Identifiers are used throughout CDA documents to uniquely identify people, places, and things.

Some identifiers are created by HL7 for use in CDA documents and related standards. These identifiers generally use OIDs. Examples of such identifiers include:

  • Every CDA template has an OID identifier in an element named templateId.
  • The CDA Header contains two identifiers for the overall document, in elements named typeId and id.
  • An XML element named setId can appear in the CDA Header or in parts of a CDA document that reference external documents to identify a set of documents or document versions.

Other identifiers come from external systems and are stored in CDA for reference. These identifiers are generally represented in CDA in an XML element named id.

Codes from code systems are also a kind of identifier in CDA, but they are handled differently from an XML syntax standpoint in CDA – and are not covered in this article.

Identifier Syntax
Whatever the identifier XML element name (templateId, setId, typeId, or id) it follows the same XML syntax which involves the following four attributes:

  • root: A required unique identifier
  • extension: An optional additional identifier that only has meaning within the scope of the root identifier
  • assigningAuthorityName: An optional human-readable name of the agency or institution that assigned the identifier (only relevant if root contains the identifier of an “assigning authority”.
  • displayable: A rarely-used attribute that is set to “true” if the identifier is recommended for display to human readers and “false” if it is a code intended only for machine processing (like a bar code).

Root and Extension
The root attribute is always present. In the simple case, it contains the unique identifier – such as an OID or GUID – and is the only attribute used.

In other  cases, the root attribute identifies a class of identifiers. For example, it may contain the OID representing an “assigning authority” – such as a government body or company – that issues identifiers. In that case, the extension attribute stores the actual identifier within the class of identifiers represented by root.

For example, root might contain an OID representing the assigning authority for US Social Security Numbers. An individual social security number would then go into extension.
When the extension attribute is used, it is always used alongside the root attribute. The unique identifier is then the combination of the two attributes.

Multiple Identifiers
In most (but not all) places in a CDA document where an identifier XML element is used, it’s permissible to have multiple instances of the XML element to capture multiple identifiers for the same person, place, or thing.

Here is an example of an identifier XML element (templateId) that holds an OID identifier:

Here is an example of an identifier XML element (id) that uses an OID in the root attribute to represent an “assigning authority” and the actual identifier issued by that assigning authority in the extension attribute:

Here is an example of an identifier XML element (id) that contains a GUID:

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