How to search a Claim using extension field - fhir-server-for-azure

Question

I have a Claim payload, in which I have added an extension block: (not sure how where the url came from)
"extension" : [{
"url" : "http://hl7.org/fhir/StructureDefinition/iso-21090-EN-use",
"valueString" : "MAPD"
}],
I want search this claim record using the extension but don't know how to do it.
I tried using GET request to https://<azure_fhir_server>/Claim?extension=MAPD but it says
{
"severity": "warning",
"code": "not-supported",
"diagnostics": "The search parameter 'extension' is not supported for resource type 'Claim'."
}
=====================
EDIT:
As suggested by #Nik Klassen, I posted the following payload to /SearchParameter
{
"resourceType" : "SearchParameter",
"id": "b072f860-7ecd-4d73-a490-74acd673f8d2",
"name": "extensionValueString",
"status": "active",
"url" : "http://hl7.org/fhir/SearchParameter/extension-valuestring",
"description": "Returns a Claim with extension.valueString matching the specified one in request.",
"code" : "lob",
"base" : [
"Claim"
],
"type" : "string",
"expression" : "Claim.extension.where(url ='http://hl7.org/fhir/SearchParameter/extension-valuestring').extension.value.string"
}
Also, did the $reindex on Claim, but the couldnt find the column lob($reindex response is below):
{
"resourceType": "Parameters",
"id": "ee8786d2-616a-4b81-8f6a-8089591b1225",
"meta": {
"versionId": "1"
},
"parameter": [
{
"name": "_id",
"valueString": "28e808d6-e420-4a33-bb0b-7cd325c8c169"
},
{
"name": "status",
"valueString": "http://hl7.org/fhir/fm-status|active"
},
{
"name": "priority",
"valueString": "http://terminology.hl7.org/CodeSystem/processpriority|normal"
},
{
"name": "facility",
"valueString": "Location/Location"
},
{
"name": "patient",
"valueString": "Patient/f8d8477c-1ef4-4878-abed-51e514bfd91f"
},
{
"name": "encounter",
"valueString": "Encounter/67062d00-2531-3ebd-8558-1de2fd3e5aab"
},
{
"name": "use",
"valueString": "http://hl7.org/fhir/claim-use|claim"
},
{
"name": "identifier",
"valueString": "TEST"
},
{
"name": "_lastUpdated",
"valueString": "2021-08-25T07:39:15.3050000+00:00"
},
{
"name": "created",
"valueString": "1957-04-12T21:23:35+05:30"
}
]
}
I read somewhere I need to create StructureDefinition, but don't know how to do that.
Basically I want to add a field "LOB" as an extension to all my resources, and search them using: GET: https://fhir_server/{resource}?lob=<value>

Answer 1

By default you can only search on fields that are part of the FHIR spec. These are listed in a "Search Parameters" section on the page for each resource type, i.e. https://hl7.org/fhir/claim.html#search. To search on extensions you will need to create a custom SearchParameter https://learn.microsoft.com/en-us/azure/healthcare-apis/fhir/how-to-do-custom-search, i.e.
POST {{FHIR_URL}}/SearchParameter
{
"resourceType" : "SearchParameter",
"id" : "iso-21090-EN-use",
"url" : "ttp://hl7.org/fhir/SearchParameter/iso-21090-EN-use",
... some required fields ...
"code" : "iso-use",
"base" : [
"Claim"
],
"type" : "token",
"expression" : "Claim.extension.where(url = 'http://hl7.org/fhir/StructureDefinition/iso-21090-EN-use').value.string"
}