Defining custom search parameters for Azure API for FHIR

The Fast Healthcare Interoperability Resources (FHIR®) specification defines a set of search parameters for all resources and search parameters that are specific to a resource(s). However, there are scenarios where you might want to search against an element in a resource that isn’t defined by the FHIR specification as a standard search parameter. This article describes how you can define your own search parameters to be used in the Azure API for FHIR.

Create new search parameter

To create a new search parameter, you POST the SearchParameter resource to the database. The code example below shows how to add the US Core Race SearchParameter to the Patient resource.

POST {{FHIR_URL}}/SearchParameter

{
  "resourceType" : "SearchParameter",
  "id" : "us-core-race",
  "url" : "http://hl7.org/fhir/us/core/SearchParameter/us-core-race",
  "version" : "3.1.1",
  "name" : "USCoreRace",
  "status" : "active",
  "date" : "2019-05-21",
  "publisher" : "US Realm Steering Committee",
  "contact" : [
    {
      "telecom" : [
        {
          "system" : "other",
          "value" : "http://www.healthit.gov/"
        }
      ]
    }
  ],
  "description" : "Returns patients with a race extension matching the specified code.",
  "jurisdiction" : [
    {
      "coding" : [
        {
          "system" : "urn:iso:std:iso:3166",
          "code" : "US",
          "display" : "United States of America"
        }
      ]
    }
  ],
  "code" : "race",
  "base" : [
    "Patient"
  ],
  "type" : "token",
  "expression" : "Patient.extension.where(url = 'http://hl7.org/fhir/us/core/StructureDefinition/us-core-race').extension.value.code"
}

Important elements of a SearchParameter:

• url: A unique key to describe the search parameter. Many organizations, such as HL7, use a standard URL format for the search parameters that they define, as shown above in the US Core race search parameter.

• code: The value stored in code is what you’ll use when searching. For the example above, you would search with GET {FHIR_URL}/Patient?race=<code> to get all patients of a specific race. The code must be unique for the resource(s) the search parameter applies to.

• base: Describes which resource(s) the search parameter applies to. If the search parameter applies to all resources, you can use Resource; otherwise, you can list all the relevant resources.

• type: Describes the data type for the search parameter. Type is limited by the support for the Azure API for FHIR. This means that you can’t define a search parameter of type Special or define a composite search parameter unless it's a supported combination.

• expression: Describes how to calculate the value for the search. When describing a search parameter, you must include the expression, even though it isn't required by the specification. This is because you need either the expression or the xpath syntax and the Azure API for FHIR ignores the xpath syntax.

Test search parameters

While you can’t use the search parameters in production until you run a reindex job, there are a few ways to test your search parameters before reindexing the entire database.

First, you can test your new search parameter to see what values will be returned. By running the command below against a specific resource instance (by inputting their ID), you'll get back a list of value pairs with the search parameter name and the value stored for the specific patient. This will include all of the search parameters for the resource and you can scroll through to find the search parameter you created. Running this command won't change any behavior in your FHIR server.

GET https://{{FHIR_URL}}/{{RESOURCE}}/{{RESOUCE_ID}}/$reindex

For example, to find all search parameters for a patient:

GET https://{{FHIR_URL}}/Patient/{{PATIENT_ID}}/$reindex

The result will look like this:

{
  "resourceType": "Parameters",
  "id": "8be24e78-b333-49da-a861-523491c3437a",
  "meta": {
    "versionId": "1"
  },
  "parameter": [
    {
      "name": "deceased",
      "valueString": "http://hl7.org/fhir/special-values|false"
    },
    {
      "name": "language",
      "valueString": "urn:ietf:bcp:47|en-US"
    },
    {
      "name": "race",
      "valueString": "2028-9"
    },
...

Once you see that your search parameter is displaying as expected, you can reindex a single resource to test searching with the element. First you'll reindex a single resource:

POST https://{{FHIR_URL}/{{RESOURCE}}/{{RESOURCE_ID}}/$reindex

Running this, sets the indices for any search parameters for the specific resource that you defined for that resource type. This does make an update to the FHIR server. Now you can search and set the use partial indices header to true, which means that it will return results where any of the resources has the search parameter indexed, even if not all resources of that type have it indexed.

Continuing with our example above, you could index one patient to enable the US Core Race SearchParameter:

POST https://{{FHIR_URL}/Patient/{{PATIENT_ID}}/$reindex

And then search for patients that have a specific race:

GET https://{{FHIR_URL}}/Patient?race=2028-9
x-ms-use-partial-indices: true

After you have tested and are satisfied that your search parameter is working as expected, run or schedule your reindex job so the search parameters can be used in the FHIR server for production use cases.

Update a search parameter

To update a search parameter, use PUT to create a new version of the search parameter. You must include the SearchParameter ID in the id element of the body of the PUT request and in the PUT call.

PUT {{FHIR_URL}}/SearchParameter/{SearchParameter ID}

{
  "resourceType" : "SearchParameter",
  "id" : "SearchParameter ID",
  "url" : "http://hl7.org/fhir/us/core/SearchParameter/us-core-race",
  "version" : "3.1.1",
  "name" : "USCoreRace",
  "status" : "active",
  "date" : "2019-05-21",
  "publisher" : "US Realm Steering Committee",
  "contact" : [
    {
      "telecom" : [
        {
          "system" : "other",
          "value" : "http://www.healthit.gov/"
        }
      ]
    }
  ],
  "description" : "New Description!",
  "jurisdiction" : [
    {
      "coding" : [
        {
          "system" : "urn:iso:std:iso:3166",
          "code" : "US",
          "display" : "United States of America"
        }
      ]
    }
  ],
  "code" : "race",
  "base" : [
    "Patient"
  ],
  "type" : "token",
  "expression" : "Patient.extension.where(url = 'http://hl7.org/fhir/us/core/StructureDefinition/us-core-race').extension.value.code"
}

The result will be an updated SearchParameter and the version will increment.

Delete a search parameter

If you need to delete a search parameter, use the following:

Delete {{FHIR_URL}}/SearchParameter/{SearchParameter ID}

Next steps

In this article, you’ve learned how to create a search parameter. Next you can learn how to reindex your FHIR server.

FHIR® is a registered trademark of HL7 and is used with the permission of HL7.