What kind of thing the recorded allergen is: a single drug, a drug class, a food, an environmental
substance or a vaccine.
It determines category, and says the thing category cannot. Every value here maps onto
exactly one of R4's four AllergyIntolerance.category values, and two of them -- drug and
class -- map onto the same one. That collision is the point: a reaction recorded against one
medicinal substance and a reaction recorded against a whole family of them are both medication,
and a cross-sensitivity check against a new prescription needs to know which it is reading.
It is a fact about the record, not a translation of the substance.AllergyIntolerance.code carries WHAT
the patient reacted to, and every coding in it says that same thing in a different vocabulary. This
says WHAT KIND, which is a different and coarser statement, so it rides an element of its own.
At most one per record. A record whose substance carries several codings still describes one
allergen and therefore one class.
MIGRATION. Write this extension. Records written before it exists carry the class as an
additional coding inside AllergyIntolerance.code.coding, under the same code system; that
carriage is not part of this specification and is not produced going forward. A reader of older
records has to read it from where it was written, and must not mistake it for a translation of the
substance while doing so. Moving the existing data is outstanding work and is not described here.
May be used to represent additional information that is not part of the basic definition of the element. To make the use of extensions safe and manageable, there is a strict set of governance applied to the definition and use of extensions. Though any implementer can define an extension, there is a set of requirements that SHALL be met as part of the definition of the extension.
Comments
There can be no stigma associated with the use of extensions by any application, project, or standard - regardless of the institution or jurisdiction that uses or defines the extensions. The use of extensions is what allows the FHIR specification to retain a core level of simplicity for everyone.
Source of the definition for the extension code - a logical name or a URL.
Comments
The definition may point directly to a computable or human-readable definition of the extensibility codes, or it may be a logical URI as declared in some other specification. The definition SHALL be a URI for the Structure Definition defining the extension.
The allergen's kind, from the allergen class vocabulary. A required binding: the set is closed and complete -- it is the five kinds this estate's allergen picker assigns -- so a value outside it is not an extension of the concept but a different one, and belongs in a different element.