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TRISS Probability Calculator

Enter the RTS, ISS, patient age and injury mechanism to estimate survival probability.

How it is calculated

TRISS turns three severity inputs into a logit and then a probability: b = b0 + b1×RTS + b2×ISS + b3×AgeIndex, Ps = 1 / (1 + e−b). RTS is the Revised Trauma Score, ISS the Injury Severity Score, and AgeIndex is 0 for a patient under 55 and 1 for 55 or older. The coefficients depend on the mechanism: blunt uses b0 = −0.4499, b1 = 0.8085, b2 = −0.0835, b3 = −1.7430, while penetrating uses b0 = −2.5355, b1 = 0.9934, b2 = −0.0651, b3 = −1.1360.

The tool shows Ps as a percentage with one decimal and b with four decimals. It only rejects negative or missing values; it does not check that RTS or ISS are inside their usual ranges, so entering a sensible range is up to you. Anything other than the penetrating option is treated as blunt. This is an educational estimate, not a clinical decision.

Worked example

Take a 34-year-old with a blunt injury, RTS 7.84 and ISS 16. The age index is 0 because the patient is under 55. For blunt trauma: b = −0.4499 + 0.8085 × 7.84 − 0.0835 × 16 − 1.7430 × 0 = −0.4499 + 6.3386 − 1.3360 = 4.5527. Then Ps = 1 / (1 + e−4.5527), and the tool reports a survival probability of 99.0%. Keeping all else equal and setting the age to 55 drops it to 94.3%.

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Common mistakes

  • Entering the patient's age band as the age index. The tool wants the actual age in years and decides the index itself: under 55 gives 0, 55 and over gives 1.
  • Choosing the wrong mechanism. Blunt and penetrating injuries use different coefficients, so the same RTS and ISS give different probabilities.
  • Entering the three raw components of RTS (Glasgow Coma Scale, blood pressure, respiratory rate) instead of the combined RTS value.
  • Entering out-of-range numbers such as an ISS above 75. The tool does not reject them and will still print a percentage.
  • Reading Ps as a prediction for one individual at the bedside. It is a model estimate from population coefficients and is meant for study and case review.

Sources

About this calculator

TRISS (Trauma and Injury Severity Score), developed by Boyd, Copes and Champion in 1987 from the Major Trauma Outcome Study, estimates a patient's probability of survival (Ps) by combining physiological and anatomical severity into a single logistic model: Ps = 1 / (1 + e^-b), where b is a weighted sum of the Revised Trauma Score (RTS, built from Glasgow Coma Scale, systolic blood pressure and respiratory rate), the Injury Severity Score (ISS, the sum of squares of the three worst Abbreviated Injury Scale regions), and an age index.

Separate coefficient sets exist for blunt versus penetrating trauma, since survival odds at a given severity differ by mechanism. Trauma centers use TRISS scores retrospectively — comparing a patient's predicted Ps against their actual outcome (the "Ws" statistic) — to benchmark care quality and flag unexpected deaths or unexpected survivals for case review, not as a real-time bedside triage tool.

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  • Editorial team

    The mechanism switch can shift the result noticeably, so pick it with care. For an RTS of 5, an ISS of 35 and an age of 34, the blunt coefficients give 66.2% while the penetrating set gives 53.8%. Teams use Ps by comparing it with what actually happened to the patient, so it suits audit and teaching rather than bedside decisions. If you use it for case review, which severity scores do you pull from your registry?

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