A nomogram to calculate the Physiological and Operative Severity Score for the enUmeration of Mortality and morbidity (POSSUM)

Abstract

The Physiological and Operative Severity Score for the enUmeration of Mortality and morbidity (POSSUM) is a well validated model for the prediction of perioperative mortality and morbidity with application to surgery and intensive care medicine. The outcome measure calculations are time-consuming, complex and potentially error-prone. Nomograms are low-cost easy-to-use graphic devices that can be used to make repeated calculations to an acceptable level of accuracy for most clinical purposes. This paper describes a nomogram to aid calculation of POSSUM scores. This graphic solution consists of two sections: a tally sheet to calculate physiological and operative severity scores (PS and OS), and a nomogram to calculate mortality and morbidity. The latter was designed using standard mathematical methods, and drafted with the aid of commercially available software. Accuracy was confirmed by using a spreadsheet to generate 120 random sets of simulated values for PS and OS, and corresponding calculated values for predicted mortality and morbidity. The outcome values in each case were derived using the nomogram and compared with the spreadsheet values using Bland-Altman analysis. Bland-Altman analysis showed close agreement between nomogram and spreadsheet. Bias of the nomogram was -0·1 percentage points for mortality and 0·1 percentage points for morbidity, with limits of agreement of -2·1 to +1·9 and -1·4 to +1·6 percentage points for mortality and morbidity respectively. Correlation coefficients were r > 0·999 and P < 0·001 for both mortality and morbidity. The nomogram provides a rapid, accurate, low-cost means of performing, visualizing and cross-checking POSSUM calculations.