As if Math Wasn’t Traumatic Enough… Calculating Injury Severity Scores

How many times have you been in the ER when a trauma patient comes in and you take a pen and paper and begin to calculate an injury severity score?

I know what you’re thinking… #aintnobodygottimeforthat… especially in a busy County ER with multiple traumas presenting simultaneously, but hear me out:

The Injury Severity Score (ISS) has been described in the literature as a good predictor of mortality in trauma patients; in fact, it’s often quoted as the only anatomical scoring system that linearly correlates with mortality. It also correlates with morbidity and hospital length of stay. ISS can be used to triage trauma patients, provide quality measurement of care for trauma patients and it can serve as an international standardization technique in research. Even if you don’t use it regularly in the ED, you need to understand it to critically read/analyze trauma literature.

Before I continue, I’ll point out that there are many versions of the injury severity score and various injury severity scales… Okay, take a deep breath:

AIS (Abbreviated Injury Scale), MAIS, (Maximum Abbreviated Injury Score), ISS (Injury Severity Score), NISS (New Injury Severity Score), TRISS (Trauma and Injury Severity Score), AP (Anatomic Profile), ASCOT (A Severity Characterization of Trauma measure), ICISS (International Classification of Diseases Injury Severity Score), RTS (Revised Trauma Score), IIS (Injury Impairment Scale), FCI (Functional Capacity Index), TMPM (Trauma Mortality Prediction Model).

I will mention the following three: AIS, ISS and NISS. Why? Because ISS is the most widely used injury scoring system, you need the AIS to calculate it and NISS is a tweaked version which has been shown to be a more accurate assessment tool.

The abbreviated injury scale (AIS) is an anatomically based scoring system in which regions of the body are scored from 1-6 in terms of “threat to life,” 1 being minor and 6 being unsurvivable. This scoring system was first described in 1969 by a group of 75 specialists from around the world, and has since then been updated, with the last update in 2008. It relies on subjective expert opinion.

Abbreviated Injury Score:

1 Minor Example, superficial laceration
2 Moderate Example, fractured sternum
3 Serious (not life-threatening) Example, open fracture of humerus
4 Severe (life-threatening) Example, perforated trachea
5 Critical (uncertain survival) Example, ruptured liver with tissue loss
6 Maximal (survival not likely) Example, Total severance of aorta

 

The injury severity score (ISS), described by Barker et al. in 1974, looks at the AIS of six regions: head and neck, face, chest, abdomen, extremity (including the pelvic girdle), and external. The sum of the squares of the highest AIS score in 3 of these regions determines the ISS score (A2+B2+C2 = ISS, where A, B and C are the highest scoring AIS injuries in 3 different regions of the body assessed). This summing of the squares was found to have a far better mortality prediction then AIS alone. The scores range from 1 to 75, any body region with an AIS of 6 (maximal, unsurvivable) automatically puts the ISS at 75.

Let’s look at an example of a 30 year-old male s/p assault:

Region Injury AIS
Head and Neck Cerebral contusion 3
Face Superficial laceration 1
Chest 1 rib fracture 2
Abdomen Grade IV Liver Laceration

Grade IV Kidney Laceration

4

 

4

Extremity/Pelvis No injury 0
External No injury 0

ISS = 42+32+22 = 29

Does anyone see a problem with this? The grade IV liver laceration and grade IV kidney laceration count as “one” because the maximal AIS of the Abdomen is 4, thereby implying that a grade IV kidney laceration is equal to a rib fracture in severity.

The problem many saw with this scoring system was exactly this: that it doesn’t take into account multiple severe injuries within the same region and it gives equal weight across all body regions. So, the NISS was developed.

The new injury severity scale (NISS) is basically the same as the ISS except that each injury counts as its own AIS score in the ISS grading, you can then take the top 3 injuries (regardless of body region) and sum the squares.

Looking at the same example as we used, the NISS = 42+42+32 = 41

Let’s look at how that correlates with mortality %:

ISS Mortality %

Age <49

Mortality %

Age 50-69

Mortality %

Age >70

5 0 3 13
10 2 4 15
15 3 5 16
20 6 16 31
25 9 26 44
30 21 42 65
35 31 56 82
40 47 62 92
45 61 67 100
50 75 83 100
55 89 100 100

(Taken from: Parker, S. www.surgical-tutor.org.uk, Accessed October 9, 2014)

In our young trauma patient using the ISS the predicted mortality would be about 21% and using the NISS, the mortality rises to about 47%.

Hope this helps you budding trauma experts in understanding the injury severity score!

 

 

References:

Barker et al. The injury severity score: a method
for describing patients with multiple injuries and evaluating emergency care. J
Trauma. 1974 Mar;14(3):187-96.

Cook et al. A comparison of the Injury Severity Score and the Trauma Mortality Prediction Model. J Trauma Acute Care Surg. 2014 Jan;76(1):47-52.

Lavoie et al. The New Injury Severity Score: a more accurate predictor of in-hospital mortality than the Injury Severity Score. J Trauma. 2004 Jun;56(6):1312-20.

Tohira et al. Systematic review of predictive performance of injury severity scoring tools. Scand J Trauma Resusc Emerg Med. 2012 Sep 10;20:63.

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1 comment for “As if Math Wasn’t Traumatic Enough… Calculating Injury Severity Scores

  1. jkhadpe
    October 16, 2014 at 1:23 pm

    Thanks for the review Sadia! I definitely agree that its important to understand these scoring systems when trying to appraise trauma research but not sure if you sold me on it in its clinical use. Seems common sense that a “life-threatening” injury is going to have a higher mortality rate than “non life-threatening”, etc.

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