A carotid POSSUM model for predicting death

Background: Risk adjustment to allow for differences in case mix aids comparative audit of carotid endarterectomy (CEA). The aim of this study was to test the predictive accuracy of P-POSSUM (Physiological and Operative Severity Score for the enUmeration of Mortality and morbidity – Portsmouth predictor equation for mortality) and V-POSSUM (P-POSSUM-based model for vascular surgery) and to develop a procedure-specific model for CEA. Methods: A total of 709 CEAs performed in two vascular units from 1992 to 2000 were analysed. The physiological and the operative severity scores were calculated; the outcome used was 30-day mortality. Predicted mortality was calculated using P-POSSUM, V-POSSUM and V-POSSUM (physiology only = PS) equations. The observed and the predicted mortality were compared. The new model was developed on a set of 350 CEAs and validated on 359 CEAs. The data sets of the two units were combined and split into two by the procedure date order. The predictive accuracy of the model was analysed with the χ2 test. Results:   Observed deaths Predicted deaths χ2 P P-POSSUM 11 31 15·8 0·003 V-POSSUM 11 37 22·9 <0·001 V-POSSUM (PS) 11 48 33·9 <0·001 P-POSSUM and V-POSSUM models showed lack of fit in predicting death.   Observed deaths Predicted deaths χ2 P Carotid model 4 6 2·0 0·728 Carotid model (PS) 4 8 3·8 0·440 There was no evidence of any lack of fit in the new model. Conclusion: P-POSSUM and V-POSSUM overpredict mortality and are unsuitable for the comparative audit of CEA. Procedure-specific models based on P-POSSUM can accurately predict death. © 2001 British Journal of Surgery Society Ltd