Evaluation of the possum and p-possum system of surgical audit in patients undergoing Laparotomy at Kenyatta national hospital

Abstract

The Physiological and Operative Severity Score for the enumeration of Mortality and Morbidity (POSSUM)and its Portsmouth predictor modification (P-POSSUM) are auditing tools designed to compare surgical outcomes independent of case mix. They use the patients' physiological and operativedata to predict morbidity and mortality. In low risk patients however, POSSUM over predictsmortality. The Portsmouth predictor modification (P-POSSUM) was developed to overcomethe over prediction of mortality by POSSUM. Evaluation of POSSUM and PPOSSUMsystems of surgical audit has mainly been done in the U.K. hospitals. Aims: to determine the applicability of POSSUM and P-POSSUM as audit tools in patients undergoing laparotomy at Kenyatta National Hospital (KNH). Methods: A prospective analysis of 166 consecutive patients undergoing laparotomy at KNH, between October 2006 and February 2007, was done. All data (12 physiological and six operative factors) collected using a data sheet were analysed. Predicted mortality rates were calculated using the POSSUM and P-POSSUM equations for mortality, while predicted morbidity rates were calculated using the POSSUM morbidity equation. The primary outcome measure was the actual 30- day mortality rate. Secondary outcome measures were the morbidity seen. Comparisons were then made between the predicted and the observed morbidity and mortality rates using both linear and exponential methods of analysis. The observed to expected morbidity and mortality (0: E) ratios were calculated for each risk group. The overall 0: E ratio for each method of analysis was then obtained. Receiver-operator characteristic curves were designed to determine the relative accuracy of each approach at prediction. The Hosmer- Lemeshow goodness of fit test was also calculated. Results: The observed mortality was 8 patients of 166 patients (4.8%) with an actual morbidity rate of 52%. The predicted POSSUM mortality was 17.1% while the predicted P-POSSUM mortality score was 7.4%. The predicted POSSUM morbidity score was 48.1 %. P-POSSUM showed a statistically significant ability to predict post operative mortality (0: E=0.67:1) while POSSUM over predicted mortality (0: E= 0.2: 1). POSSUM morbidity score appeared accurate in predicting actual morbidity (0: E=1 :1). Area Under Curve (AUC) of Receiver Operator Curves (ROC) of POSSUM mortality and morbidity equation were 0.7440 and 0.6758 respectively while that of P-POSSUM mortality equation was 0.7775 showing good discrimination of all the three scoring systems