1 University Department of Anaesthesia and 2 Trauma and Orthopaedics Directorate, Queen's Medical Centre, Nottingham NG7 2UH, UK
* Corresponding author. E-mail: Iain.Moppett{at}nottingham.ac.uk
Accepted for publication December 2, 2004.
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Abstract |
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Methods. A cohort study was conducted in Queen's Medical Centre, Nottingham over a period of nearly 2 yr. Complete data from 1164 patients were analysed to compare the mortality predicted by POSSUM and the observed mortality. POSSUM risk of death was calculated using the original POSSUM equation, with modifications to the operative score appropriate for orthopaedic surgery.
Results. POSSUM predicted 181 (15.6%) deaths and the observed mortality was 119 (10.2%). The area under the receiver operating characteristic curve was 0.62, indicating poor performance by the POSSUM equation.
Conclusion. POSSUM overpredicts mortality in hip fracture patients. It should be used with caution whether as an audit tool or for preoperative triage.
Keywords: audit, POSSUM scoring ; complications, fractured neck of femur ; complications, mortality
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Introduction |
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In the current health-care climate, there is increasing use of scoring systems. First, they are used to compare operative outcome both between and within units. Secondly, scoring systems are used to identify high-risk patients before surgery, to inform the consent process, and to triage the use of higher-level care. This preoperative use of POSSUM appears anecdotally to be increasing despite being outside the original validation of the system. Patients presenting with fractured neck of the femur form a large, high-risk group for which POSSUM would seem useful. However, POSSUM has not been formally evaluated for postoperative or preoperative use in fractured neck of femur surgery.
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Methods |
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All data were entered into a Microsoft Excel XP® spreadsheet and POSSUM scores generated using automated calculations. A random sample of data (20 records) was checked manually by one of the investigators (I.K.M.), as were the highest 5% and lowest 5% of POSSUM scores, both for accuracy of input data and correct calculation of physiological, operative and total scores. Individual physiological values which deviated markedly from normal were cross-checked with hospital records. No errors were found. Previous internal cross-checking of this audit data has found an error rate of <3%.
Each physiological datum is given a score between 1 and 8 using the original POSSUM system (Table 1),1 giving a minimum and maximum physiological score of 12 and 96. The operative data are scored using a modification of the original POSSUM system to allow for orthopaedic operations (Table 2),2 again between 1 and 8, giving a minimum of 6 and maximum of 48. Routine surgery for fractured neck of femur (dynamic hip screw, hemiarthroplasty) is scored as major (4 points); revision surgery is major+ (8 points). The total physiological and operative scores are then entered into a logistic regression equation, which gives a risk of death.
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Results |
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Discussion |
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Overall, we found that POSSUM overestimated the risk of death, particularly in those patients with a higher predicted risk of dying. The ROC area under the curve of 0.62 indicates a poor test, particularly in the context of AUC values for POSSUM in general surgery of >0.97 and general orthopaedics of >0.85.5 Other workers have found a similar overprediction of POSSUM when used in both general surgery and specific operative subgroups.812 Our results are based on a single centre, so there is the possibility that our results are at variance with national/international experience. This cannot be excluded; however, the 30-day mortality in this series of 10.2% is in line with national statistics and other published data.13
In the original POSSUM, the physiological data were collected close to the surgery, whereas we collected the data on hospital admission. Given the relatively short period from admission to operation, this is unlikely to have a marked impact on results. One might expect an improvement in physiological score following admission and/or resuscitation, which could explain some of the overprediction from POSSUM. Conversely, patients may deteriorate after admission, which would lead to underestimation of physiological dysfunction. These issues apply to all scoring systems and to date there appears to be no consensus on the best time to estimate risk.14 Clearly, the presence of missing data may also have altered the results. However, including all patients operated on does not change the overall 30-day mortality significantly (10.5 vs 10.2%). High-risk patients may also have been identified early and provided with appropriate high-level perioperative anaesthetic, orthopaedic and nursing care, hence reducing their mortality. This is unlikely, given the similarity of overall mortality between this and other series. Also in our centre, all patients with fractured neck of femur have surgery on dedicated trauma theatres, performed by anaesthetists and surgeons of specialist registrar grade and above. Because of the homogeneity of surgery in this group of patients, the surgical component of POSSUM has a very narrow range, so the variability in total POSSUM is largely due to changes in physiological (preoperative) score. Some authors have found that using purely the physiological component of POSSUM performs as well as the combined scores in predicting mortality in ruptured abdominal aortic aneurysm4 and major arterial surgery.3 4 In our series, this performs poorly. There are probably several reasons for this.
The patient population is generally elderly. Fifty-nine per cent of them were >80 yr. Hence, the presence of abnormalities on blood tests or examination may be more normal than in a general, unselected population and hence not a specific marker of at-risk patients. The weighting of individual components may also be inappropriate for this population, with undue emphasis on certain aspects.14 Previously published work has found strong associations between serum albumin or haemoglobin concentrations on admission and mortality in this group.15 16 Whatever the reason, POSSUM is not identifying the correct risk factors for this patient group.
Recently published work has suggested the use of a simplified scoring system, which performs better than POSSUM in a general surgical population.17 We did not formally test this new score with our data. However, for this relatively homogeneous population, it is likely to perform badly. The score uses four factors: age, ASA grading, mode of surgery (elective vs emergency) and severity (three-point scale). All surgery for fractured neck of the femur is emergency surgery, and all the operations are classed as Grade II. Most patients are ASA II or III and most are >70 yr of age. Thus, there is very little variation possible within the Donati score.
Does it matter whether POSSUM is valid for use in this population? Surgical and anaesthetic audit is an appropriate part of modern medical practice. Good audit requires some degree of gold standard against which to compare results. The temptation is to extrapolate validated audit tools into other fields. We would suggest that POSSUM does not reliably perform this role. Using POSSUM to identify high-risk patients before surgery is also not reliable. POSSUM overpredicts mortality in fractured neck of the femur, and should not be used as a comparative audit tool for this group of patients. Its role as a preoperative assessment tool is also limited. Further work combining established risk factors for this population, such as concentrations of haemoglobin16 and serum albumin15 may provide a better predictive tool.
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Footnotes |
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References |
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