations of the IMPACT model (core, extended, and lab) were assessed using multiple regression analyses and indicated by the area under the receiver operating characteristic curve (AUC). A sample of 587 patients was available for analysis; the mean age was 37.8±17 years. The median 6-month GOS was 3 (IQR 3); 6-month mortality was 41%. The prognostic models were composed of age, motor score, and pupillary reactivity (core model), Marshall grade on head CT and secondary insults (extended), and laboratory values (lab); all of these displayed good prediction ability for unfavorable outcome and mortality (unfavorable outcome AUC=0.76, 0.79, 0.76; mortality AUC=0.78, 0.83, 0.83, respectively). All model iterations displayed adequate calibration for predicting unfavorable outcome and mortality. Prospective, independent validation supports the IMPACT prognostic model's prediction of patient 6-month functional status and mortality after severe TBI. The IMPACT prognostic model is an effective instrument to assist TBI study design and analysis.