Calculate the Injury Severity Score from AIS ratings across 6 body regions. Includes NISS, RTS, TRISS survival probability, and mortality estimates.
The Injury Severity Score (ISS) Calculator computes the internationally standardized anatomical trauma severity measure used by every trauma center, EMS system, and injury research database worldwide. Developed by Baker et al. in 1974, the ISS remains the gold standard for classifying trauma severity and is the primary criterion for defining "major trauma" (ISS ≥ 16).
The ISS is calculated by summing the squares of the highest Abbreviated Injury Scale (AIS) severity codes from the three most severely injured body regions. This mathematical structure gives disproportionate weight to severe injuries (AIS 4-5), which appropriately reflects their impact on mortality and resource utilization. Any single AIS score of 6 (deemed unsurvivable) automatically sets the ISS to 75 — the maximum possible score.
This calculator also computes the New ISS (NISS), which uses the three highest AIS scores regardless of body region (addressing a known limitation of ISS for patients with multiple severe injuries in the same region). When GCS and systolic blood pressure are provided, it calculates the Revised Trauma Score (RTS) and TRISS probability of survival, combining anatomical injury severity with physiologic derangement and patient age.
ISS is required for trauma registry reporting, quality improvement programs, clinical research, and inter-hospital benchmarking. Accurate ISS coding ensures proper resource allocation, appropriate trauma center utilization, and meaningful outcomes comparison across institutions. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain. Use this clarification to avoid ambiguous interpretation.
ISS = (AIS₁)² + (AIS₂)² + (AIS₃)², where AIS₁, AIS₂, AIS₃ are the highest AIS scores from three different body regions. NISS = sum of squares of the three highest AIS scores regardless of region. RTS = 0.9368(GCS code) + 0.7326(SBP code) + 0.2908(RR code). TRISS = 1/(1+e^(-b)), b = -1.2470 + 0.9544(RTS) - 0.0768(ISS) - 1.9052(age>54).
Result: ISS = 29 (Very severe trauma)
ISS = 4² + 3² + 2² = 16 + 9 + 4 = 29. This exceeds the major trauma threshold (≥16), corresponding to estimated 20-40% mortality. The patient would benefit from a Level I trauma center.
The ISS was developed by Susan Baker and colleagues in 1974 as an improvement over simple AIS-based injury description. Baker recognized that trauma patients often have multiple injuries, and a system was needed to quantify overall severity rather than describing each injury independently. The key insight was that the three most severely injured body regions, with squared scoring, provided optimal mortality prediction.
The AIS itself originated in 1969 as a standardized lexicon for describing individual injuries. Now maintained by the Association for the Advancement of Automotive Medicine (AAAM), the AIS dictionary contains over 2,000 injury codes, each assigned a severity level from 1 (minor) to 6 (currently untreatable/unsurvivable).
Every designated trauma center in the United States maintains a trauma registry with ISS data, as required by the American College of Surgeons (ACS) Committee on Trauma verification standards. ISS is used for:
- **Triage decisions**: ISS ≥ 16 identifies patients who benefit most from specialized trauma center care - **Quality benchmarking**: TQIP (Trauma Quality Improvement Program) risk-adjusts mortality by ISS, among other variables - **Research stratification**: Clinical trials use ISS strata to ensure balanced enrollment - **Resource allocation**: ISS correlates with ICU days, ventilator days, blood product usage, and total hospital costs
While ISS remains the global standard, newer scoring systems aim to address its limitations. The NISS (Osler et al., 1997) improves prediction for patients with multiple injuries in the same region. The Trauma and Injury Severity Score (TRISS) adds physiologic data and age. The International Classification of Diseases-based Injury Severity Score (ICISS) uses ICD diagnosis codes instead of AIS, enabling large-database research without manual AIS coding. Machine learning models incorporating continuous vital signs and imaging data are being developed but have not yet replaced ISS in clinical practice.
ISS takes the highest AIS from three different body regions, while NISS takes the three highest AIS scores regardless of region. NISS better predicts mortality for patients with multiple severe injuries concentrated in one region (e.g., two critical abdominal injuries).
Squaring gives disproportionate weight to more severe injuries. An AIS 5 contributes 25 points while an AIS 3 contributes only 9 — appropriately reflecting the exponentially greater impact of critical injuries on survival.
ISS ≥ 16 is the internationally accepted threshold for major trauma. This triggers trauma team activation, Level I/II trauma center criteria, mandatory trauma registry inclusion, and quality benchmarking. Some systems use ISS ≥ 15.
Yes — patients with ISS ≥ 16 have significantly better outcomes when treated at designated trauma centers. ACS-TQIP uses ISS as a primary severity stratification variable for benchmarking trauma center performance.
ISS can only capture one injury per body region; it does not weight injuries equally across regions (a severe brain injury and severe limb injury both contribute 25 points); and it does not account for the interaction between injuries. NISS partially addresses the first limitation.
TRISS combines anatomical (ISS), physiologic (RTS), and demographic (age) information to estimate survival probability. It is the primary metric for unexpected outcomes analysis — comparing actual vs. predicted survival to identify patients who survived against odds or died unexpectedly.