A second coding for one member of a pre-expanded ValueSet, carried on that member beside its own
system and code. Repeats: a member with two secondary codings carries two.
Elements / Details
Every element this profile touches, with its full definition. Element names in the tables link here.
A second coding for one member of a pre-expanded ValueSet, carried on that member beside its own
system and code. Repeats: a member with two secondary codings carries two.
SUPERSEDED BY A PUBLISHED EXTENSION. Write
http://hl7.org/fhir/5.0/StructureDefinition/extension-ValueSet.expansion.contains.property
instead. R4's ValueSet.expansion.contains has no element for a per-member property, which is why
this extension was minted. R5 adds expansion.contains.property -- and HL7 publishes that element
back to R4 as an extension, ACTIVE, fhirVersion 4.0.1, contexted on
ValueSet.expansion.contains, in hl7.fhir.uv.xver-r5.r4. It was already available when this one
was written.
It is a strict superset, and the difference is the part that matters. The published extension
carries a code child naming WHAT THE PROPERTY IS, beside a value child that admits code,
Coding, string, integer or boolean -- and a subProperty for nested pairs. This extension
has only a value, so what the value MEANS has to be inferred from valueCoding.system.
That inference fails on a case this guide already has. The allergen picker's list annotates each
member with an allergen class, and the problem picker's list annotates each member with a
classification edition. Both are axes over expansion members; the moment two axes are drawn from ONE
vocabulary, system no longer tells them apart and there is nowhere to say which is which. The
published extension names the axis explicitly and does not have that failure mode.
ADOPTING IT NEEDS A DEPENDENCY.hl7.fhir.uv.xver-r5.r4 is not currently pinned by this guide,
and it must be declared before a validator can resolve the canonical. That declaration belongs with
the change that moves the writer, not ahead of it -- the package carries over 1,400
StructureDefinitions and pinning it for an extension nothing yet writes would add weight for no
reader.
What it buys. These lists ship pre-expanded and are filtered in the client, so the second code
travels with the member the user picks. The picker can then emit a complete CodeableConcept --
the member's own coding, followed by each secondary coding -- rather than resolving a map after the
fact. The allergen class rides this way on the allergen
picker's list, so an entry knows what kind of allergen it is at the moment it is recorded; the
problem picker carries a classification coding the same way, the edition named by that coding's own
system.
Read it by the value's system -- and this is the limitation that supersedes it. The extension
URL says only that a second code is present; what the code IS comes from valueCoding.system.
Repeats are in no guaranteed order and a member may carry more than one, so infer nothing from
position either. Two axes drawn from one vocabulary are indistinguishable under this rule, which is
exactly what the published extension's code child exists to state.
It annotates a member; it does not decide membership. A concept is in the set because
compose includes it. This adds a code to a member and never adds or removes one.
MIGRATION. Nexus EMR writes this extension today at
https://fhir.nexus.health/StructureDefinition/valueset-secondary-coding (FHIR-374): it is live in
seeded expansions and in code, and is under migration to the canonical declared here. That address
shares the final segment valueset-secondary-coding with the canonical, so matching that segment
reads both -- but it does NOT read the second address below.
A SECOND PRE-MIGRATION ADDRESS, which no suffix rule reaches. The earliest form of this
extension was spelled for one classification and is written at
https://fhir.nexus.health/StructureDefinition/valueset-secondary-icd10 (FHIR-374). It is the same
extension under a narrower name -- the same value, the same reading rule, and the classification it
carries is named by valueCoding.system rather than by the URL -- so this guide declares one
canonical for both rather than a second artifact for the special case. It is the address the seeded
problem picker's classification codings are filed under today. It has no segment in common
with the canonical and must be matched exactly; a reader built on the suffix alone misses every
member filed under it. Write the canonical, and neither of these.
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.
A second coding for this expansion member. system states which vocabulary the code is drawn from and is the only thing that identifies what the code means; it is not constrained here, because a curated list chooses the vocabulary its members are annotated with.