How this instrument works
The Injury Severity Score takes the three most severely injured of six defined body regions — head/neck, face, thorax, abdomen, extremities/pelvis, and external — each already scored on the Abbreviated Injury Scale from 1 (minor) to 6 (unsurvivable). ISS squares each of the three scores and adds the results together. A patient scored 5, 4, and 3 across their worst three regions produces 25 + 16 + 9 = 50, not 5 + 4 + 3 = 12: the arithmetic is deliberately punishing toward multiple severe injuries rather than gentle toward them.
Squaring rather than simply summing is the point of the design. A patient with two regions scored 5 loses far more physiologic reserve than a patient with one region scored 5 and one scored 1 — the body isn't managing two independent problems, it's fighting for the same blood pressure and clotting factors twice over. Baker, O'Neill, Haddon, and Long introduced the score in 1974 in the Journal of Trauma, testing how well Abbreviated Injury Scale codes predicted survival across more than two thousand records, and found that squaring and summing the top three regional scores tracked mortality better than any simple additive alternative they tried.
There's also a ceiling rule: an AIS of 6 in any single region — the scale's own definition of a wound not compatible with survival — overrides the arithmetic entirely and sets ISS to its maximum of 75, whatever the other two regions read. Totals run from 0 to 75, and most trauma systems draw the line for major trauma at anything above 15. Worth stating plainly: this isn't built for a patient or a bystander to fill in after an accident. AIS coding requires the separate Abbreviated Injury Scale manual and training most people never receive; ISS itself is a trauma-registry and triage-quality instrument, built for coders and researchers comparing case mix and outcomes across hospitals, not for scoring your own injuries at the roadside.
- Score the single most severely injured body region on the Abbreviated Injury Scale (1–6) and enter it as A.
- Score the second-worst region the same way and enter it as B.
- Score the third-worst region and enter it as C — only the three worst of the six defined regions count; the rest are ignored.
- Read ISS: the sum of the three squares, or automatically 75 if any of A, B, or C is a 6.
Worked example — AIS 5, 4, 3
Three regions score 5, 4, and 3 on the Abbreviated Injury Scale. Square each: 5² = 25, 4² = 16, 3² = 9. Add them: 25 + 16 + 9 = 50 — a very severe, multi-system injury, well past the major-trauma threshold of 15.
Change the worst region from 5 to 6 and the rule changes entirely: an AIS of 6 means unsurvivable by the scale's own definition, so ISS jumps straight to its ceiling of 75 regardless of the other two regions — even 6, 3, 2, which would square and sum to only 49, still scores the full 75. Compare a milder case: regions scoring 4, 3, and 2 square and sum to 16 + 9 + 4 = 29, serious but well below the first example's 50.
Questions
Why does ISS square the numbers instead of just adding them?
Because two severe injuries together tend to cost a patient more than plain addition would suggest. Compare two regions both scored 5 against one region scored 5 and another scored 1: a simple sum gives 10 versus 6 — worse, but not dramatically so. Square each score first and the gap widens: 25+25=50 versus 25+1=26, very nearly double. Squaring amplifies the penalty for a second severe injury far more than a second mild one, which mirrors how a patient with two failing systems tends to fare worse than raw addition implies.
What does an ISS of 50 actually mean for a patient?
It signals a very severe, high-mortality multi-system injury — published mortality curves generally show scores in the 40s and 50s carrying substantial risk of death despite treatment, though the exact figure varies by age, mechanism, and study population. ISS doesn't diagnose or predict any one person's outcome; it's a standardized severity descriptor used to compare case mix and track trauma-center performance across large numbers of patients, not a personal prognosis.
Why does one AIS 6 force the score straight to 75?
Because AIS 6 is defined, by the Abbreviated Injury Scale itself, as an injury essentially incompatible with survival — the coding manual doesn't leave room for a '6 that turns out fine.' Continuing to square and sum in that situation could sometimes land below 75 (a 6, 3, and 2, for instance, squares and sums to only 49), understating the severity. The override simply forces the ceiling whenever that single worst-case injury is present, so the score never undersells a wound the scale itself calls unsurvivable.
Can I calculate my own ISS after an injury?
Not meaningfully without AIS training. The score depends entirely on first assigning an accurate 1–6 severity code to each body region, which requires the separate Abbreviated Injury Scale coding manual and the judgment of someone trained to use it — typically a trauma registrar, surgeon, or emergency physician working from imaging and operative findings. Plugging in guessed severity numbers returns an arithmetically correct total that doesn't reflect anyone's actual condition.
What counts as 'major trauma' on this scale?
Most trauma systems and registries use a score above 15 as the working threshold for major trauma, a cutoff used to benchmark trauma-center performance and set research inclusion criteria rather than to guide any single patient's bedside care. It isn't a hard clinical rule — someone scored just under 15 can still be critically injured, and someone just over it may do fine — but it's the convention most published trauma-outcome studies use to define their case-severity cutoff.
How is AIS different from ISS?
AIS scores one injury in one body region, from 1 (minor) to 6 (unsurvivable); it comes from a separate coding manual maintained by the Association for the Advancement of Automotive Medicine. ISS takes the three worst AIS-scored regions out of six and combines them, by squaring and adding, into a single whole-body severity figure. AIS describes a wound; ISS describes a patient.
References
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.