Statistical & Clinical Analysis
Portsmouth P-POSSUM Surgical Mortality Formula: Physiology & Operative Logit
Calculate predicted 30-day surgical inpatient mortality (%) from 12-factor Physiology Score (12–88) and 6-factor Operative Severity Score (6–48) using Prytherch’s Portsmouth P-POSSUM logistic equation.
Why Portsmouth P-POSSUM (-9.065 + 0.1692*Phys + 0.1550*Op) fixed original POSSUM’s 2×–6× overprediction in low-risk surgery!
Graham Copeland’s original 1991 POSSUM audit score used exponential risk-band grouping rather than individual-patient linear logistic analysis—causing it to overpredict mortality 2-fold overall and 6-fold in low-risk elective surgery! David R. Prytherch, Bruce M. Whiteley, et al. (1998, British Journal of Surgery — Portsmouth Hospital, 10,000 surgical procedures) retained the exact same 12-variable Physiology Score (minimum 12, maximum 88) and 6-variable Operative Severity Score (minimum 6, maximum 48) while recalibrating the linear predictor to ln(R / (1 - R)) = -9.065 + 0.1692 * Physiology + 0.1550 * Operative!
Notice that every patient starts at minimum scores Physiology = 12 and Operative = 6 (never 0!): at Physiology = 24 and Operative = 14 (major colectomy in an elderly cardiac patient), P-POSSUM logit is -2.8342 and predicted 30-day mortality is 5.55%—dropping to 0.36% in an uncomplicated elective procedure (Physiology = 14, Operative = 7)!
| Surgical Audit Case (Physiology 12–88, Operative 6–48) | P-POSSUM Linear Logit | Predicted 30-Day Mortality (%) | NELA / Surgical Audit Action |
|---|---|---|---|
| Physiology = 24, Operative = 14 (Urgent Colectomy) | -2.8342 | 5.55% (>= 5% High-Risk Cutoff!) | Consultant Anesthesia + Post-Op HDU/ICU |
| Physiology = 14, Operative = 7 (Elective Hernia) | -5.6112 | 0.36% | Standard Surgical Ward Recovery |
| Physiology < 12 or Operative < 6 (Impossible Floor!) | Error Guard | "Physiology >= 12 and Operative >= 6 Required" | Minimum-Score Audit Guard |
Portsmouth P-POSSUM Predicted 30-Day Mortality Probability (%)
Evaluates 100 / (1 + EXP(-(-9.065 + 0.1692 * B2 + 0.1550 * B3))), enforcing POSSUM minimum floors Physiology >= 12 and Operative >= 6.
=IF(OR(B2 < 12, B2 > 88, B3 < 6, B3 > 48), "Physiology >= 12 and Operative >= 6 Required", ROUND(100 / (1 + EXP(-(-9.065 + 0.1692 * B2 + 0.155 * B3))), 2))P-POSSUM Log-Odds Linear Predictor ln(R / (1 - R))
Outputs the linear log-odds -9.065 + 0.1692 * Physiology + 0.1550 * Operative.
=IF(OR(B2 < 12, B2 > 88, B3 < 6, B3 > 48), "Physiology >= 12 and Operative >= 6 Required", ROUND(-9.065 + 0.1692 * B2 + 0.155 * B3, 4))Try the example
Use the original values or change them to test your understanding.