Using CQL with FHIR, published by HL7 International / Clinical Decision Support. This guide is not an authorized publication; it is the continuous build for version 1.0.0 built by the FHIR (HL7® FHIR® Standard) CI Build. This version is based on the current content of https://github.com/HL7/cql-ig/ and changes regularly. See the Directory of published versions

Conformance

This topic specifies conformance requirements for systems that support authoring, publishing, distribution, and implementation of FHIR Knowledge Artifacts that make use of Clinical Quality Language (CQL).

Library Resources

In addition to the use of CQL directly in expression-valued elements, CQL content used within knowledge artifacts can be included through the use of a Library resource. These libraries can then be referenced from FHIR resources such as PlanDefinition and Measure using the library element (as well as the cqf-library extension for resources that do not declare a library element). The content of the CQL library is included using the content element of the Library.

Conformance Requirement 4.1 (Library Resources):

1. Content conforming to this implementation guide SHALL use FHIR Library resources to represent CQL libraries in FHIR.
2. For distribution to environments that support CQL compilation directly, FHIR Library resources SHOULD include CQL content. a. FHIR Library resources that include CQL content SHALL conform to the CQLLibrary profile

For distribution to environments that support ELM execution directly, FHIR Library resources MAY include ELM content in XML or JSON format. See the Using ELM topic for conformance requirements related to the use of ELM for distribution and implementation of CQL logic.

Library Name and URL

Conformance Requirement 4.2 (Library Name and URL):

1. The identifying elements of a library SHALL conform to the following requirements:
   • Library.url SHALL be <CQL namespace url>/Library/<CQL library name>
   • Library.name SHALL be <CQL library name>
   • Library.version SHALL be <CQL library version>
2. For libraries included in FHIR implementation guides, the CQL namespace is defined by the implementation guide as follows:
   • CQL namespace name SHALL be IG.packageId
   • CQL namespace url SHALL be IG.canonicalBase
3. CQL library source files SHOULD be named <CQLLibraryName>-<version>.cql
4. To avoid issues with characters between web ids and names, library names SHALL NOT have underscores.

The prohibition against underscores in CQL library names is required to ensure compliance with the canonical URL pattern (because URLs by convention should not use underscores). In addition, many publishing environments will use the canonical tail (i.e. the name of the library) as the logical id of the Library resource, which does not allow underscores per the FHIR specification.

FHIR Type Mapping

Conformance Requirement 4.3 (FHIR Type Mapping):

1. CQL defined types SHALL map to types in FHIR according to the following mapping:

1. In addition:
   • List types SHALL have elements of types that can be mapped to FHIR according to this mapping
   • Tuple types SHALL have elements of types that can be mapped to FHIR according to this mapping

For example, the following CQL expression results in a List<FHIR.Observation>:

In the Library resource, this is represented as a parameter:

{
    "extension": [{
      "url": "http://hl7.org/fhir/StructureDefinition/cqf-cqlType",
      "valueString": "List<FHIR.Observation>"
    }],
    "name": "FHIRObservationListExample",
    "use": "out",
    "min": 0,
    "max": "*",
    "type": "Observation"
  }

Note the parameter is multi-cardinality to indicate this is a list-valued expression. Also note the use of the cqf-cqlType extension to relay the CQL type.

When invoked through an operation (such as $cql or Library/$evaluate), this would be represented as multiple entries in the resulting Parameters resource:

{
  "name": "FHIRObservationListExample",
  "resource": {
    "resourceType": "Observation",
    "id": "blood-glucose",
    "status": "final",
    ...
  }
}, {
  "name": "FHIRObservationListExample",
  "resource": {
    "resourceType": "Observation",
    "id": "blood-pressure",
    "status": "final",
    ...
  }
}, {
  "name": "FHIRObservationListExample",
  "resource": {
    "resourceType": "Observation",
    "id": "bmi",
    "status": "final",
    ...
  }
}

Note that for an empty list, the cqf-isEmptyList extension is used:

{
  "extension": [{
    "url": "http://hl7.org/fhir/StructureDefinition/cqf-cqlType",
    "valueString": "List<FHIR.Observation>"
  }],
  "name": "FHIRObservationEmptyListExample",
  "_valueBoolean": {
    "extension": [{
      "url": "http://hl7.org/fhir/StructureDefinition/cqf-isEmptyList",
      "valueBoolean": true
    }]
  }
}

Note that the extension is provided on the value element, and an arbitrary choice of boolean is selected; there is no value to provide, the result is an empty list, so this is just a way to provide the cqf-isEmptyList extension (because parameters in a FHIR Parameters resource must have a value element).

For a complete example illustrating all possible type mappings, refer to the Type Mapping Example and Type Mapping Evaluation Result Example

Parameters and Data Requirements

Conformance Requirement 4.4 (Parameters and Data Requirements):

1. Parameters to CQL libraries SHALL be either CQL-defined types that map to FHIR types, or FHIR resource types, optionally with profile designations.
2. Top level expressions in CQL libraries SHALL return either CQL-defined types that map to FHIR types (as defined in 2.19), or FHIR resource types, optionally with profile designations
3. Tuple types are represented in FHIR as a parameter that has parts corresponding to the elements of the tuple type. List types are represented in FHIR as a parameter that has a cardinality of 0..*.
4. Libraries used in computable artifacts SHALL use the parameter element to identify input parameters as well as the type of all top-level expressions as output parameters.
5. Libraries used in computable artifacts SHALL use the dataRequirement element to identify any retrieves present in the CQL, according to the following mapping:

For example, given the following CQL:

define Conditions: [Condition]

The corresponding data requirement is:

{
  "type": "Condition",
  "profile": [ "http://hl7.org/fhir/StructureDefinition/Condition" ]
}

When the retrieve includes a terminology filter, the codeFilter element is used to communicate the filter:

valueset "Inpatient Encounters": 'http://cts.nlm.nih.gov/fhir/ValueSet/2.16.840.1.113883.3.117.1.7.1.292'
...
define Encounters: [Encounter: "Inpatient Encounters"]

{
  "type": "Encounter",
  "codeFilter": [ {
    "path": "type",
    "valueSet": "http://cts.nlm.nih.gov/fhir/ValueSet/2.16.840.1.113883.3.117.1.7.1.292"
  } ]
}

The cqf-isSelective extension MAY be used to identify selective data requirements (i.e. data requirements that are likely to be the most selective of the data of interest for the artifact:

{
  "extension": [ {
      "url": "http://hl7.org/fhir/StructureDefinition/cqf-isSelective",
      "valueBoolean": true
  } ],
  "type": "Encounter",
  "codeFilter": [ {
    "path": "type",
    "valueSet": "http://cts.nlm.nih.gov/fhir/ValueSet/2.16.840.1.113883.3.117.1.7.1.292"
  } ]
}

Although this extension may be used by artifact authors as a way to indicate expected selectivity of a data requirement, it will more typically be used by implementers and downstream packaging repositories to indicate selectivity of a data requirement given known data heuristics in particular datasets.

The cqf-fhirQueryPattern extension MAY be used to recommend a FHIR RESTful query that can be used to satisfy the data requirement:

{
  "extension": [ {
    "url": "http://hl7.org/fhir/StructureDefinition/cqf-fhirQueryPattern",
    "valueString": "Encounter?subject=Patient/&type:in=http://cts.nlm.nih.gov/fhir/ValueSet/2.16.840.1.113883.3.117.1.7.1.292"
  } ],
  "type": "Encounter",
  "profile": [ "http://hl7.org/fhir/us/qicore/StructureDefinition/qicore-encounter" ],
  "codeFilter": [ {
    "path": "type",
    "valueSet": "http://cts.nlm.nih.gov/fhir/ValueSet/2.16.840.1.113883.3.117.1.7.1.292"
  } ]
}

Systems that can infer more selective requirements from additional restrictions applied in the CQL after the retrieve MAY include those requirements to provide more selective data requirements. For example:

define "Completed Inpatient Encounters": 
  [Encounter: "Inpatient Encounters"] E
    where E.status = 'finished'

The status restriction is represented using the cqf-valueFilter extension:

{
  "extension": [ {
    "extension" : [
      {
        "url" : "path",
        "valueString" : "status"
      },
      {
        "url" : "comparator",
        "valueCode" : "eq"
      },
      {
        "url" : "value",
        "valueString" : "finished"
      }
    ],
    "url" : "http://hl7.org/fhir/StructureDefinition/cqf-valueFilter"
  } ],
  "type": "Encounter",
  "codeFilter": [ {
    "path": "type",
    "valueSet": "http://cts.nlm.nih.gov/fhir/ValueSet/2.16.840.1.113883.3.117.1.7.1.292"
  } ]
}

Elements that are referred to in the CQL MAY be communicated using the mustSupport element:

define "Inpatient Encounters During Measurement Period": 
  [Encounter: "Inpatient Encounters"] E
    where E.period during "Measurement Period"

{
  "type": "Encounter",
  "mustSupport": [ "period" ],
  "codeFilter": [ {
    "path": "type",
    "valueSet": "http://cts.nlm.nih.gov/fhir/ValueSet/2.16.840.1.113883.3.117.1.7.1.292"
  } ]
}

When using profile-informed authoring, the retrieve will have a templateId corresponding to the profile:

{
  "type": "Encounter",
  "profile": [ "http://hl7.org/fhir/us/qicore/StructureDefinition/qicore-encounter" ],
  "codeFilter": [ {
    "path": "type",
    "valueSet": "http://cts.nlm.nih.gov/fhir/ValueSet/2.16.840.1.113883.3.117.1.7.1.292"
  } ]
}

When referencing extensions that are surfaced as elements in profile-informed authoring, the mustSupport uses the .extension() function in FHIRPath, and the rendered-value extension is used to provide a human-readable rendering, corresponding to the sliceName of the extension:

using QICore
...
define SDEEthnicity: Patient.ethnicity

{
  "type": "Patient",
  "profile": [ "http://hl7.org/fhir/us/qicore/StructureDefinition/qicore-patient" ],
  "mustSupport": [ "extension('http://hl7.org/fhir/us/core/StructureDefinition/us-core-ethnicity')" ],
  "_mustSupport": [
    {
      "extension": [{
        "url": "http://hl7.org/fhir/StructureDefinition/rendered-value",
        "valueString": "ethnicity"
      }]
    }
  ]
}

In the case that dynamic CQL construction is required, implementers should take care to sanitize inputs from any parameters used in the construction of dynamic CQL to avoid injection attacks.

RelatedArtifacts

Conformance Requirement 4.5 (Related Artifacts):

1. Libraries used in computable artifacts SHALL use the relatedArtifact element to identify includes, code systems, value sets, and data models used by the CQL library:

CQL Version

Conformance Requirement 4.6 (Specifying CQL Version):

1. The version of CQL/ELM used for content in a library SHOULD be specified using the version parameter of the text/cql and application/elm+xml, application/elm+json media types. a. If specified, the value of the version parameter SHALL correspond to the major and minor version of a published release of the CQL specification (https://cql.hl7.org/history.html)
2. Resource narratives for Libraries and knowledge artifacts that use CQL SHOULD include the CQL/ELM version if it is specified in the media type.

For example, the following media types indicate version 1.5 of the CQL specification.

• text/cql; version=1.5
• application/elm+xml; version=1.5
• application/elm+json; version=1.5

Using Expressions

CQL can be used in expression-valued elements in the following ways:

1. To specify an unqualified expression name in the “primary” library for an artifact
2. To specify a qualified expression name in a library referenced by an artifact
3. To directly specify an inline expression

To distinguish these use cases, the language element of the expression value is used as specified in the Using Expressions topic of the FHIR specification.

The “primary library” for an artifact is determined as follows:

1. If the resource type has a library element (e.g. PlanDefinition.library), and there is one and only one library specified, that is the primary library
2. If the resource has one and only one cqf-library extension, that is the primary library

If there is more than one library specified in the resource, then expression identifiers must be qualified with the name of the library (see Conformance Requirement 2.3 (Nested Libraries)).

Must Support

Certain elements in the profiles defined in this implementation guide are marked as Must Support. This flag is used to indicate that the element plays a critical role in defining, sharing, and implementing artifacts, and implementations SHALL understand and process the element.

In addition, because artifact specifications typically make use of data implementation guides (e.g. International Patient Summary(IPS), US Core, QI-Core), the implications of the Must Support flag for profiles used from those implementation guides must be considered.

For more information, see the definition of Must Support in the base FHIR specification.

Conformance Requirement 4.7 (Must Support Elements):

For resource instances claiming to conform to profiles from this IG, Must Support on any profile data element SHALL be interpreted as follows:

• Authoring systems and knowledge repositories SHALL be capable of populating all Must Support data elements.
• Evaluating systems SHALL be capable of processing resource instances containing Must Support data elements without generating an error or causing the evaluation to fail.
• In situations where information on a particular data element is not present and the reason for absence is unknown, authoring and repository systems SHALL NOT include the data elements in the resource instance. For example, for systems using ‘9999’ to indicate unknown data values, do not include ‘9999’ in the resource instance.
• When consuming resource instances, evaluating systems SHALL interpret missing data elements within resource instances as data not present for the artifact.
• Submitting and receiving systems using knowledge artifacts to perform data exchange or artifact evaluation operations SHALL respect the must support requirements of the profiles used by the artifact to describe the data involved in the operation.