What Is Severity of Illness?

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  • Written By: Mary McMahon
  • Edited By: Shereen Skola
  • Last Modified Date: 05 November 2018
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Severity of Illness (SOI) is an assessment of the acuity of a patient’s illness, based on functional measures like symptoms and damage to organ systems. This can help with health care planning decisions determining treatment options and can also assist with risk assessments, where care providers may consider how well a patient would respond to a given treatment, if at all. Hospitals may be required to maintain statistics on this and other metrics for the purpose of measuring performance and determining the amount of insurance and government assistance payouts.

A survey form can be used to collect information about severity of illness. Some facilities use a standardized measure developed by researchers while others may devise their own. On the form, medical providers note the specifics of the patient’s condition and rate factors relating to the illness. They can look at whether it is treatable or not, the intensity of the symptoms, and how many organ systems or tissues are involved.


Using this information, it is possible to assign a score which can be used to place the patient in a minor, moderate, major, or extreme category. Someone otherwise healthy who reports to the doctor with a cough, for instance, probably has a minor severity of illness and will respond well to treatment. Conversely, a car accident victim with multiple crush injuries to the legs and severe brain damage has an extreme severity of illness. The risk of mortality increases with higher scores, because patients are more at risk of complications.

Some hospitals and providers may make triage decisions on the basis of severity of illness. Resources tend to be allocated to people who are more likely to survive, especially in shortages. At the scene of an accident with numerous victims, for instance, people in the severe category may not be prioritized because they would consume a high volume of resources and might not recover. Calculated decisions of this nature can be difficult to make, but may improve overall outcomes.

In comparisons between facilities to look at outcomes and performance, it is important to account for the case mix index. A facility that sees primarily critically ill patients transferred from other hospitals, for example, might have a very high mortality rate when compared to another location that mostly handles people with moderate to major illness. Accounting for the case mix, looking at how many patients fall into each category, might show that the high-mortality facility actually has better overall outcome statistics because of the high quality of care available.



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