Using FHIR to expose a Patient Identity Registry lookup service – part 1

Now that we’ve covered a bit of the theory and background to FHIR, let’s look at a concrete example of where we might apply these principles. The Use Case we’re going to look at is to provide a REST based lookup service in front of a patient Identity Server.

Some notes before we start:

  • We assume that there is already an existing registry that we are providing a front end for. It handles all the assigning of identifiers, managing merges & un-merges and such like. In other words this is a read-only service (at the moment).
  • Every patient will have a single identifier (although that may change over time). We’re going to assume that this has national scope so we’ll call it a National Health Identifier – or NHI. This won’t apply in all domains of course, but it simplifies the issue for now.
  • So that we can create full URL’s, we need a server name. We’re also going to need some namespaces (more on that shortly) so, in a nod to Dilbert, we’re going to assume that the country we are building this for is elbonia.

The scenarios we will support are:

  • Retrieving a list of matching patients based on criteria such as Name, Gender, Date of Birth and such like – what you’d use if you were wanting to find someones NHI.
  • If we do have an NHI identifier, retrieving the resource (or resources) that have that NHI.

This is going to be a rather long post, so I’ll split it into 2 separate posts:

  • In this one we’ll talk in some detail about the resource, and cover the special importance of the datatype of the properties of the resource, and how to manage merging where we have duplicates.
  • In the second post, we’ll talk about the behavior of our registry service.

First, which resource to use? Well, the obvious candidate is the patient resource. It has the base demographics we need, plus an identifier (in fact multiple identifiers).

This is the first time that we’ve looked at a resource in any depth so it’s worth spending a bit of time on it. Each resource has a specific page in the spec, and each page has a number of sections:

  • A description of what the resource is intended to represent. The size and content varying according to the complexity of the resource.
  • A UML representation of the resource – a class diagram – containing the core properties with the property name, multiplicity and datatype. The datatype of the property is especially interesting as it can range from a single string to a more complex structure – an object in its own right. We will see more of this structure as we explore the patient resource, but it’s important to realize that the datatypes are the same for all resources.
  • A XML representation of the properties with the same information as the class diagram – with the addition of a brief description of each property. The name is hyperlinked to a formal description, and the datatype/s hyperlinked to the detailed description of the datatype. (Many resource properties allow multiple datatypes – you choose which one is the most appropriate when creating – or updating – the resource).
  • The terminology bindings for those properties that are bound to a code set or terminology.
  • Other constraints – generally those that cannot be represented by XSD schema.
  • Any other comments of note.
  • The specific search parameters defined for that resource.

Each resource will also have a specific XML schema and schematron file – these can  all be downloaded from the spec in a single zip file.

Of the main representations, I find the XML format the easiest to follow, so let’s look at some of the properties for the patient resource (at least those that have special points to discuss).


First up is the identifier property. This will hold our NHI identifier (though it can have others as well – e.g. a drivers license). If you look at the XML, you will see that the datatype is also called Identifier (it is in a green font, and is a hyperlink). Click on that link – it will take you to the description of what an identifier datatype is, and what properties it has.

There are a few that are of interest to us (and note that it is extensively hyperlinked to get more information as you need it).

  • The use property describes how this identifier is going to be used. We’re the official registry so ‘official’ seems most appropriate – but we note with interest that ‘temp’ or temporary might also apply in some scenarios.
  • The label is for humans – so let’s use NHI for that
  • The system needs to define the namespace within which the actual identifier value is valid – and unique. It’s a URI, so let’s base it on the address for the Elbonian Ministry of Health – so we get http://nhi.moh.elbonia.govt (Note that this doesn’t have to be a ‘real’ address, so long as it is globally unique).
  • The actual value of the identifier – eg PRP1660, or whatever we choose to use. FHIR makes no comment about the format of the identifier – that’s up to the implementation. We might decide, for example, to incorporate a checksum digit to help with error checking.
  • The period over which the identifier is valid. That could be useful if we have to ‘retire’ an identifier, but for now we’re going to assume that if our service returns an identifier it is currently valid and is intended to remain so, so we won’t use it for now.
  • We could use the assigner to give more data about the organization that maintains the registry.

Next property is the name of the patient. It has a datatype of HumanName, and contains properties such as family name, given name and use. There is also a text property that can be used for the complete name. Whether the text property and/or the individual items are used is at the discretion of the resource author, although profiles can be used to control usage to some extent.

This is a good place to mention that you could – if you needed to – use extensions to add other properties to datatypes. For example, you might want to add a salutation to the name. I’ll talk more about extensions in another post.


This has the datatype of codeableconcept. Codeableconcept is an incredibly important datatype as it represents coded data and is extensively used within FHIR. It deserves its own post, so for the moment I’ll just point out that there is a particular set of codes that can be used here (if you look in the terminology bindings section you will see a reference to the set).

Patient identification is a critical part of recording healthcare information – and one that is prone to error, for example a single person being registered multiple times. The link property in FHIR us used to assert that two (or more) patient resources are, in fact the same person.

And now is a good time to describe the difference between resource ID and the identifier.

As described in previous posts, all resources have an ID – the resource ID – which  along with the server and resource type comprise the unique URI of the resource. The resource ID cannot change, and so something like the registry identifier is not a good candidate for the resource ID because it can change. You should think of the resource ID like a database primary key – let the server assign it, and attach no semantic meaning to it. (A client can also assign the ID – though you need to be careful if doing this).

An example might help.

Suppose you create 2 patient resources. One has the ID of 100 with an identifier of PRP1660, and the other has an ID of 200 with an identifier of WER4568. After storing clinical data against these resources (and using the resource ID as the linking mechanism) you discover that they are actually the same person. You determine that the identifier you wish to keep is PRP1660, and wish to ‘shift’ all the records currently pointing to WER4568 to point to PRP1660, and inactivate WER4568. You cannot (or should not) change the ID in the resource reference of all the other resources that currently point to WER4568. The correct action is to set the link of each patient resource to the other one (thus they point to each other), and set the active property of WER4568 to false. This should be done in a transactionally safe manner.

In this way it is obvious to any subsequent view of the record what has happened, and which is the correct resource.

Before merging

Identifier ID Active link
PRP1660 100 True (or absent)
WER4568 200 True (or absent

After merging:

Identifier ID Active link
PRP1660 100 True (or absent) 200
WER4568 200 False 100

(Note that the link is actually a resource reference, and so has a slightly more complex format than a single string)

And while we’re talking about errors, what do you do if – rather than having duplicated patients, you just set the subject link of another resource (say a condition resource) to the wrong patient? Well, that is what the versioning facility in FHIR is for – you create an updated version of the condition resource with the ID of the correct patient resource, thus maintaining the history of change.

The other properties are all important but I won’t talk any further about them here, as their meaning and use should be evident from their description and from the discussion above.

It’s also worth pointing out that in a real scenario, if there were properties that we needed and weren’t in the FHIR specification (for example the reason for a merge, or to store realm specific information like iwi), then we can always use the extension mechanism to add them.

Well, that is all we’re going to say about patient – next post will describe how we use the patient resource in our registry REST service

About David Hay
I'm an independent contractor working with a number of Organizations in the health IT space. I'm an HL7 Fellow, Chair Emeritus of HL7 New Zealand and a co-chair of the FHIR Management Group. I have a keen interest in health IT, especially health interoperability with HL7 and the FHIR standard. I'm the author of a FHIR training and design tool - clinFHIR - which is sponsored by InterSystems Ltd.

One Response to Using FHIR to expose a Patient Identity Registry lookup service – part 1

  1. Pingback: #FHIR XDS: revisiting the patient identity. | Hay on FHIR

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