Injury severity score - ISS (Baker et al 1974)

ISS defined as the sum of the squares of the single highest Abbeviated injury score (AIS)  in each of the three most severely injured body regions.

Six regions (head, face, chest, abdomen, extremities including pelvis, external structures)

Of particular note, the use of only the single most severe AIS injury per body region is used.

ISS ranges from 1 to 75, an ISS of 75 is assigned to anyone with an AIS of 6.


Limitations of ISS:

  •  ISS limits total number of contributing injuries to three regions.

  • Considers at most one injury per body region. Viz multiple severe injuries in one region

  • In patients with injuries in several body regions, the ISS is often constrained to consider a second, perhaps less severe injury in a second body region rather than a second, more severe injury in the first body region. in response to this the NISS was developed.

  • Takes no account of physiological variables

  • Gives equal weight to each body region

New injury severity score - NISS (Osler et al 1997)

New ISS was developed in 1997 as a simple modification of the ISS. The New ISS is defined as the sum of the squares of the AIS of each of the patient’s three most severe AIS injuries, regardless of the body region in which they occur.

An example may make the differences between ISS and NISS scoring more clear.

A patient involved in an RTA sustains a steering wheel compression injury to the abdomen. At laparotomy, a small bowel perforation (AIS score = 3) is first discovered. The ISS is now 9, as is the NISS. Next, a moderate liver laceration is discovered (AIS score = 3). The ISS remains 9, but the NISS increases to 18. Next, a moderate pancreatic laceration with duct involvement is encountered (AIS score = 3). The ISS still remains 9, whereas the NISS increases again to 27. A bladder perforation is next discovered (AIS score = 4). The ISS now increases to 16, whereas the NISS continues its climb to 34. Next, a bimalleolar fibular fracture (AIS score = 2) is discovered. The ISS increases to 20, but the NISS remains unchanged at 34. The NISS thus behaves in a way that is more consistent with a trauma surgeon's instincts than does the ISS: as injuries increase in number, death becomes more likely, even if these injuries are accumulating in a single body region. Furthermore, adding a trivial injury (fibular fracture) to a different body region should not significantly affect the likelihood of death

NISS better predicts survival and is easier to calculate than ISS

Limitations of NISS:

  • More accurate for penetrating injury
  • No account for physiological variables

Baker SP, O’Neill B, Haddon W Jr, Long WB. The injury severity score: a method for describing patients with multiple injuries and evaluating emergency care. J Trauma. 1974; 14: 187–196

Baker SP, O’Neill B. The injury severity score: an update. J Trauma. 1976; 16: 882–885

Osler T, Baker SP, Long W: A modification of the injury severity score that both improves accuracy and simplifies scoring. J Trauma 1997; 43: 922–925 - ISS calculator

Last updated 14/09/2004
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