Prospective Evaluation of In-hospital Mortality with the P-POSSUM Scoring System in Patients Undergoing Major Digestive Surgery

Background The P-POSSUM score, the most well known of predictive scores for postoperative mortality, requires validation for population and setting. Methods Validation methods included discrimination (C-index statistic), observed: expected (O:E) ratio, calibration with the Hosmer-Lemeshow test, and subgroup analysis (emergency surgery, cancer, age, organs). The study included 3,881 multisite patients undergoing major digestive surgery in France. Results Discrimination via the receiver operating characteristic curve was good (C-index = 0.87). The overall O: E ratio was 1 (95% confidence interval ([95 % CI]: 0.88-1.13), and therefore the quality of the surgical performance is within normal ranges. The O: E ratio, calculated by risk ranges, showed overestimation in the low risk range, especially in the 3 % to 6 % and 6 % to 10 % ranges. Calibration was poor (p < 0.001). The model deviated from the normal pattern of calibration, with mortality lower than expected in the high-risk range. Subgroup analysis found reasonable to good discrimination of populations (C-index ranging from 0.78 to 0.93 except for liver surgery [0.67]) while calibration of individuals remained poor (p < 0.001 to 0.02). Conclusions Good discrimination, as well as nonsignificant overall O: E values, makes P-POSSUM a valuable tool when it is used for surgical audit to compare mortality between populations for major digestive surgery. Conversely, poor calibration (goodness-of-fit), especially in subgroup analysis, and underestimation or overestimation of O: E ratios considerably limits the value of P-POSSUM for prediction of mortality in individuals. Therefore P-POSSUM should not be used to predict outcomes for one particular patient.