What is the Injury Severity Score?
The Injury Severity Score (ISS) is an established medical score used to assess trauma severity. It was introduced by Baker, O'Neill, Haddon, and Long in 1974 and has since become the most widely used anatomical scoring system for trauma patients worldwide. The ISS provides a single numeric score ranging from 1 to 75 that correlates with mortality, morbidity, hospital stay, and other measures of trauma severity.
The ISS is based on the Abbreviated Injury Scale (AIS), which classifies individual injuries by body region and severity. The score is calculated from the three most severely injured body regions, making it a composite measure that captures the overall burden of anatomical injury. Trauma centers, registries, and research studies around the world rely on ISS for patient triage, quality assessment, and outcome prediction.
An ISS of 75 is the maximum possible score and is automatically assigned when any single body region receives an AIS score of 6 (unsurvivable). The threshold for major trauma is generally considered to be an ISS of 16 or greater, which is used in many trauma system definitions and research protocols as a benchmark for severe polytrauma.
The Abbreviated Injury Scale (AIS)
The Abbreviated Injury Scale is the foundation of the ISS. First published in 1971 by the Association for the Advancement of Automotive Medicine (AAAM), the AIS classifies individual injuries on a 6-point ordinal scale based on threat to life. The scale has been updated multiple times, with the most recent revision being AIS 2015 (Update 2018).
| AIS Code | Severity | Description |
|---|---|---|
| 0 | No injury | No detectable injury to the body region |
| 1 | Minor | Superficial injuries, minor lacerations |
| 2 | Moderate | Injuries requiring medical attention but not life-threatening |
| 3 | Serious | Injuries that are not immediately life-threatening but require hospitalization |
| 4 | Severe | Life-threatening injuries with probable survival |
| 5 | Critical | Life-threatening injuries with uncertain survival |
| 6 | Unsurvivable | Injuries incompatible with life (ISS automatically = 75) |
The six body regions used in ISS assessment are: Head/Neck, Face, Chest, Abdomen, Extremities/Pelvis, and External/Skin. Each region is assigned the highest AIS score among all injuries within that region. It is important to note that while the AIS scale goes up to 6, the ISS calculator typically only uses values 0 through 5 for input, since an AIS of 6 automatically results in a maximum ISS of 75.
ISS Calculation Method
The ISS is computed from the three most severely injured body regions. The steps are straightforward:
- Assign the highest AIS severity code to each of the six body regions
- Identify the three body regions with the highest AIS scores
- Square each of these three highest scores
- Sum the three squared values
Where AIS1, AIS2, and AIS3 are the three highest AIS scores from the six body regions, sorted in descending order. The minimum non-zero ISS is 1 (a single region with AIS = 1), and the maximum is 75 (either three regions with AIS = 5, giving 25 + 25 + 25 = 75, or any region with AIS = 6). Only certain ISS values are mathematically possible: 0, 1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14, 16, 17, 18, 19, 20, 21, 22, 24, 25, 26, 27, 29, 30, 32, 33, 34, 35, 36, 38, 41, 42, 43, 45, 48, 49, 50, 51, 54, 57, 59, 66, and 75.
Trauma Classification
ISS values are grouped into severity categories that correlate with clinical outcomes. These categories help clinicians quickly assess the overall severity of a patient's injuries and guide treatment decisions.
| ISS Range | Category | Estimated Mortality |
|---|---|---|
| 1 – 8 | Minor | < 1% |
| 9 – 15 | Moderate | 1 – 2% |
| 16 – 24 | Serious | 3 – 8% |
| 25 – 40 | Severe | 10 – 30% |
| 41 – 49 | Critical | 30 – 50% |
| 50 – 75 | Maximal (often unsurvivable) | 50 – 75%+ |
A patient is generally classified as having major trauma when the ISS is 16 or greater. This threshold is widely used in trauma system design and research as a cut-off for defining severe injury. Many trauma registries and quality improvement programs use ISS ≥ 16 as an inclusion criterion for major trauma patients.
ISS Severity Diagram
Limitations of ISS
Despite its widespread use, the ISS has several well-known limitations that clinicians should be aware of:
- Only considers one injury per region: The ISS uses only the highest AIS score from each body region. This means that a patient with multiple serious injuries in the same region (e.g., two AIS-3 injuries in the chest) will have the same ISS contribution as a patient with a single AIS-3 chest injury. Multiple injuries in one region are effectively ignored.
- Non-linear scale: Because the ISS uses squared values, the relationship between ISS and mortality is not linear. Small differences in AIS scores can lead to large differences in ISS, and some ISS values are not mathematically possible.
- Body region weighting: The ISS treats all body regions equally, but in reality, injuries to the head or chest carry different prognostic significance than injuries to the extremities or face at the same AIS level.
- Does not account for patient factors: Age, comorbidities, physiological response, and mechanism of injury are not incorporated into the ISS. These factors significantly affect outcomes but are not captured by an anatomical scoring system alone.
- Inter-rater variability: The accuracy of the ISS depends on correct AIS coding, which requires trained personnel and can be subject to inter-rater variability, especially for complex injuries.
- Limited sensitivity to certain injury patterns: Penetrating trauma and isolated severe head injuries may not be well-characterized by the ISS alone. Supplementary scoring systems are often recommended.
NISS: The New Injury Severity Score
The New Injury Severity Score (NISS) was proposed by Osler, Baker, and Long in 1997 to address one of the main limitations of the ISS. Unlike the ISS, which uses the highest AIS score from each body region, the NISS uses the three highest AIS scores regardless of body region. This means the NISS can account for multiple severe injuries in the same body region.
Studies have shown that the NISS is at least as good as the ISS in predicting mortality, and in some populations (particularly those with penetrating injuries or multiple injuries in the same region), the NISS outperforms the ISS. However, the traditional ISS remains more widely used in trauma registries and research due to its historical prevalence and established benchmarks.
Other complementary trauma scoring systems include the Revised Trauma Score (RTS), which measures physiological parameters, and the Trauma and Injury Severity Score (TRISS), which combines ISS with RTS and patient age to provide a more comprehensive outcome prediction. Many trauma centers now use a combination of anatomical and physiological scoring systems for comprehensive patient assessment.
Worked Example
Consider a patient involved in a motor vehicle collision with the following injuries:
- Head/Neck: Subdural hematoma — AIS 4 (Severe)
- Face: Nasal fracture — AIS 1 (Minor)
- Chest: Rib fractures with hemothorax — AIS 3 (Serious)
- Abdomen: No injury — AIS 0
- Extremities: Femur fracture — AIS 3 (Serious)
- External: Abrasions — AIS 1 (Minor)
Sorting the AIS scores in descending order: 4, 3, 3, 1, 1, 0. The top three are 4, 3, and 3.
An ISS of 34 falls in the Severe category (25–40), indicating major trauma with an estimated mortality of 10–30%. This patient would meet the major trauma threshold (ISS ≥ 16) and would typically be managed at a Level I or Level II trauma center.
Frequently Asked Questions
What is the maximum ISS score?
The maximum ISS is 75. This occurs when three body regions each have an AIS of 5 (25 + 25 + 25 = 75) or when any single region is assigned an AIS of 6 (unsurvivable), which automatically sets the ISS to 75.
What defines major trauma?
Major trauma is generally defined as an ISS of 16 or greater. This threshold is widely used in trauma registries, research studies, and trauma system design as a benchmark for severe injury requiring specialized care.
Can ISS predict mortality?
ISS correlates with mortality but is not a precise predictor on its own. Mortality depends on many factors including age, comorbidities, mechanism of injury, and quality of care. ISS is best used in combination with other scoring systems like RTS or TRISS for outcome prediction.
How does ISS differ from NISS?
The ISS uses the highest AIS score from each of the six body regions, then squares and sums the top three. The NISS uses the three highest AIS scores regardless of body region. NISS may be more accurate for patients with multiple injuries in the same region, while ISS is more established in trauma registries.
Is AIS 6 used in ISS calculations?
An AIS of 6 (unsurvivable) is not squared and summed like other scores. Instead, any injury rated AIS 6 automatically assigns the maximum ISS of 75, regardless of injuries in other body regions.
Who should calculate the ISS?
ISS should be calculated by trained healthcare professionals, trauma registrars, or researchers familiar with the AIS coding system. Accurate AIS coding is essential for a meaningful ISS, and the AIS dictionary published by the AAAM provides the standardized coding guidelines.