MHD-rev2-vol-3

HL7/FHIR Mapping Overview

Many of the Document Sharing concepts defined in IHE profiles are directly supported by FHIR. FHIR has the concept of a 80/20 rule where they are willing to customize FHIR to support 80% of the features and leave the last 20% to use the defined FHIR extension mechanisms. IHE Document Sharing defines some attributes using the extension mechanism.

FHIR Servers

There is also a difference in architecture between IHE Document Sharing and HL7 FHIR. Looking at XDS there are two actors that would be typically thought of as servers, the Document Repository and Document Registry. In FHIR these would be considered two FHIR servers which are customized to maintain different content and provide different processing. This is not an issue for the MHD profile since MHD focuses on extensions to the Document Source and Document Consumer areas of the profile but it may be important later.

Resource Extensions

There is a difference in the handling of certain object types. In IHE Document Sharing the concepts of author and patient are defined as a small collection of attributes to the DocumentEntry, SubmissionSet, and Folder objects. In FHIR these concepts are defined by the Practitioner and Patient resource types. These objects are intended to be created and stored on a server before a DocumentEntry equivalent object is submitted. So where IHE Document Sharing intends these to be atomically contained within the DocumentEntry, SubmissionSet, and Folder objects FHIR builds a finer mesh of smaller objects and depends on references instead of inclusion.

To accommodate the more composed style of IHE Document Sharing objects, FHIR allows normally separate, referenced objects to be included inside other objects. This adapts the FHIR style of referencing to the IHE Document Sharing style of composition. This inclusion of objects use the FHIR contains mechanism. A contains element can be used to package a normally referenced object inside the referencing object. The handling of the contained content at the receiving server is beyond the scope of the FHIR specification.

References to other objects

IHE Document Sharing, originally defined on the OASIS ebXML Registry standard, uses independent Association objects to link other objects. The Association objects contain a from-reference a to-reference and a type indicating the purpose of the linkage. Associations are first class objects; they can contain other metadata attributes to refine the meaning of the references they provide.

FHIR references are not built from independent objects but from link attributes defined within the linked object. These references are single direction, an object containing a reference to another object.

Major object types

The major objects defined by IHE Document Sharing map to FHIR resources as follows.

Document Sharing Object	FHIR Resource
DocumentEntry	DocumentReference
SubmissionSet	DocumentManifest
Folder	List
Association	Embedded reference