The injury severity score is assigned using a publicly-available Stata program for Injury Classification (ICDPIC) which uses ICD-9-CM diagnosis codes to assign a severity score (ranging from 1 to 75 with 75 being the most severe). The Stata program is available at http://ideas.repec.org/c/boc/bocode/s457028.html.

The Injury Severity Score (ISS) retained in the HCUP data element INJURY_SEVERITY is based on an anatomical scoring system that provides an overall score for patients with multiple injuries. Each injury is assigned an Abbreviated Injury Scale (AIS) score, allocated to one of six body regions [Head, Face, Chest, Abdomen, Extremities (including pelvis), and External]. Only the highest AIS score in each body region is used. The three most severely injured body regions have their score squared and added together to produce the ISS score. The ISS correlates linearly with mortality, morbidity, hospital stay and other measures of severity. More information on the ISS is available at https://aci.health.nsw.gov.au/get-involved/institute-of-trauma-and-injury-management/data/injury-scoring/injury_severity_score.

In the range of 800-999 for ICD-9CM injury diagnoses, ICDPIC does not calculate an injury severity score for late effects of injuries (diagnoses 905-909), effects of foreign body (930-939), burns (940-949), certain early complications of trauma (958), poisoning by drugs, toxic and other effects (960-995). It should be noted that the range of injury codes scored by ICDPIC is narrower than the range used to assign the HCUP injury flags (INJURY and MULTINJURY); therefore, some records identified as having an injury may not have an injury severity score.