Understanding the P-POSSUM Formula
The P-POSSUM system is an improved version of the POSSUM scoring system, specifically refined to address the original's tendency to overestimate postoperative mortality in lower-risk patients. The "P" stands for Portsmouth, the city where the modification was developed. The core of the P-POSSUM system lies in its formula, which is used to calculate the predicted mortality risk for a patient undergoing surgery.
The formula for predicting mortality risk, denoted as R, is based on the patient's Physiological Score (PS) and Operative Severity Score (OSS). The logarithmic formula is as follows:
$$ \ln \frac{R}{1-R} = -9.37 + (0.19 \times \text{PS}) + (0.15 \times \text{OSS}) $$
To find the actual predicted risk percentage (R), you must exponentiate the result and complete the final steps of the logistic regression calculation. Fortunately, clinicians typically use online calculators to quickly perform this complex calculation, based on data entered for the patient's variables.
The Physiological Score (PS) Factors
The physiological score is determined by evaluating 12 different preoperative factors, assigning a score of 1, 2, 4, or 8 based on the severity of the patient's condition. The scores for all 12 factors are then added together to get the total PS.
- Age: Scores are assigned based on age ranges (e.g., ≤60, 61-70, ≥71). For geriatric patients over 70, this can lose some discriminatory power compared to a continuous variable.
- Cardiac Signs: Based on clinical assessment and chest radiograph, looking for signs of heart failure or cardiac irregularity.
- Respiratory Signs: Assessed through patient history and chest radiograph, checking for dyspnea or chronic obstructive airway disease (COAD).
- Systolic Blood Pressure: Graded based on ranges, with lower and higher values indicating greater risk.
- Pulse: Evaluated by heart rate, with scores increasing for rates outside the normal range.
- Glasgow Coma Scale (GCS): This neurological assessment tool is used, with lower scores indicating more severe impairment.
- Hemoglobin: Scores are based on blood test results, with both low and very high levels increasing the score.
- White Cell Count (WCC): Based on blood test results, with a higher count indicating infection or inflammation.
- Urea: A measure of kidney function based on blood test results.
- Sodium: An electrolyte balance measure based on blood test results.
- Potassium: An electrolyte balance measure based on blood test results.
- Electrocardiogram (ECG): Checks for abnormal rhythms, Q waves, or ST/T wave changes.
The Operative Severity Score (OSS) Factors
Like the physiological score, the operative severity score is derived from six factors related to the surgical procedure itself. These are also scored exponentially (1, 2, 4, 8) and summed to determine the total OSS.
- Operative Severity: A classification of the surgery's magnitude (minor, moderate, major, major+).
- Multiple Procedures: Scores are assigned based on the number of separate procedures performed.
- Total Blood Loss: Quantifies the blood lost during the operation.
- Peritoneal Soiling: Grades the level of abdominal cavity contamination with pus, blood, or bowel contents.
- Presence of Malignancy: Scored based on the presence and extent of any cancer (none, primary only, nodal metastases, distant metastases).
- Mode of Surgery: Reflects the urgency of the procedure (elective, emergency <24h, emergency <2h).
Comparing P-POSSUM and Original POSSUM
While P-POSSUM and the original POSSUM use the same 18 variables to calculate their respective physiological and operative scores, they utilize different formulas and predictive models. The primary difference is the recalibration of the coefficients to provide a more accurate prediction of mortality, particularly for low-risk surgical patients where the original system was known to overestimate risk.
| Feature | Original POSSUM | P-POSSUM |
|---|---|---|
| Purpose | Predicts both morbidity and mortality. | Predicts mortality only. |
| Mortality Formula | $$\ln \frac{R}{1-R} = -7.04 + (0.13 \times \text{PS}) + (0.16 \times \text{OSS})$$ | $$\ln \frac{R}{1-R} = -9.37 + (0.19 \times \text{PS}) + (0.15 \times \text{OSS})$$ |
| Accuracy | Tends to overestimate mortality in low-risk patients. | Provides a more accurate prediction of mortality, better calibrated for all risk levels. |
| Coefficients | Lower coefficients for PS and OSS in the mortality formula. | Higher coefficient for PS, lower for OSS, and a different intercept in the mortality formula. |
How the P-POSSUM Score is Used in Clinical Practice
The P-POSSUM score is a valuable tool for surgical audit and risk assessment, though it does not replace a clinician's judgment. Its primary uses include:
- Informed Consent: Provides an objective, quantitative estimate of surgical mortality risk, which can be communicated to patients and their families during the informed consent process.
- Risk Stratification: Helps classify patients into different risk groups, aiding clinicians in making decisions about the appropriate level of perioperative care and management.
- Surgical Audit: Allows hospitals and surgical units to compare their outcomes to expected mortality rates, helping to assess and improve the quality of surgical care.
- Research: Serves as a standardized measure for comparing surgical outcomes across different studies and populations.
Limitations and Considerations
Despite its improved accuracy over the original POSSUM system, P-POSSUM has limitations. Some studies have noted that the score can still display a poor fit between predicted and observed mortality in specific patient populations or surgical specialties. These variations can depend on factors like patient demographics, specific fracture types, or the hospital's specific patient selection criteria. Therefore, it should be used alongside other clinical risk factors and adjusted for specific patient populations to achieve the best predictive accuracy.
Furthermore, for very low-risk patients, the score might still exhibit limitations. The tool's primary strength lies in comparing outcomes across large groups of patients rather than predicting a definitive outcome for any one individual. For further reading on surgical risk assessment and clinical outcomes, the British Journal of Surgery provides a wealth of research on the topic.
Conclusion
The P-POSSUM score formula represents a significant refinement in surgical risk assessment, offering a more reliable prediction of postoperative mortality compared to its predecessor. By integrating a patient's physiological status and the severity of their operative procedure, it provides a quantitative measure for assessing risk. While clinicians often use online tools for calculation, understanding the underlying formula and its constituent variables is crucial for interpreting the score. Its value lies in facilitating informed clinical decision-making, counseling patients, and improving the quality of surgical audit.
