Comparison of Possum and P-Possum as Audit Tools in Patients Undergoing Emergency Laparotomy for Secondary Bacterial Peritonitis

Background : Though POSSUM and P-POSSUM have been proposed as accurate tools of audit, our initial experience has not been encouraging. Therefore, a prospective study was conducted to find their accuracy for predicting outcome in peritonitis patients who underwent emergency laparotomy. Methods : 172 patients treated in single surgical unit over two years were included. Expected morbidity and mortality, computed by POSSUM and P-POSSUM equations using linear as well as exponential methods of analysis, were compared with observed outcome by observed: expected (O:E) ratios. X2-test was done to draw statistical significance; P<0.050 was taken as significant. Results : POSSUM significantly over-predicted mortality with linear as well as exponential methods with O:E ratios being 0.32 (X2=57.35, 1 d.f. P<0.001) and 0.25 (X2=111.26, 1 d.f. P<0.001), respectively. P-POSSUM also significantly over-predicted mortality by linear as well as exponential methods with O:E ratios being 0.55 (X2=11.37, 1 d.f. P<0.001) and 0.27 (X2=92.30, 1 d.f. P<0.001), respectively. POSSUM significantly over-predicted morbidity by linear and exponential analysis with O:E being 0.76 (X2=47.94, 1 d.f. P<0.001) and 0.81 (X2=23.27, 1 d.f. P<0.001), respectively.