Understanding Injury Severity Scores: How Medical Teams Measure Trauma

What Does an Injury Severity Score Measure?

It tells you how severe someone's injuries are based on documented injuries. This is especially useful when a patient has injuries in multiple areas of their body.

Each injury will be scored on an Abbreviated Injury Scale (AIS) before determining the Injury Severity Score. This scale rates how severe the injury is to that area of the body. There is a rating of 1 to 6. 1 is the least severe, and 6 is a maximal injury that is classified as currently untreatable.

As you can see the AIS only rates the injuries themselves. The Injury Severity Score takes a few of those scores and determines how severe the patient's injuries are as a whole. Neither of these are pain scales. You can be in a lot of pain from an injury and still have a lower AIS score.

Documentation is important after an accident, but does not determine fault. Minnesota uses comparative fault when it comes to dividing fault in an accident. If you want to learn more about how your documented injuries may impact your Minneapolis personal injury claim, speak with the team at Meshbesher & Spence.

How Is the ISS Calculated?

First, injuries are assigned to these body regions:

• Head & Neck
• Facial
• Thorax
• Abdomen, pelvic contents
• Upper & Lower Extremity, pelvic girdle
• External, skin, burns

Take the highest AIS score for each body region. Then square and add the highest scores from three different body regions. If there are less than three regions with injuries leave the blank as a zero.

If a patient had these injuries they would have an Injury Severity Score of 17.

3² + 2² + 2² = 17

Squaring the number puts more importance on more severe injuries. An AIS score of 2 would only add 4 to the Injury Severity Score.

Another injury in the same region does not add points when its AIS rating is lower.

The maximum Injury Severity Score is 75. You would need three AIS 5 injuries in different body regions to get that. If a patient has an injury with an AIS of 6 they will automatically receive a 75 Injury Severity Score.

Doctors will be able to determine an AIS score for each injury based on test results, surgery and other documentation. It is not something you can rate yourself on.

How Should You Read the Final Score?

In general, a higher score suggests more severe anatomical injuries. Some will group them like this:

• 1–8: Less Severe
• 9–15: Moderate
• 16+: Major Trauma

This will vary from source to source and does not tell you what the outcome will be for that patient. Having an Injury Severity Score of 17 does not mean you have a 17% chance of dying. An increase of 3 points will not have the same effect on the Injury Severity Score throughout the entire range.

A patient with the same Injury Severity Score as another could have a completely different prognosis due to their injuries, age and other factors.

That is why doctors will look at the patient's injuries and overall health instead of just relying on the Injury Severity Score.

When Do Trauma Teams Use ISS?

When a patient comes into a hospital for traumatic injuries they need to be treated for what is happening in that moment. Doctors will assess if the patient is breathing, has a pulse, bleeding, consciousness etc. They are not going to wait until they have an Injury Severity Score to treat someone for a life threatening injury.

Once all of the patient's injuries are identified they can calculate the Injury Severity Score. That takes time and usually will not happen in the first few minutes of an emergency visit.

There could also be injuries found during surgery or testing that weren't detected when the patient came in initially. Having all the documentation available will allow doctors to better code the patients injuries.

Hospitals will use this to compare patient data that may have similar injury profiles. It is also used by researchers when studying mortality rates and other data.

The Glasgow Coma Scale looks at a patient's eyes, verbal response and movement to determine how conscious they are. Doctors will also look at vital signs to see how the body is functioning. All of these will give more information about the patient than just their anatomy.

What Does an ISS Leave Out?

While the formula can be helpful, it leaves out some things.

Only one score per body region is used to calculate the Injury Severity Score. Other serious chest injuries may not increase the score.

The Injury Severity Score does not take into account the patient's age, blood pressure, preexisting conditions or how they respond to treatment. It also does not factor in how someone will do at their job, family life or day to day activities.

It cannot predict time away from work.

If someone broke their hand they may not be able to go back to their job that requires fine motor skills. The long term effects can be devastating with a lower Injury Severity Score.

A September 2026 study came up with a new prediction model using 4,521,790 patients from the ACS TQIP from 2017 to 2021. In separate validation data, the model more accurately distinguished in-hospital deaths than the Injury Severity Score.

Researchers recommended further evaluation, but it isn't yet known whether it will replace the Injury Severity Score.

How Does ISS Relate to an Accident Claim?

Knowing your Injury Severity Score can be beneficial when reading through medical documents after an accident. It can also help when your case is being evaluated by an insurance company or in your personal injury claim.

However, your claim will also look at who was at fault, how much treatment you will need in the future and how your life has changed since your accident.

Under Minnesota's comparative fault rule, damages are reduced by the claimant's share of fault. Recovery is generally barred if that share exceeds the defendant's fault.

You may also be able to prove that your injuries affected your ability to do your job, enjoy hobbies and other things with additional documentation.

If your Injury Severity Score does not seem to reflect your current situation, speak with your doctor about what injuries were included in that calculation.

Wrapping It Up