Sum the squares of the three highest AIS scores from different body regions.
The Injury Severity Score is the sum of squares of the highest AIS (Abbreviated Injury Scale) grades from the three most severely injured body regions. ISS ranges from 1 to 75. An AIS of 6 in any region automatically sets ISS to 75 (unsurvivable). ISS ≥ 16 is considered major trauma. ISS correlates with mortality, morbidity, and hospital stay.