Risk Prediction in Surgery

Introduction

POSSUM stands for Physiological and Operative Severity Score for the enUmeration of Mortality and Morbidity. It was developed by Copeland et al in 1991 and has since been applied to a number of surgical groups including orthopaedic patients, vascular surgery (AAA, carotid endarterectomy etc), head and neck surgery and GI/Colorectal surgery (see references).

POSSUM is becoming more widely used in the UK as surgical culture moves more towards outcome measures and providing the patient (as the end user of service) with as much information as possible to make fully informed consent. Furthermore a system that uses risk adjusted prediction is going to become an essential tool for clinical governance reviews to 'prove' a units performance and also for an individual consultant surgeons appraisal process for much the same reason.

Surgeons are more aware of POSSUM than other scoring systems used by anaesthetists for example who ASA for general risk prediction and APACHE for critically ill patients on ITU. ASA is too simplistic and highly subjective whilst APACHE is too complex for general use. POSSUM lies somewhere between these two systems, closer to APACHE and requires only 12 physiological and 6 operative parameters for its calculation. POSSUM can even be used in the pre-clerking clinic to give an estimate of risk providing those clinicians handling the data understand the implications.

POSSUM used exponential analysis and a report from Whiteley et al 1998 claimed that POSSUM over predicted death in their group of patients especially in low-risk patients. In an effort to counteract this effect the original POSSUM equation was modified leading to the Portsmouth predictor equation for mortality (P-POSSUM) utilising the same physiological and operative variables. This method used linear analysis. Further studies have since shown the use of POSSUM and P-POSSUM to predict mortality equally well. Even the P-POSSUM model still overpredicts mortality in low-risk groups, but is a better 'fit' than POSSUM. Furthermore, there have been reports of overprediction in different surgical specialities. This has led some to produce specialty-specific POSSUM such as V-POSSUM for use in elective vascular surgery (Prytherch 2001).

How does POSSUM work?

The original description of POSSUM was as a tool to compare morbidity and mortality in a wide range of general surgical procedures in order to facilitate surgical audit and the comparison of units performance. The idea was to adjust risk of a surgical procedure based on the patients physiological condition and therefor allow a more accurate comparison of units (or individuals) performance. The original developers examined 62 physiological parameters and used multi-variate analysis to identify the most powerful predictors. This reduced the 62 to 12 physiological and 6 operative parameters. Other factors did improve the model slightly but so strongly duplicate the 18 above that they offered no additive predictive power. Each of the 18 factors were weighted to a value of 1,2,4 or 8 depending on measurement simplifying the calculation.

POSSUM formula:

Ln R/1-R = -7.04 + (0.13 x physiological score) + (0.16 x operative severity score)

P-POSSUM formula:

Ln R/1-R = -9.065 + (0.1692 x physiological score) + (0.1550 x operative severity score)

where R = predicted risk of mortality

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