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Vocabulary Work GroupMaturity Level: 5Ballot Status: Trial Use

The Units of Measure Coding System (UCUM) is recommended for use with the Quantity data type.

SourceUCUM is made available by the Regenstrief Institute, Inc and The UCUM Organization at http://unitsofmeasure.org
SystemThe URI http://unitsofmeasure.org identifies UCUM codes
VersionWhere a version is used, it should be the standard UCUM version e.g. 1.9. There is no need to use version in the Coding data type, only in Value sets that use UCUM codes
CodeValid expressions using the case sensitive symbols
DisplayThere is no defined display; the UCUM code is used directly for the display
InactiveA few codes in UCUM are marked as deprecated (e.g. ppb, pptr)
SubsumptionNo Subsumption relationships are defined by UCUM
Filter PropertiesTwo filter properties are defined as described below

UCUM is Copyright © 1999-2013 Regenstrief Institute, Inc. and The UCUM Organization, Indianapolis, IN. All rights reserved. See TermsOfUse for details.

UCUM is inherently a compositional code system; almost all UCUM codes are expressions composed using the UCUM expression syntax. Many servers, however, do not implement full support for the syntax, and rely on a large library of pre-built valid UCUM codes (FHIR includes a common UCUM codes value set for this purpose).

UCUM us always documented as a compositional code system. If a server does not support the full grammar, it should document that in its Terminology Capabilities Statement.

This section documents the property filters that can be used with the UCUM code system in value set composition statements.

DescriptionRestricts the expression to a describe a particular UCUM base property
Property Nameproperty
Operations Allowed=
Values Allowed[string name of property]
CommentsRestricts expressions to any expression that is comparable to a base unit with a matching property value. Note: this does not match the property on units, only base units
DescriptionAllows for any expression that is comparable to the named unit
Property Namecanonical
Operations Allowed= / in
Values AllowedUCUM expression
CommentsThis allows any expression that is comparable to the given unit(s)

Implicit value sets are those whose specification can be predicted based on the grammar of the underlying code system, and the known structure of the URL that refers to them. There is one set of implicit value sets defined for UCUM: By Canonical Value.

If any value set resources exist with an identifier that conforms to the URL patterns specified below, the content of the resource must conform to the template provided. Profiles and other value set references are allowed to reference these value sets directly.

The value set identifier http://unitsofmeasure.org/vs is a value set that contains all UCUM codes.

A value set with an identifier of "http://unitsofmeasure.org/vs/[expression]" must conform to this template, where [expression] is a valid UCUM expression:

<ValueSet xmlns="http://hl7.org/fhir">
  <text>
    <status value="generated"/>
    <div xmlns="http://www.w3.org/1999/xhtml">
      [some html that identifies that this value set 
      includes all UCUM expressions that are comparable to the provided expression]
    </div>
  </text>
  <identifier value="http://unitsofmeasure.org/vs/[expression]"/>
  <version value="[optional - but recommended - UCUM version]"/>
  <name value="Ucum Expressions comparable to [expression]"/>
  <description value="Ucum Expressions comparable to [expression]"/>
  <status value="active"/>
  <date value="[optional date of UCUM release]"/>
  <compose>
    <include>
      <system value="http://unitsofmeasure.org"/>
      <filter>
        <property value="comparable"/>
        <op value="="/>
        <value value="[expression]"/>
      </filter>
    </include>
  </compose>
</ValueSet>