Injury Severity Scores
On this page
Direct answer
Injury scoring turns trauma into numbers that triage, predict and compare. The Abbreviated Injury Scale (AIS) grades each injury 1 (minor) to 6 (unsurvivable) across six body regions; the Injury Severity Score (ISS) squares the three highest AIS grades from three different regions and sums them. ISS 16 or more defines major trauma, and any AIS-6 injury scores the maximum 75 by convention. The Revised Trauma Score grades the physiology — GCS, systolic pressure and respiratory rate coded 0-4 and weighted (RTS = 0.9368 × GCS + 0.7326 × SBP + 0.2908 × RR, maximum 7.84) — and TRISS combines RTS, ISS, age and mechanism into a probability of survival used to audit trauma care.
What you must remember
- AIS regions (six): head and neck, face, chest, abdomen and pelvic contents, extremities and external — ISS can take only one injury, the worst, from each region.
- ISS formula: sum of squares of the three highest AIS scores in three different regions; range 1-75; an AIS-6 in any region sets ISS at 75 automatically.
- Severity bands: ISS under 9 mild, 9-15 moderate, 16-24 severe, 25-75 very severe; 16 is the conventional threshold for major trauma and full trauma-team activation.
- The ISS blind spot: multiple injuries in one region are invisible — two AIS-4 abdominal injuries still score 4² = 16 — so the New Injury Severity Score (NISS, Osler 1997) sums the three highest AIS anywhere and predicts mortality better in penetrating and head injury.
- RTS coded values: GCS 13-15 = 4, 9-12 = 3, 6-8 = 2, 4-5 = 1, 3 = 0; systolic BP 89 or more = 4, 76-89 = 3, 50-75 = 2, 1-49 = 1, nil = 0; respiratory rate 10-29 = 4, over 29 = 3, 6-9 = 2, 1-5 = 1, nil = 0.
- TRISS: logistic combination of RTS, ISS, age over 55 and mechanism yielding a probability of survival (Ps); deaths with Ps above 0.5 flag potentially preventable deaths — the engine of trauma audit.
- Neighbours worth naming: MGAP (mechanism, GCS, age, arterial pressure) for field triage and the Kampala Trauma Score for resource-limited settings.
Worked calculation
A rider hits a truck: GCS 11, blood pressure 80 mmHg systolic, respiratory rate 28. Injuries: subdural haematoma (AIS head 4), flail chest with contusion (AIS chest 4), splenic laceration (AIS abdomen 3), open femur fracture (AIS extremity 3). ISS takes the three highest AIS from different regions: 4² + 4² + 3² = 16 + 16 + 9 = 41 — very severe, trauma-team territory. Had there also been a major liver injury (AIS abdomen 5), ISS would still count only the worst abdominal injury, while NISS would climb to 5² + 4² + 4² = 57 — why NISS outperforms ISS when injuries cluster. Now the physiology: GCS 11 codes 3, SBP 80 codes 3, RR 28 codes 4, so RTS = (0.9368 × 3) + (0.7326 × 3) + (0.2908 × 4) = 2.81 + 2.20 + 1.16 = 6.17 of a maximum 7.84 — deranged but salvageable. TRISS then combines RTS 6.17, ISS 41, age and blunt mechanism into a mid-range survival probability — the benchmark for the next, similar patient.
Perspective: exam angles and Indian usage
NEET-PG asks this topic as arithmetic: "ISS of injuries graded 3, 3, 2, 2 in four regions?" (3² + 3² + 2² = 22), or "which score cannot rise when two injuries share a region?" (ISS). The viva favourite is why ISS misbehaves with penetrating trauma — the same-region blind spot. Indian relevance is concrete: trauma registries increasingly report ISS-banded outcomes, so a resident presenting an M&M case states the ISS and RTS, and TRISS probability where the registry calculates it.
Frequently asked questions
How is the Injury Severity Score calculated?
Square the three highest AIS grades from three different body regions and add them; ISS ranges up to 75, with any AIS-6 injury assigned the maximum by convention.
What is the cut-off ISS that defines severe or major trauma?
ISS of 16 or above, the level at which mortality rises steeply and highest-level trauma team activation is conventional.
How does NISS differ from ISS?
NISS sums the squares of the three highest AIS scores regardless of body region, capturing multiple injuries to one region that ISS ignores, and predicts mortality better in penetrating and head trauma.
What is the Revised Trauma Score and its maximum value?
A physiologic score from coded Glasgow Coma Scale, systolic blood pressure and respiratory rate (RTS = 0.9368 GCS + 0.7326 SBP + 0.2908 RR, all coded), with a maximum of 7.84.
What does TRISS add to ISS and RTS?
Logistic combination of RTS, ISS, age and mechanism into an individual probability of survival, used to audit trauma deaths as expected or potentially preventable.