This page is part of the US Core (v3.2.0: STU4 Ballot 1) based on FHIR R4. The current version which supercedes this version is 5.0.1. For a full list of available versions, see the Directory of published versions

General Guidance

This section outlines important definitions, interpretations, and requirements common to all US Core actors used in this guide. The conformance verbs - SHALL, SHOULD, MAY - used in this guide are defined in FHIR Conformance Rules.

U.S. Core Data for Interoperability

The US Core Profiles were originally designed to meet the 2015 Edition certification criterion for Patient Selection 170.315(g)(7), and Application Access – Data Category Request 170.315(g)(8). They were created for each item in the 2015 Edition Common Clinical Data Set (CCDS). The Location, Organization, and Practitioner Profiles are not called out specifically in the certification criteria but are included because they are directly referenced by other profiles. The US Core Profiles are informed by the prior Data Access Framework and the Argonaut Data Query Implementation Guides. However, the profiles here are stand alone and include requirements from the U.S. Core Data for Interoperability (USCDI) v1.

The table below lists the US Core Profile and FHIR Resources used for the corresponding USCDI Data elements:

Changes Between Major Versions of FHIR

With each major version in FHIR, the core data models have undergone changes. The FHIR core specification provides a base resource differential to help implementers navigate version changes.

Referencing US Core Profiles

The search expectations and US Core Profiles have been developed and tested using logical FHIR ids. Therefore a reference to a US Core resource SHOULD include a logical id (Reference.reference), not an identifier (Reference.identifier).

Many of the profiles in this guide reference other FHIR resources that are also US Core Profiles. This is defined in the formal profile definitions. For example, US Core CareTeam Profile references US Core Patient. For any other references to base FHIR resources¹ or not formally defined in a US Core Profiles, the referenced resource SHOULD be a US Core Profile if a US Core Profile exists for the resource type. For example, although Condition.asserter is not constrained by this guide, the reference to Patient or Practitioner SHOULD be a valid US Core Patient or US Core Practitioner. US Core Resources in the differential view and marked as “Must Support” follow the Must Support rules described in Conformance Expectations. Other resources allowed in the base FHIR specification may be referenced even though the current publication framework does not display them. For example, RelatedPerson is an allowed target reference in DocumentReference.author.

Contained Resources

Contained resources SHOULD NOT be used when responding to a query. The only time contained resource can be used is when the source data exists only within the context of the FHIR transaction. For example, the Medication List Guidance page describes how a contained Medication in MedicationRequest is used for representing the medication. If referencing a contained resource in a US Core Profile, the contained resource SHOULD be a US Core Profile if a US Core Profile exists for the resource type. Further guidance about the general use case for contained can be found in the base FHIR specification.

Missing Data

If the source system does not have data for a Must Support data element, the data element is omitted from the resource as described above. If the source system does not have data for a required data element (in other words, where the minimum cardinality is > 0), the core specification provides guidance which is summarized below:

1. For non-coded data elements, use the DataAbsentReason Extension in the data type
   • Use the code unknown - The value is expected to exist but is not known.

   Example: Patient resource where the patient name is not available.

   {
     "resourceType" : "Patient",
          ...
          "name": [
            {
              "extension": [
                {
                  "url": "http://hl7.org/fhir/StructureDefinition/data-absent-reason",
                  "valueCode": "unknown"
                }
              ]
            }
          ]
           "telecom" :
           ...
        }
   

2. For coded data elements:
   • example, preferred, or extensible binding strengths (CodeableConcept datatypes):
     • if the source systems has text but no coded data, only the text element is used.
     • if there is neither text or coded data:
       • use the appropriate “unknown” concept code from the value set if available
       • if the value set does not have the appropriate “unknown” concept code, use unknown from the DataAbsentReason Code System.
   • required binding strength (CodeableConcept or code datatypes):
     • use the appropriate “unknown” concept code from the value set if available
     • For the following status elements no appropriate “unknown” concept code is available, therefore the element must be populated:
       • AllergyIntolerance.clinicalStatus
       • Condition.clinicalStatus
       • DocumentReference.status
       • Immunization.status
       • Goal.lifecycleStatus

       If one of these a status code is missing, a 404 http error code and an OperationOutcome SHALL be returned in response to a query for the resource.

Using UCUM codes in the Quantity datatype

Both the US Core Vital Signs Profile and US Core Laboratory Result Observation Profile bind the valueQuantity datatypes to the UCUM code system. A FHIR UCUM Codes value set that defines all UCUM codes is in the FHIR specification. This guidance specifies how to represent the Quantity datatype when the correct UCUM units are missing or the units are missing altogether which will likely occur in the real world.

UCUM code provided

 "valueQuantity": {
    "value": 26.0,
    "unit": "g/mL",
   "system": "http://unitsofmeasure.org",
   "code": "g/mL"
  }

free text units only:

• If UCUM units are not available then represent units in the unit element.

 "valueQuantity": {
    "value": 26.0,
    "unit": "RR",
     }

no units

 "valueQuantity": {
    "value": 26.0
 }

Language Support

There is a basic need be able to access records in your language, and the data provider should do their best to translate (safely) to the language being requested. Understanding that this will be variably complete depending on the nature of the record. For example translating the following elements is relatively straightforward:

• Coding.display
• Generated narrative
• Attachment.title

The following guidelines outline how to request and return a resource in the requested language.

• Clients MAY request language/locale using the http Accept-Language header.
• Servers SHOULD make reasonable efforts to translate what can be safely translated.
• Servers SHOULD populate the Resource’s language element with a code which is based on the underlying language of record, not the requested language.

  • Servers SHALL use the http://hl7.org/fhir/StructureDefinition/language extension when the language of a display, etc is known to be different to the stated (or inferred) language.

    Example

        <?xml version="1.0" encoding="UTF-8"?>
        <Patient xmlns="http://hl7.org/fhir">
            <id value="language-example-1"/>
            <meta>
            [...snip...]
            </meta>
            <language value="es"/>
            <!--0..1 Language of the resource content in this case Spanish-->
            <text>
            [...snip...]
            </text>
            <extension url="http://hl7.org/fhir/us/core/StructureDefinition/us-core-race">
                <extension url="ombCategory">
                    <valueCoding>
                        <system value="urn:oid:2.16.840.1.113883.6.238"/>
                        <code value="2106-3"/>
                        <display value="White">
                            <!--Human Language extension-->
                            <extension url="http://hl7.org/fhir/StructureDefinition/language">
                                <valueCode value="en"/>
                                <!--English is different from stated language-->
                            </extension>
                        </display>
                    </valueCoding>
                </extension>
            [...snip...]
    

• Servers SHALL use the http://hl7.org/fhir/StructureDefinition/translation extension when the server is providing additional translations by its own choice or in response to a different Accept-Language than what the resource is stored in.

  Example

        <?xml version="1.0" encoding="UTF-8"?>
        <Patient xmlns="http://hl7.org/fhir">
            <id value="language-example-2"/>
            <meta>
            [...snip...]
            </meta>
            <language value="en"/>
            <!--0..1 Language of the resource content: English-->
            <text>
            [...snip...]
            </text>
            <extension url="http://hl7.org/fhir/us/core/StructureDefinition/us-core-race">
                <extension url="ombCategory">
                    <valueCoding>
                        <system value="urn:oid:2.16.840.1.113883.6.238"/>
                        <code value="2106-3"/>
                        <display value="White">
                        <!-- Translation -->
                        <extension
                             url="http://hl7.org/fhir/StructureDefinition/translation" >
                         <extension url="lang"> <!-- 1..1 Code for Language -->
                          <valueCode value="es"/><!-- 0..1 Value of extension -->
                         </extension>
                         <extension url="content"> <!-- 1..1 Content in other Language -->
                          <valueString value="Blanca">
                         </extension>
                        </extension>
                        </display>
                    </valueCoding>
                </extension>
                [...snip...]
  

• Servers SHOULD make it known what languages are supported in their CapabilityStatement(s) using this extension²:

  http://hl7.org/fhir/5.0/StructureDefinition/extension-CapablilityStatement.acceptLanguage

For further guidance on language and locale for generation of the resource narrative, see the Multi-language support for Narratives in the core specification.

Read (Fetch) Syntax

For fetching a resource interactions on profile pages are defined with the following syntax:

GET [base]/[Resource-type]/[id] {parameters}

• GET is the HTTP verb used for fetching a resource
• Content surrounded by [] is mandatory for the client to supply, and will be replaced by the string literal identified.
  • base: The Service Root URL (e.g. “https://fhir-open-api-dstu2.smarthealthit.org”)
  • Resource-type: The name of a resource type (e.g. “Patient”)
  • id: The Logical Id of a resource(e.g. “24342”)
• Content surrounded by {} is optional for the client to supply, and will be replaced by the string literal identified.
  • parameters: URL parameters as defined for the particular interaction (e.g.”?_format=xml”}

For more information see the FHIR RESTful API

Search Syntax

For searching a resource, interactions on profile pages are defined with the following syntax:

GET [base]/[Resource-type]?[parameter1]{:m1|m2|...}={c1|c2|...}[value1{,value2,...}]{&[parameter2]{:m1|m2|...}={c1|c2|...}[value1{,value2,...}]&...}

• GET is the HTTP verb used for fetching a resource
• Variables surrounded by “[]” are mandatory for the client to supply, and will be replaced by the string literal identified.
• Variables surrounded by “{}” are optional for the client to supply, and will be replaced by the string literal identified.
  • base: The Service Root URL (e.g. “https://fhir-open-api-dstu2.smarthealthit.org”)
  • Resource-type: The name of a resource type (e.g. “Patient”)
  • parameter: the search parameters as defined for the particular interaction (e.g.”?patient=Patient/123”)
  • value: the search parameter value for a particular search
    • When searching using the token type searchparameter (how to search by token), the syntax {system|}[code] means that the system value is optional for the client to supply.:
      • The client SHALL provide at least a code value and MAY provide both the system and code values.
      • The server SHALL support both.
    • When searching using the reference type searchparameter (how to search by reference), the syntax {Type/}[id] means that the Type value is optional for the client to supply:
      • The client SHALL provide at least a id value and MAY provide both the Type and id values.
      • The server SHALL support both.

  • {:m1

    m2

    …}: The list of supported search parameter modifiers

  • {c1

    c2

    …}: The list of supported search parameter comparators

  • {,value2,…}: Optional multiple ‘OR’ Values

  • {&parameter2={:m1

    m2

    …}={c1

    c2

    …}[value1{,value2,…}&…}: Optional multiple ‘AND’ search parameters

In the simplest case, a search is executed by performing a GET operation in the RESTful framework:

GET [base]/[Resource-type]?name=value&...

For this RESTful search, the parameters are a series of name=[value] pairs encoded in the URL. The search parameter names are defined for each resource. For example, the Observation resource the name “code” for search on the LOINC code. For more information see the FHIR RESTful Search API

Note that the patient may be implicit in the context in some implementations (e.g. using SMART). Then the patient parameter can be omitted:

GET [base]/[Resource-type]{?other-parameters}

Search for Servers Requiring Status

Servers are strongly encouraged to support a query for resources without requiring a status parameter. However, if business requirements prohibit this they SHALL follow the guidelines here.

For searches where the client does not supply a status parameter, an implementation’s business rules may override the FHIR RESTful search expectations and require a status parameter to be provided. These systems are allowed to reject such requests as follows:

• SHALL return an http 400 status
• SHALL return an OperationOutcome specifying that status(es) must be present.
• SHALL support search with status if status required
• SHALL NOT restrict search results ( i.e. apply ‘hidden’ filters) when a client includes status parameters in the query.

  • If a system doesn’t support a specific status code value that is queried, search results SHOULD return an http 200 status with search bundle containing resources matching the search criteria and an OperationOutcome warning the client which status code value is not supported.

  • For example, in a query enumerating all the AllergyIntolerance.verificationStatus statuses to a system that supports concepts unconfirmed, confirmed, entered-in-error but not refuted, the search parameter is referring to an unsupported code since refuted is not known to the server.

Click Here to See a Rejected Search Due to Missing Status Example

GET [base]/Encounter?patient=1137192

HTTP/1.1 400 Not Found
[other headers]
{
  "resourceType": "OperationOutcome",
  "id": "no-status",
  "issue": [
    {
      "severity": "error",
      "code": "business-rule",
      "details": {
        "text": "A \"status\" search parameter is required for this search"
      },
      "diagnostics": "valid statuses for Encounter include planned | arrived | triaged | in-progress | onleave | finished | cancelled | entered-in-error | unknown"
    }
  ]
}

• SHALL document this behavior in its CapabilityStatement for the “search-type” interaction in CapabilityStatement.rest.resource.interaction.documentation.
• Follow the deleted data guidance above.

Searching Multiple Patients

Currently, most EHRs permit queries that provide a single patient id, but do not support the comma separated query or a query where the patient parameter is omitted as described in the standard FHIR REST API. Instead, a user facing app can perform multiple “parallel” queries on a list of patient ids. Alternatively, the FHIR Bulk Data Access (Flat FHIR) specification can be used to perform a “back end” system level query to access a large volumes of information on a group of individuals or when trying to identify and query against an unknown population such as when looking for population based research data.

However, neither specification defines how a user facing provider app is able to seek realtime “operational” data on multiple patients (such as all patients with recent lab results). Opportunities to add this capability to this guide are discussed in Future of US Core

Compartment Based Search

This IG does not support patient compartment based searches.

Across Platform Searches

US Core servers are not required to resolve full URLs that are external to their environment.

Guidance On Limiting The Number Of Search Results

In order to manage the number of search results returned, the server may choose to return the results in a series of pages. The search result set contains the URLs that the client uses to request additional pages from the search set. For a simple RESTful search, the page links are contained in the returned bundle as links. See the managing returned resources in the FHIR specification for more information.