Evaluation of POSSUM and P-POSSUM as predictors of mortality and morbidity in patients undergoing laparotomy at a referral hospital in Nairobi, Kenya

Author Information

Kimani MM1,2, Kiiru JN3, Matu MM3, Chokwe T1,2, Saidi H1,2

  1. University of Nairobi, College of Health Science

  2. Kenyatta National Hospital, Department of surgery

  3. Kenya Medical Research Institute, Centre for Microbiology Research

Corresponding author:

Dr. Moses Kimani, P.O. Box 54270- 00200 Nairobi Kenya, Tel: +254724400193, Email: mwangikimanimoses@gmail.com

Abstract

Background: The Physiological and Operative Severity Score for enUmeration of Morbidity and Mortality (POSSUM) and its Portsmouth modification (P-POSSUM) were developed for comparative audit in surgical patients. This study evaluated applicability of these systems in estimating mortality and morbidity risks in a cohort of patients undergoing laparotomy at the national referral hospital in Nairobi, Kenya.

Methods: Data of 166 patients undergoing laparotomy was subjected to POSSUM and P-POSSUM scoring systems and analyzed using linear and exponential methods. The discrimination power of POSSUM and P-POSSUM as predictors of surgical outcome was measured using the receiver–operating characteristic (ROC) curve.
Results: The overall observed to expected (O:E) ratio using linear analysis was 0.29:1 (POSSUM) and 0.67:1 (P-POSSUM) while exponential analysis gave an O:E of 0.2:1 (POSSUM) and 0.4:1 (P-POSSUM). The predicted morbidity using POSSUM was 1.09:1 (linear analysis) and 1:1 (exponential analysis). Hosmer–Lemeshow analysis did not show a significant lack of fit with the observed mortality when applied to POSSUM and P-POSSUM. The area under ROC curve was 0.74 (POSSUM) and 0.78 (P-POSSUM).

Conclusion: Our results support the suitability of P-POSSUM and POSSUM scoring systems to stratify morbidity and mortality risk in our study population.

Introduction

Surgical audit has become an essential requirement in modern surgical practice for ensuring the quality of care(1). By evaluating the procedures used for diagnosis and treatment and to lower the risks of patients undergoing surgery, both individual surgeon and institutional outcome trends can be analyzed (1).

Audits are common quality indicator tools in hospitals in the developed countries but rarely done in many developing countries (2). Patients are therefore subjected to surgery without due consideration of factors that may complicate the outcomes of these operations. In such setups, crude morbidity and mortality rates are used as indicators of quality of care. However, these approaches may be misleading since lack of adjustments for different settings leads to erroneous results when outcomes of surgery are compared between different units and hospitals (3,4). In order to standardize surgical audit procedures, a number of scoring systems have been developed for the comparison of case mixes.

The Physiological and Operative Severity Score for the enUmeration of Mortality and Morbidity (POSSUM) (5) and its Portsmouth predictor modification (P-POSSUM) (6) audit systems use the patients’ physiological and operative data to predict morbidity and mortality and compare surgical outcomes independent of case mix (5,6). The two systems are built on the accepted principle that the mode of presentation, physiological condition of the patient and extent of the surgical procedure performed are predictors of patient outcome. This can be useful in monitoring the improvement of surgical outcomes in patients ranked to be at high risk of mortality and morbidity following surgery.

Our local experience with outcome predictions in any surgical field using standardized schemes is limited. Neither POSSUM nor P-POSSUM have been applied in any Kenyan hospitals and therefore, little is known about their applicability as indicators of quality of healthcare delivery to patients undergoing surgical procedures in Kenya. This study aimed to evaluate the applicability of POSSUM and P-POSSUM scoring systems in predicting post-surgical outcomes in patients undergoing laparotomy in a referral hospital in Nairobi, Kenya.

Patients and Methods

Kenyatta National Hospital (KNH) is the central referral and teaching hospital in Kenya. It is located in the city of Nairobi. Nairobi has an estimated population of about 3 million people. Patients undergoing laparotomy at the hospital are managed by a tier of doctors from interns, senior house officers, surgical residents, registrars to full time consultants in General Surgery. However, all surgeries are performed by the residents, registrars and consultants.

The institution has the capacity to undertake major surgical procedures on a 24-hour basis. We conducted a single centre prospective study over a period of 6 months in patients undergoing laparotomy from the emergency unit and those admitted to the general surgical wards, intensive and high dependency units at KNH. A total of 166 patients, undergoing either elective or emergency surgery, and meeting the recruitment criteria, were enrolled in the study.

Selection of the patients was done from the point of admission – emergency laparotomy patients were selected from the accident and emergency department while those who underwent elective laparotomy were recruited from different hospital wards. Patient data was collected from hospital files. Patients and their relatives provided further information during the consent process. The primary outcomes were the actual 30-day mortality rates for the overall group and for different risk strata (6,7). Actual (observed) deaths were then compared to predicted (expected) mortality generated using the POSSUM and P-POSSUM equations. The results were analyzed using linear and exponential methods as described by Hobson et al (8). Secondary outcomes included actual and predicted rates of morbidities (surgical site infection, electrolyte imbalance, burst abdomen, intestinal obstruction, and fecal fistulas among others) for the same group of patients, as originally described by Copeland et al (5). Pre-operative scores were done as described by Copeland et al (5) using 12 physiological factors while 6 operative scores were made at the end of the operation. Predicted morbidity and mortality rates were calculated using both POSSUM and P-POSSUM equations as described by Wijesinghe et al (9).

The discrimination power and accuracy of POSSUM and P-POSSUM in predicting death was measured using the receiver–operating characteristic (ROC) curve. An area under the ROC curve of less than 0.7 show poor discrimination, that of between 0.7 and 0.8 indicates a reasonable discriminatory power while a value above 0.8 suggests perfect discrimination. Statistical analyses and ROC curve plotting were done using the SPSS Version 15.0 (SPSS, Chicago, IL, USA). The Hosmer–Lemeshow (H–L) goodness of fit test (10) was used to assess the differences between predicted and observed mortality rates for POSSUM and P-POSSUM. A value of p ≤ 0.05 was considered statistically significant.

Results

Of the 166 patients enrolled, 111 (67%) were males. The ages ranged from 13 to 82 years (mean 35 years). Majority (67%) were enrolled for emergency laparotomy. Eight (4.8%) patients died. The overall predicted mortality of the study group was calculated at 16.7% using the POSSUM linear analysis method (table 1) and 50% using POSSUM exponential analysis (table 2).

This gave an observed to expected (O:E) ratio of 0.29:1 and 0.2:1 respectively. When similar calculations were made using the P-POSSUM scheme, the predicted mortality was 7.2% (linear analysis) and 13.9% (exponential analysis) giving an Observed to Expecte