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We have a query about Patient’s allergies that are being recording in different clinical systems

Query: Few clinical systems manage allergies with three broad categories – Allergy, Intolerance and Adverse reaction. Can adverse reaction be a category on its own? If so can we understand with a real time example?

While going through FHIR standard’s definition of Allergy resource, they categorize allergies with two types - Allergy and intolerance.

As per FHIR references, as extracted from the below link

https://www.hl7.org/fhir/allergyintolerance.html

1. A record of a clinical assessment of an allergy or intolerance; a
   propensity, or a potential risk to an individual, to have an adverse
   reaction on future exposure to the specified substance, or class of
   substance.


2. 
   

   Adverse reactions may be:

   
   
   

     An allergy (typically type I hypersensitivity, plus other "allergy- 
   
     like" reactions, including pseudoallergy).
   
     An intolerance (typically non-immune adverse reactions that are not 
   
     determined or perceived to be allergic or "allergy-like", and are 
   
     to some degree idiosyncratic and/or individually specific [i.e. are 
   
     not a reaction that is expected to occur with most or all patients 
   
     given similar circumstances])
   
   

   
   

From the above citations we can arrive as Allergy can only be categorized by two types – Allergy and Intolerance. Any reaction for an allergy/ intolerance is an adverse reaction.

Is it clinically valid to classify allergy with two categories? ‘Adverse reaction’ need not be a category on its own?

The notion of AllergyIntolerance is that there's an increased risk of something bad happening to the patient if they're exposed to a particular substance. Having had an adverse reaction in the past when exposed to a substance is a reason for asserting an increased risk. However, that prior adverse reaction will either have been an allergic reaction or it will be "something else" - and all "something else" reactions are considered to be intolerances. If, rather than capturing a situation of "increased risk" you want to instead track particular individual reactions to try to determine causality, suggest looking at the AdverseEvent resource which focuses on that. In practice, you might well have both - an AllergyIntolerance to ensure there's a flag noting the potential risk to the patient that will trigger decision support when prescribing, etc. plus an AdverseEvent resource to handle the reporting/analysis associated with a particular reaction.

This is an important issue to understand because of the prominent place allergy and adverse reactions play in clinical care.

The two categories "allergy" and "intolerance" are historical, longstanding. Most non-clinicians and even the majority of clinicians have little insight into the mechanisms that underly allergic reactions, and tend to classify all reactions to which a particular individual has a propensity to from a substance as "allergy". The advancement of knowledge over the past few decades has led to an understanding that there are a number of non-immunologic mechanisms for "allergic reactions" some of which appear much like an Type I hypersensitivity reaction (anaphylaxis or hay fever) and some of which do not. The historical existence of only two categories ensconced in legacy standards, leaves only the category=intolerance for everything known to be non-immunologic. The important thing overall is that an individual has been determined to have the attribute of sensitivity to a particular substance (or class).

Many adverse reactions are neither allergy nor intolerance: drug-drug interactions that result in a physiologic reaction, overdose, surgical mishaps, radiation burns, transfusion reactions, etc. For that matter, if an individual has an allergic or intolerance reaction and has never had one before, they have an adverse reaction but they don't have "an allergy" until after that reaction and someone makes a judgement that they are likely to have future reactions if exposed to that substance.