Background Scoring systems are very useful tools in surgical practice, they remain essential outcome predictive tools in the critically ill surgical patients allowing contributory factors to effect their weight in outcome prediction. The POSSUM (Physiologic al and Operative Severity Score for the enumeration of Mortality and morbidity) and its modification, the Portsmouth predictor equation for mortality (PPOSSUM) have been shown to be predict outcome in peritonitic patients. This study aimed to evaluate their role in our clinical setting and additionally show the best timing for parameter assessment utilized in the scoring system which has been variable in literature. Methods This was a prospective, observational, hospital based, study in a single centre which evaluated the POSSUM and P-POSSUM scoring systems at two distinct timings for both scores (on presentation prior to resuscitation- Score 1) and at induction of anaesthesia (after resuscitation- Score 2). The scores were categorized and the observed: expected ratios (O:E ratio) were calculated for each sub-group. An O:E ratio of 1 was the best prediction while values below 1 signified over-prediction and above 1 under-prediction. As the data was non-parametric in distribution, the Wilcoxon signed rank test was used for paired continuous variables and the Mann Whitney-U test for independent samples. The sensitivity, specificity, positive and negative predictive values were calculated. The Receiver-Operating Characteristics curve was plotted and the area under the curve (AUC) was calculated to assess the discriminatory power of the different scoring approaches. Statistical significance of differences in expected and observed rates obtained utilizing the same score measured at different times as well as between different scores applied at the same time was analysed. A p-value <0.05 was considered significant and the results are presented in tables and figures. Results It was a young (average age 33 years (S.D 20.5)), predominantly male population aged 17-81 years. The most common cause of peritonitis was acute appendicitis (27.3%) followed by perforated gastric (21.8%) and duodenal ulcers (18.2%). The mean pulse rate (114 beats/min) and respiratory rates (33 cycles/min) were slightly higher at presentation compared to after resuscitation (respectively 110 beats/min) and 27 cycles/min) and a similar trend was observed in the physiological and total scores. There were statistically significant differences in the expected values using the different scoring approaches to predict morbidity and mortality. A gross morbidity rate of 61.8% was observed with surgical site- infections (36%) being the most common post-operative complication. The POSSUM scores 1 and 2 expected morbidity rates respectively averaged 80% and 78%. Additionally, although they were very sensitive, they were not specific (47.2% and 50.0% for scores 1 and 2 respectively). Both scoring approaches significantly over predicted morbidity with O: E ratios of 0.67 and 0.68 with area under the receiver-operating characteristics curve (AUC) of 0.664 (P=0.015) and 0.649 (P=0.064) which suggests a poor fit. The gross mortality rate was 30.9% in this study. Four scoring approaches to mortality prediction evaluated predicted a rate of 27.6-51.1% The scores derived from physiological variables measured on presentation to the hospital had more favourable fit (AUC 0.8 with P<0.001) compared to when the same scoring systems were calculated from variables after resuscitation (AUC 0.7 (P<0.001). However, while both POSSUM and P-POSSUM scores 1 were very sensitive, POSSUM score 1 was less specific (68.4%) and significantly over-predicted mortality (O: E =0.59) compared to the P-POSSUM 1 score (0: E =0.85, specificity of 92.1% and AUC 0.8). Overall, the use of P-POSSUM score 1 (physiological score component derived from pre-resuscitation variables) was the scoring option which best predicts mortality in this study. Conclusion In conclusion, resource allocation is a key aspect of surgical care in resource-challenged health care settings and this scoring system provide an objective guide to managing teams. The POSSUM score was not a useful predictor of morbidity, the P-POSSUM score 1 demonstrated a good fit. It is a simple tool that utilizes basic investigative tools to predict mortality in a patient and can be readily applied to clinical practice. This study has shown that utilizing physiological parameters present on admission to the emergency room prior to resuscitation better fits our health care setting