Internal comparative audit in a thoracic surgery unit using the physiological and operative severity score for the enumeration of mortality and morbidity (POSSUM)

OBJECTIVE: The aim of the study was to use the physiological and operative severity score for the enumeration of mortality and morbidity (POSSUM) to assess the performance of our thoracic surgery unit during two successive periods of activity. METHODS: From 1992 through 1997, 801 candidates for thoracic procedures at our institution were considered for the study. After validation of the logistic regression model, including the POSSUM score, observed and POSSUM-predicted morbidities were compared within two groups of patients divided by year of operation (group 1: 1992-1994, n=362; group 2: 1995-1997, n=439) by means of the z-test for the comparison of a proportion with an expected value. RESULTS: The POSSUM-predicted morbidity was significantly lower than the observed one in the first period of activity of our unit (19.6 vs. 24.3%, respectively; z-test for the comparison of a proportion with an expected value, 2.25; P=0.01), whereas no difference was found in the second period (20.5 vs. 19.1%, respectively; z-test for the comparison of a proportion with an expected value, -0.71; P=0.76). CONCLUSIONS: The result suggests a worse-than-expected performance of our unit in the first period of activity, showing that POSSUM can be reliably applied as an instrument of internal comparative audit in a thoracic surgery unit.