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.

FHIR Resources

The FHIR specification documents pre-defined objects that are to be aggregated to construct a health record. These objects are called Resources. The construct of a profile based on FHIR is to define aggregations and extensions to these objects. The primary resources to support document sharing are the DocumentReference and DocumentManifest resources which correspond to the IHE Document Sharing DocumentEntry and SubmissionSet objects.

Traditional FHIR is based on a REST-style transport where each resource is submitted or retrieved separately. This fits poorly with the existing IHE transactions used in document sharing for submitting content. FHIR also defines a composite submission based on the ATOM protocol that is similar to the MTOM protocol used by existing document sharing profiles.

Resource Extensions

FHIR resources can be extended. An extension is a way to add content to a resource beyond its basic definition.

Contained Resources

While the FHIR resources DocumentReference and DocumentManifest map to the IHE definitions of DocumentEntry and SubmissionSet, there are differences in the specification of some attributes. IHE defines patient and author information as attributes of the DocumentEntry and SubmissionSet objects where FHIR defines these as separate resources. FHIR resources are never nested, they are referenced so it is not possible to build up a DocumentEntry in FHIR with the same nesting rules. Instead a DocumentEntry maps to a composite of a DocumentReference resource, zero or more Practitioner resources representing the authors, and a Patient resource holding the patient information. This composite of FHIR resources, equivalent to a DocumentEntry, is constructed using a technique in FHIR called contained resources. This is a special set of rules for imposing a nested style of resource composition in a specification that is based on reference and not composition. This submission style can be used with the ATOM protocol but not the REST protocol.

To accommodate the more composed style of IHE Document Sharing objects, FHIR allows normally separate, referenced objects to be bundled into a FHIR resource in a contained object. In this partial example:

<DocumentReference>
   <contained>
      <Patient>
         ...
      </Patient>
   </contained>
   <contained>
      <Practitioner/>
   </contained>
</DocumentReference>

The Patient element

ATOM Feed

The ATOM Feed is used as the bundling mechanism for submissions. It is the only mechanism in FHIR that allows a collection of objects to be sent and have have inter-object relationships coded. From an IHE Document Sharing perspective this accomplishes two things. First it allows DocumentManifest and DocumentReference objects to be bundled together. This is accomplished in ebRIM using the RegistryObjectList object. Second, the FHIR specification requires a different metadata construction style than ebRIM. With ebRIM the document specifics, the author specifics, and the patient specifics are all bundled into one object. This happens with the SubmissionSet, DocumentEntry, and Folder objects. In FHIR the author and patient information is normally submitted as separate objects. A document submission would reference these objects, not include them.

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

Open Issues

MHD001 - Current metadata is referenced as Document Sharing ... Good reference?

MHD002 - In our current binding we use the term DocumentEntry. At the moment in MHD I refer to the equivalent in the FHIR binding as DocumentReference. What wording style is going to move forward?

MHD003 - The volume 3 material is being written as a reference and not a tutorial. Tutorial material should be kept separate.

MHD004 - I am excluding FHIR example XML (and JSON) from our document instead referencing specific pages on the HL7 FHIR website.

MHD005 - The MHD Document Recipient must map Practitioner, Patient, and RelatedPerson into Document Sharing authorPerson elements. The MHD Document Responder will have to choose how it converts authorPerson into one of Practitioner, Patient, and RelatedPerson. One of the following choices must be made in writing the profile: accept the mapping of RelatedPerson to authorPerson to Practitioner (for example) or add new annotations to the ebRIM format so these can be distinguished. Does the XCN data type allow for the coding of the distinction?

Object Types

This section only offers guidance on how to code FHIR to meet the existing IHE Document Sharing requirement. See Volume 3 of the ITI Technical Framework section 4.2.3 for the IHE Document sharing requirements.

DocumentReference

IHE Document Sharing DocumentEntry corresponds to FHIR DocumentReference.

The text element represents an HTML rendition of the DocumentReference. This example shows that the HTML has been generated (as opposed to hand generated?).

<DocumentReference>
   <text>
      <status value="generated"/>
      <div  xmlns="http://www.w3.org/1999/xhtml">
      </div>
   </text>
 </DocumentReference>

The attributes of the DocumentReference are nested inside the DocumentReference element.

author attribute

The Document Sharing information model requires at least an authorPerson, authorTelecommunication, or authorInstitution sub-attribute shall be present when the author attribute is included in the document metadata. This implies that authorRole and authorSpeciality can only be coded if one of authorPerson, authorTelecommunication, or authorInstitution is coded. Furthermore, authorPerson can be coded as either a human or machine author. The following mappings shall be used between ebRIM binding and FHIR binding:

DS Model	FHIR resource
human	Practitioner or Patient or RelatedPerson
device	Device
telecom	Can only be coded as an element inside Practitioner, Patient, or RelatedPerson
institution	Can only be coded as an organization element inside Practitioner, Patient, and Device. It cannot be coded as part of RelatedPerson.

Note: The MHD Document Recipient must map Practitioner, Patient, and RelatedPerson into Document Sharing authorPerson elements. The MHD Document Responder will have to choose how it converts authorPerson into one of Practitioner, Patient, and RelatedPerson.

Since author is implemented by referencing an independent resource in FHIR and Document Sharing information model requires that it be included in the submission, it is coded in the contained section of the DocumentReference element. An example of this coding in both XML and JSON can be found here.

availabilityStatus attribute

availabilityStatus is coded as the FHIR element status with the following value mapping:

ebRIM	FHIR
urn:oasis:names:tc:ebxml-regrep:StatusType:Approved	current
urn:oasis:names:tc:ebxml-regrep:StatusType:Deprecated	superceeded