The Injury Severity Score (ISS) is an anatomical summary score based on the Abbreviated Injury Scale (AIS) that describes overall injury burden in patients with multiple injuries. It is calculated once formal AIS coding is available, not during initial resuscitation or prehospital triage.

The score was introduced by Baker and colleagues in 1974 using hospital and medical-examiner data for more than 2,000 injured persons. Higher ISS values correlated strongly with mortality, allowing comparison of death rates and evaluation of trauma care across hospitals and systems.

ISS is now widely used in national and regional trauma registries to:

describe case-mix and anatomical injury severity across six body regions,
support risk adjustment and benchmarking of mortality, ICU admission and resource use, and
define "+major trauma+" for audit and quality indicators, commonly at ISS ≥16, although local thresholds may vary.

Many registries group ISS into broad bands to summarise risk. The bands used in this calculator (1–8 minor, 9–15 moderate, 16–24 severe/major, ≥25 very severe) are similar to groupings used in several state-level trauma data systems. Example data from a large Korean regional trauma centre show a steep mortality gradient across these bands:

ISS band	Description	Example in-hospital mortality*
1–8	Minor injury	~1%
9–15	Moderate injury	~3%
16–24	Severe/major trauma	~12%
25–40	Very severe trauma	~26%
41–75	Extremely severe trauma	~48%

*Approximate figures from a single Korean regional trauma centre over five years; event rates will vary between systems.

Limitations and cautions:

ISS only counts the worst injury in each of three body regions; patients with multiple severe injuries in one region (eg complex head, thoracic or limb trauma) may have their overall anatomical severity underestimated.
Each body region is weighted equally, even though injuries in some regions carry higher mortality risk than others, prompting development of alternatives such as the New ISS, TRISS, ICISS and weighted ISS.
Performance and useful thresholds vary across subgroups, including isolated head injury, major burns, penetrating trauma and paediatric trauma, so ISS should be interpreted alongside physiology, mechanism and comorbidity.
Changes between AIS versions (eg AIS98, AIS08, AIS15) can shift which ISS threshold best approximates "+major trauma+"; many registries continue to use ISS ≥16 for consistency but may adopt adjusted cut-points in AIS08/15 data.

For Australian practice, ISS is primarily a trauma-registry and audit tool. Use it to understand injury patterns and outcomes, not as a stand-alone triage rule in the resuscitation bay or prehospital setting.