The HL7 FHIR standard makes use of extensions to keep the standard simple. An extension allows for the inclusion of data that does not fit into the standard’s defined data elements. Within a given resource, extensions can be used to define new elements or they can be used to extend the data included in a defined element.
Extensions are defined on a server that should be available to anyone who will be receiving resources with the extension. Extensions are discovered using a URL. To add an extension to a resource, the following format would be utilized:
Extensions will be used widely in HL7 FHIR in an effort to stay focused on common workflows and reduce the complexity of the core standard. Workflows that are unique in nature would employ extensions to satisfy the requirements of the workflow. HL7 says that there can be no stigma associated with extensions and encourages all users to embrace them so the standard can remain simple while addressing all workflows.
For example, in the United States it is common for medical records to include religion; however, this is not the practice worldwide. The FHIR standard, which is a worldwide standard, will not include religion. The United States can, however, have its own extension for religion. A religion extension for FHIR might look like this:
The extension itself could be served up by a United States organization, such as the ONC or HL7. Anyone could reference the extension through the URL and identify the proper coded value for the patient’s religion in the patient resource.