How is injury severity score calculated? A comprehensive guide for healthcare professionals

September 24, 2025 •

3 min read

The Injury Severity Score (ISS) is a critical tool in trauma care, with a score over 15 defining a major trauma. Learning how is injury severity score calculated involves first assessing the Abbreviated Injury Scale (AIS) for a patient's injuries across different body regions.

Quick Summary

The Injury Severity Score is determined by first assessing a patient's injuries using the Abbreviated Injury Scale, assigning severity scores to different body regions. The scores from the three most severely injured regions are then squared and summed to produce the final ISS, a key metric for trauma outcomes.

Key Points

ISS relies on AIS: The Injury Severity Score is calculated using the Abbreviated Injury Scale (AIS), which rates the severity of individual injuries from 1 (minor) to 6 (unsurvivable).
Regions are key: The human body is divided into six specific regions for ISS calculation: Head/Neck, Face, Chest, Abdomen/Pelvic Contents, Extremities/Pelvic Girdle, and External.
Top three regions only: The formula uses only the highest AIS score from each of the three most severely injured body regions, ignoring other less severe injuries.
Scores are squared: The highest AIS scores from the three regions are squared and then added together to get the final ISS. Squaring the values gives more weight to more severe injuries.
AIS 6 means maximum score: If a patient has even one injury with an AIS score of 6, their ISS is automatically set to the maximum score of 75, as it is considered unsurvivable.
ISS vs. NISS: The ISS has a limitation in evaluating multiple severe injuries within the same body region, which the New Injury Severity Score (NISS) addresses by considering the three most severe injuries regardless of location.
Threshold for major trauma: An ISS greater than 15 is typically used to define a major trauma, indicating a high risk of morbidity and mortality.

Understanding the Fundamentals of Injury Severity Scoring

The Injury Severity Score (ISS) is a standardized anatomical scoring system used in trauma care and research to quantify injury severity and predict patient outcomes. Unlike tools for immediate triage, the ISS is typically calculated after all injuries are identified and assessed, often following definitive diagnostic procedures. The basis for the ISS calculation is the Abbreviated Injury Scale (AIS).

The Role of the Abbreviated Injury Scale (AIS)

The AIS is a standardized system that rates individual injuries on a scale of 1 to 6 based on their severity and threat to life. The scores are:

AIS Score 1: Minor
AIS Score 2: Moderate
AIS Score 3: Serious
AIS Score 4: Severe
AIS Score 5: Critical
AIS Score 6: Maximal (currently considered unsurvivable)

The AIS classifies injuries within six specific body regions:

Head or Neck
Face
Chest
Abdomen or Pelvic Contents
Extremities or Pelvic Girdle
External

Step-by-Step Guide to Calculating the ISS

Calculating the ISS involves several steps:

Identify and Score All Injuries: Document all injuries and assign an AIS score to each one.
Determine Highest AIS per Region: For each of the six body regions, select the single highest AIS score.
Select the Three Most Injured Regions: Identify the three body regions with the highest AIS scores.
Square and Sum: Square the highest AIS score from each of the three most severely injured regions and add them together using the formula: $ISS = A^2 + B^2 + C^2$, where A, B, and C are the highest AIS scores from those three regions.
AIS 6 Override: If any single injury has an AIS score of 6, the ISS is automatically set to 75, regardless of other injuries.

Practical Example of an ISS Calculation

Consider a patient with an intracranial hemorrhage (Head AIS 4), rib fractures (Chest AIS 3), broken femur (Extremity AIS 4), nasal fracture (Face AIS 1), and liver laceration (Abdomen AIS 5). The highest AIS scores per region are: Head 4, Face 1, Chest 3, Abdomen 5, Extremity 4, External 0. The three highest are Abdomen (5), Head (4), and Extremity (4). The calculation is $ISS = 5^2 + 4^2 + 4^2 = 25 + 16 + 16 = 57$.

Limitations and Alternatives to the ISS

The ISS has limitations, particularly its inability to account for multiple severe injuries within the same body region, potentially underestimating severity. This led to the development of the New Injury Severity Score (NISS), which sums the squares of the three highest AIS scores regardless of the body region, providing a potentially more accurate picture of concentrated severe trauma.


Feature	Injury Severity Score (ISS)	New Injury Severity Score (NISS)
Regions Considered	Highest AIS from the 3 most severely injured of 6 body regions.	Highest AIS from the 3 most severely injured injuries, regardless of region.
Calculates Multiple Injuries in One Region?	No.	Yes.
Range	1 to 75.	1 to 75.
Major Trauma Threshold	> 15.	Similar, but can be higher.

Another system is the Revised Trauma Score (RTS), which uses physiological data like Glasgow Coma Scale, systolic blood pressure, and respiratory rate, often used for immediate triage. The ISS and RTS are often combined in the TRISS method to predict survival probability. Trauma professionals select scoring systems based on their specific needs. For further reading, consult resources like Trauma.org.

Conclusion

Calculating the Injury Severity Score is a crucial process in trauma care, based on the Abbreviated Injury Scale and specific body regions. It quantifies anatomical injury severity by squaring and summing the highest AIS scores from the three most injured regions. While a valuable tool for research and outcome prediction, the ISS's limitation in handling multiple severe injuries in one region highlights the importance of alternative scores like the NISS. These scoring systems provide a standardized method for assessing and communicating the impact of trauma.

Frequently Asked Questions

The Abbreviated Injury Scale (AIS) is a system for rating the severity of individual injuries on a 1-6 scale. The ISS uses the AIS scores to determine the overall severity for a patient with multiple injuries. It involves taking the highest AIS scores from the three most severely injured body regions and squaring them.

No, a key limitation of the ISS is that it only considers the single most severe injury within each of the six body regions. For example, if a patient has two severe injuries in the chest, only the highest AIS score is used for the ISS calculation. The New Injury Severity Score (NISS) was developed to address this limitation.

The maximum possible score for the ISS is 75. This score is automatically assigned if any single injury is rated an AIS of 6, which indicates an unsurvivable injury.

Major trauma is typically defined as having an Injury Severity Score (ISS) greater than 15. This threshold is often used in research and healthcare to identify patients with potentially life-threatening injuries who may require specialized trauma center care.

The six body regions are: Head or Neck, Face, Chest, Abdomen or Pelvic Contents, Extremities or Pelvic Girdle, and External.

Calculating the ISS requires a comprehensive assessment of all injuries, which is often not possible during immediate triage in the field or emergency room. It is typically calculated after more definitive diagnostics, like CT scans and surgical findings, are available.

The ISS is an anatomical score, focusing on the physical location and severity of injuries. The RTS is a physiological score, based on a patient's vital signs like Glasgow Coma Scale, systolic blood pressure, and respiratory rate. These two scores are often used together in the TRISS methodology to predict patient outcomes.