The Abbreviated Injury Scale (AIS) and Injury Severity Score (ISS): A Structured Approach to Trauma Severity Assessment
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Introduction
Trauma care requires rapid, standardized assessment of injury severity to guide triage, management, prognosis, and research. To achieve this, trauma systems worldwide use the Abbreviated Injury Scale (AIS) and the derived Injury Severity Score (ISS). Together, these tools provide an objective, anatomically based measure of trauma severity and correlate strongly with morbidity and mortality.
Abbreviated Injury Scale (AIS)
Definition
The Abbreviated Injury Scale (AIS) is an anatomical scoring system developed by the Association for the Advancement of Automotive Medicine (AAAM). It assigns a numerical severity score (1–6) to individual injuries within specific body regions.
Each injury is coded based on:
Body region
Specific organ or structure injured
Severity of injury
⚠️ Important: AIS scores apply to individual injuries, not the patient as a whole.
AIS Severity Levels (1–6)
AIS Score | Severity Level | Clinical Meaning |
1 | Minor | Superficial injury, no threat to life |
2 | Moderate | Requiring treatment, not life-threatening |
3 | Serious | Significant injury, not immediately life-threatening |
4 | Severe | Life-threatening, survival probable |
5 | Critical | Survival uncertain |
6 | Maximal | Currently untreatable / unsurvivable |
📌 Exam Pearl:Any AIS 6 injury automatically assigns an ISS of 75, regardless of other injuries.
Major Body Regions (ISS Categories)
AIS categorizes injuries into six major body regions, which are also used for ISS calculation:
Head & Neck
Brain, skull, cervical spine, carotid vessels, larynx
Face
Eyes, nose, mouth, facial bones
Chest (Thorax)
Ribs, thoracic spine, lungs, heart, diaphragm
Abdomen & Pelvis
Abdominal organs (liver, spleen, kidneys, intestines), lumbar spine
Extremities & Pelvic Girdle
Arms, legs, pelvic bones, shoulder girdle
External / Body Surface
Skin, burns, abrasions, lacerations
Examples of Organ-Specific AIS Scores
Head & Neck Injuries
Injury | Typical AIS |
Minor scalp laceration | 1–2 |
Cerebral concussion | 2 |
Cerebral contusion | 2–4 |
Diffuse axonal injury (severe) | 4–5 |
Chest Injuries
Injury | Typical AIS |
Single rib fracture | 2 |
Fractured sternum | 2 |
Pulmonary contusion (severe) | 4 |
Perforated trachea | 4 |
Abdominal Injuries
Injury | Typical AIS |
Retroperitoneal hematoma | 2 |
Splenic laceration (moderate) | 3 |
Ruptured liver with tissue loss | 5 |
📌 Clinical Pearl: AIS reflects anatomical damage, not physiology (vital signs). Hypotension does not increase AIS unless anatomical severity is higher.
Injury Severity Score (ISS)
Definition
The Injury Severity Score (ISS) quantifies overall trauma severity by combining the most severe injuries across different body regions.
How ISS Is Calculated
Identify the highest AIS score in each body region
Select the three most severely injured regions
Square each AIS score
Add them together:
ISS = (AIS_1)^2 + (AIS_2)^2 + (AIS_3)^2
Special Rule
Any AIS 6 injury → ISS = 75 (maximum score)
Example Calculation
A trauma patient has:
Head injury: AIS 4
Chest injury: AIS 3
Abdominal injury: AIS 2
ISS = 4^2 + 3^2 + 2^2 = 16 + 9 + 4 = 29
📌 Interpretation:ISS ≥ 16 = Major trauma
Clinical Significance of ISS
ISS Range | Trauma Severity | Clinical Meaning |
1–8 | Minor | Usually survivable, limited intervention |
9–15 | Moderate | Requires hospitalization |
≥16 | Severe (Major Trauma) | High mortality risk |
50–75 | Critical | Extremely high mortality |
AIS vs ISS: Key Differences
Feature | AIS | ISS |
Unit of scoring | Individual injury | Whole patient |
Based on | Anatomy | Combined anatomy |
Scale | 1–6 | 1–75 |
Predicts mortality | Indirectly | Strongly |
Used in | Coding, research | Triage, outcomes |
Limitations (Exam-Relevant)
❌ Does not consider:
Age
Physiologic parameters (BP, GCS)
Comorbidities
Multiple injuries in same body region (only the highest counts)
📌 This is why ISS is often combined with physiologic scores (e.g., TRISS, GCS) in real trauma systems.
High-Yield
✅ AIS is anatomical, not physiological
✅ ISS uses only one injury per body region
✅ ISS ≥ 16 = major trauma
✅ Any AIS 6 → ISS = 75 automatically
✅ Widely used in trauma registries and outcome prediction
Summary
The Abbreviated Injury Scale (AIS) provides a standardized method to grade individual injuries, while the Injury Severity Score (ISS) integrates the most severe injuries across body regions to estimate overall trauma severity and mortality risk. Mastery of AIS and ISS is essential for trauma assessment, exam success, and real-world clinical decision-making.