Injury Severity Score

aka ISS

Principal means of predicting trauma outcome

  • Computed from Abbreviated Injury Scale (AIS) scores associated with individual injuries which range from 1 to 6
  • ISS defined as “the sum of the squares of the highest AIS grade in the three most severely affected areas” – because 6 is nearly always fatal the scale therefore goes from 0 to 75

There are six areas in the ISS

  1. Face
  2. Head & neck
  3. Chest
  4. Abdomen & pelvic contents
  5. Bony pelvis & limbs
  6. Body surface

The AIS90 dictionary is used to score every injury & the ISS then calculated

There are several shortcomings:

  • Based on the highest AIS score among the injuries in a particular body region – thus it may underestimate the severity of the condition in the patients with multiple injuries in one or more body regions
  • Diverse injury combinations with distinct survival probabilities can have the same or nearly the same ISS values – e.g. isolated severe head injury AIS=5: ISS=25 given same weighting as abdominal injury (eg liver laceration) AIS=4: ISS=16 + extremity injury (eg open radius fracture) AIS=3: ISS=9
  • ISS gives same weighting to injuries with the same AIS severity in different body regions

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.