Differences between FHIR Ids and Identifiers

In the context of HL7 FHIR (Fast Healthcare Interoperability Resources), the terms "id" and "identifier" refer to specific elements used for identifying resources within the FHIR data model. For a newbie, these terms can be confusingly similar, but they serve distinct purposes.  

Look at the below Patient resource for August T. Faulkner:

The resource has an id of “1” — generated by the FHIR server when the resource was created.
Patient August T. Faulkner also has a identifier (Medical Record Number) — possibly provided by the hospital — of 78510398960

FHIR Resource ID

• Every resource stored in FHIR must have an id
• It is typically assigned by the server when a resource is created, but it can also be specified by the client under certain circumstances.
• It is often used as part of the resource's URL, making it easy to reference and retrieve.
• It does not change for the lifetime of the resource
• It is unique to that resource type in that FHIR server

Here’s how to look up the above patient using the resource’s id:

• /Patient/1
• /Patient?_id=1

FHIR Resource Identifier

Patient August T. Faulkner also has a Medical Record Number — possibly provided by the hospital — of 78510398960.

• This is added to the same FHIR resource as an identifier.
• Identifiers in FHIR are business identifiers. Unlike the FHIR resource id they are not created by the FHIR server and have no connection to the server.
• Identifiers are typically composed of a system (URI), which defines the namespace or identifier scheme, and a value (string), which represents the actual identifier within that system.
• Resources may have multiple identifiers to capture various identification schemes.

Examples of other identifiers used for patients are:

• Social security number
• Health card number
• Passport number

Here’s how you look up the patient August T. Faulkner using his Medical Record Number:

• /Patient?identifier=78510398960
• /Patient?identifier=http://my.localhospital.ie/fhir/patient-MRN|78510398960

Thanks