Overview of search in Azure API for FHIR


Azure API for FHIR will be retired on September 30, 2026. Follow the migration strategies to transition to Azure Health Data Services FHIR service by that date. Due to the retirement of Azure API for FHIR, new deployments won't be allowed beginning April 1, 2025. Azure Health Data Services FHIR service is the evolved version of Azure API for FHIR that enables customers to manage FHIR, DICOM, and MedTech services with integrations into other Azure services.

The Fast Healthcare Interoperability Resources (FHIR®) specification defines the fundamentals of search for FHIR resources. This article will guide you through some key aspects to searching resources in FHIR. For complete details about searching FHIR resources, refer to Search in the HL7 FHIR Specification. Throughout this article, we'll give examples of search syntax. Each search will be against your FHIR server, which typically has a URL of https://<FHIRSERVERNAME>.azurewebsites.net. In the examples, we'll use the placeholder {{FHIR_URL}} for this URL.

FHIR searches can be against a specific resource type, a specified compartment, or all resources. The simplest way to execute a search in FHIR is to use a GET request. For example, if you want to pull all patients in the database, you could use the following request:

GET {{FHIR_URL}}/Patient

You can also search using POST, which is useful if the query string is too long. To search using POST, the search parameters can be submitted as a form body. This allows for longer, more complex series of query parameters that might be difficult to see and understand in a query string.

If the search request is successful, you’ll receive a FHIR bundle response with the type searchset. If the search fails, you’ll find the error details in the OperationOutcome to help you understand why the search failed.

In the following sections, we’ll cover the various aspects involved in searching. Once you’ve reviewed these details, refer to our samples page that has examples of searches that you can make in the Azure API for FHIR.

Search parameters

When you do a search, you'll search based on various attributes of the resource. These attributes are called search parameters. Each resource has a set of defined search parameters. The search parameter must be defined and indexed in the database for you to successfully search against it.

Each search parameter has a defined data types. The support for the various data types is outlined below:


There is currently an issue when using _sort on the Azure API for FHIR with chained search. For more information, see open-source issue #2344. This will be resolved during a release in December 2021.

Search parameter type Azure API for FHIR FHIR service in Azure Health Data Services Comment
number Yes Yes
date Yes Yes
string Yes Yes
token Yes Yes
reference Yes Yes
composite Partial Partial The list of supported composite types is described later in this article
quantity Yes Yes
uri Yes Yes
special No No

Common search parameters

There are common search parameters that apply to all resources. These are listed below, along with their support within the Azure API for FHIR:

Common search parameter Azure API for FHIR FHIR service in Azure Health Data Services Comment
_id Yes Yes
_lastUpdated Yes Yes
_tag Yes Yes
_type Yes Yes
_security Yes Yes
_profile Yes Yes
_has Partial Yes Support for _has is in MVP in the Azure API for FHIR and the OSS version backed by Azure Cosmos DB. More details are included under the chaining section below.
_query No No
_filter No No
_list No No
_text No No
_content No No

Resource-specific parameters

With the Azure API for FHIR, we support almost all resource-specific search parameters defined by the FHIR specification. The only search parameters we don’t support are available in the links below:

You can also see the current support for search parameters in the FHIR Capability Statement with the following request:

GET {{FHIR_URL}}/metadata

To see the search parameters in the capability statement, navigate to CapabilityStatement.rest.resource.searchParam to see the search parameters for each resource and CapabilityStatement.rest.searchParam to find the search parameters for all resources.


The Azure API for FHIR does not automatically create or index any search parameters that are not defined by the FHIR specification. However, we do provide support for you to define your own search parameters.

Composite search parameters

Composite search allows you to search against value pairs. For example, if you were searching for a height observation where the person was 60 inches, you would want to make sure that a single component of the observation contained the code of height and the value of 60. You wouldn't want to get an observation where a weight of 60 and height of 48 was stored, even though the observation would have entries that qualified for value of 60 and code of height, just in different component sections.

With the Azure API for FHIR, we support the following search parameter type pairings:

  • Reference, Token
  • Token, Date
  • Token, Number, Number
  • Token, Quantity
  • Token, String
  • Token, Token

For more information, see the HL7 Composite Search Parameters.


Composite search parameters do not support modifiers per the FHIR specification.

Modifiers & prefixes

Modifiers allow you to modify the search parameter. Below is an overview of all the FHIR modifiers and the support in the Azure API for FHIR.

Modifiers Azure API for FHIR FHIR service in Azure Health Data Services Comment
:missing Yes Yes
:exact Yes Yes
:contains Yes Yes
:text Yes Yes
:type (reference) Yes Yes
:not Yes Yes
:below (uri) Yes Yes
:above (uri) Yes Yes
:in (token) No No
:below (token) No No
:above (token) No No
:not-in (token) No No

For search parameters that have a specific order (numbers, dates, and quantities), you can use a prefix on the parameter to help with finding matches. The Azure API for FHIR supports all prefixes.

Search result parameters

To help manage the returned resources, there are search result parameters that you can use in your search. For details on how to use each of the search result parameters, refer to the HL7 website.

Search result parameters Azure API for FHIR FHIR service in Azure Health Data Services Comment
_elements Yes Yes
_count Yes Yes _count is limited to 1000 resources. If it's set higher than 1000, only 1000 will be returned and a warning will be returned in the bundle.
_include Yes Yes Included items are limited to 100. _include on PaaS and OSS on Azure Cosmos DB don't include :iterate support (#2137).
_revinclude Yes Yes Included items are limited to 100. _revinclude on PaaS and OSS on Azure Cosmos DB don't include :iterate support (#2137). There's also an incorrect status code for a bad request #1319
_summary Yes Yes
_total Partial Partial _total=none and _total=accurate
_sort Partial Partial sort=_lastUpdated is supported on Azure API for FHIR and the FHIR service. For Azure API for FHIR and OSS Azure Cosmos DB databases created after April 20, 2021, sort is supported on first name, last name, birthdate, and clinical date.
_contained No No
_containedType No No
_score No No


By default _sort sorts the record in ascending order. You can use the prefix '-' to sort in descending order. In addition, the FHIR service and the Azure API for FHIR only allow you to sort on a single field at a time.

By default, the Azure API for FHIR is set to lenient handling. This means that the server will ignore any unknown or unsupported parameters. If you want to use strict handling, you can use the Prefer header and set handling=strict.

Chained & reverse chained searching

A chained search allows you to search using a search parameter on a resource referenced by another resource. For example, if you want to find encounters where the patient’s name is Jane, use:

GET {{FHIR_URL}}/Encounter?subject:Patient.name=Jane

Similarly, you can do a reverse chained search. This allows you to get resources where you specify criteria on other resources that refer to them. For more examples of chained and reverse chained search, refer to the FHIR search examples page.


In the Azure API for FHIR and the open source backed by Azure Cosmos DB, there's a limitation where each subquery required for the chained and reverse chained searches will only return 1000 items. If there are more than 1000 items found, you’ll receive the following error message: “Subqueries in a chained expression can't return more than 1000 results, please use a more selective criteria.” To get a successful query, you’ll need to be more specific in what you are looking for.


As mentioned above, the results from a search will be a paged bundle. By default, the search will return 10 results per page, but this can be increased (or decreased) by specifying _count. Within the bundle, there will be a self link that contains the current result of the search. If there are additional matches, the bundle will contain a next link. You can continue to use the next link to get the subsequent pages of results. _count is limited to 1000 items or less.

The next link in the bundle has a continuation token size limit of 3KB. You have flexibility to tweak the continuation token size between 1 to 3KB, using header "x-ms-documentdb-responsecontinuationtokenlimitinkb".

Currently, the Azure API for FHIR only supports the next link in bundles, and it doesn’t support first, last, or previous links.

Next steps

Now that you've learned about the basics of search, see the search samples page for details about how to search using different search parameters, modifiers, and other FHIR search scenarios.

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