Injury Severity Score (ISS)

The Injury Severity Score (ISS) is the gold standard anatomical scoring system for assessing overall severity of injuries in trauma patients with multiple injured body regions. Developed by Baker et al. in 1974, the ISS is based on the Abbreviated Injury Scale (AIS) and provides a single numerical score that correlates with mortality, morbidity, hospital length of stay, and other measures of trauma severity.

The ISS is calculated by dividing the body into six anatomical regions: head/neck, face, chest (thorax), abdomen/pelvic contents, extremities/pelvic girdle, and external (skin/subcutaneous). For each region, the highest AIS severity code is identified. The AIS grades individual injuries on a scale of 1 (minor) to 6 (unsurvivable).

The ISS is then calculated by taking the three most severely injured body regions, squaring each of their highest AIS scores, and summing these squares. This mathematical approach means the ISS ranges from 1 to 75, with the squaring function giving disproportionate weight to more severe injuries, reflecting the non-linear relationship between injury severity and clinical outcomes.

A special rule applies when any single body region has an AIS score of 6 (unsurvivable): the ISS is automatically assigned as 75, regardless of other injuries. This reflects that a single unsurvivable injury dominates the prognosis.

The ISS has been extensively validated and remains central to trauma system evaluation worldwide. An ISS of 15 or greater is commonly used as the threshold for defining major trauma. ISS correlates linearly with mortality, hospital length of stay, ICU requirements, and long-term functional outcomes.

Despite its widespread use, the ISS has recognized limitations. It accounts for only the worst injury in each body region, potentially underestimating severity in patients with multiple significant injuries within the same region. The New ISS (NISS) uses the three highest AIS scores regardless of body region to address this limitation.

The ISS also does not account for physiological parameters, patient age, or pre-existing conditions. For comprehensive outcome prediction, the ISS is typically combined with the Revised Trauma Score (RTS) in the TRISS methodology, which incorporates both anatomical and physiological variables along with age and injury mechanism.

This calculator implements the standard ISS calculation, accepting the highest AIS score for each of the six body regions and computing the sum of squares of the three highest regional scores with the automatic 75 rule for AIS 6 injuries.