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CDA Documents may reference information contained in other documents. While CDA Release 2.0 supports references in content via the <linkHtml> element, this is insufficient for many EMR systems as the link is assumed to be accessible via a URL, which is often not the case. In order to link an external reference, one needs the document identifier, and access to the clinical system wherein the document resides. For a variety of reasons, it is desirable to refer to the document by its identity, rather than by linking through a URL.
- The identity of a document does not change, but the URLs used to access it may vary depending upon location, implementation, or other factors.
- Referencing clinical documents by identity does not impose any implementation specific constraints on the mechanism used to resolve these references, allowing the content to be implementation neutral. For example, in the context of an XDS Affinity domain the clinical system used to access documents would be an XDS Registry and one or more XDS Repositories where documents are stored. In other contexts, access might be through a Clincial Data Repository (CDR), or Document Content Management System (DCMS). Each of these may have different mechanisms to resolve a document identifier to the document resource.
- The identity of a document is known before the document is published (e.g., in an XDS Repository, Clincial Data Repository, or Document Content Management System), but its URL is often not known. Using the document identity allows references to existing documents to be created before those documents have been published to a URL. This is important to document creators, as it does not impose workflow restrictions on how links are created during the authoring process.
Fortunately, CDA Release 2.0 also provides a mechanism to refer to external documents in an entry, as shown below.
<entry> <act classCode='ACT' moodCode='EVN'> <templateId root='184.108.40.206.4.1.193220.127.116.11.1.4.4'/> <id root='' extension=''/> <code nullFlavor='NA' /> <text><reference value='#study-1'/></text> <!-- For CDA --> <reference typeCode='REFR|SPRT'> <externalDocument classCode='DOC' moodCode='EVN'> <id extension='' root=''/> <text><reference value='http://foo..'/></text> </externalDocument> </reference> <!-- For HL7 Version 3 Messages <sourceOf typeCode='REFR|SPRT'> <act classCode='DOC' moodCode='EVN'> <id extension='' root=''/> <text><reference value='http://foo…'</text> </act> </sourceOf> --> </act> </entry>
<act classCode='ACT' moodCode='EVN'>
The external reference is an act that refers to documentation of an <act> (classCode='ACT'), that previously occurred (moodCode='EVN').
The <templateId> element identifies this <act> as a reference act, allowing for validation of the content. As a side effect, readers of the CDA can quickly locate and identify reference acts. The templateId must have root='18.104.22.168.4.1.19322.214.171.124.126.96.36.199.4'.
<id root=' ' extension=' '/>
The reference is an act of itself, and must be uniquely identified. If there is no explicit identifier for this act in the source EMR system, a GUID may be used for the root attribute, and the extension may be omitted. Although HL7 allows for multiple identifiers, this profile requires that one and only one be used.
The reference act has no code associated with it.
In order to connect this external reference to the narrative text which it refers, the value of the <reference> element in the <text> element is a URI to an element in the CDA narrative of this document.
<externalDocument classCode='DOC' moodCode='EVN'>
External references are listed as either supporting documentation (typeCode='SPRT') or simply reference material (typeCode='REFR') for the reader. If this distinction is not supported by the source EMR system, the value of typeCode should be REFR. For CDA, the reference is indicated by a <reference> element containing an <externalDocument> element which documents (classCode='DOC') the event (moodCode='EVN'). For HL7 Version 3 Messages, the reference is represented with the element <sourceOf> and the external document is representated with a <act> element, however semantics, and attributes remain otherwise without change.
<id extension=' ' root=' '/>
The identifier of the document is supplied in the <id> element.
<text><reference value=' '/></text>
A link to the original document may be provided here. This shall be a URL where the referenced document can be located. For CDA, the link should also be present in the narrative inside the CDA Narrative in a <linkHTML> element.