Situational Awareness for Novel Epidemic Response
0.1.0 - STU Ballot

Situational Awareness for Novel Epidemic Response, published by HL7 International Public Health Workgroup. This is not an authorized publication; it is the continuous build for version 0.1.0). This version is based on the current content of and changes regularly. See the Directory of published versions

Computing Measures

Several studies have shown that automation and standardization for electronic reporting improves data quality, timeliness, and the quantity of available data in public health reporting 12. Developers of a system focusing on reporting for pandemic management in British Columbia note the challenges of one-off reporting solutions, including operational disruptions to address changing needs3.

Among institutions, barriers to reporting include having available systems to report to, and challenges in interfacing to facility Health IT infrastructure and extracting data from EHR systems4.

Thus, measures should be automatable where feasible, and integratee with existing Health IT system capabilities. Automation of measurement is possible when:

  1. There is a common information model:
    1. A commonly understood information model and terminology that enables data collection requirements to be specified consistently across disparate systems, or
    2. A general information model which can be eaily adapted via local translations.
  2. There are common workflows that will not significantly impact the interpretation of data. For example, the workflow for reporting that a patient is on a ventilator could rely on orders for ventilation, or on flowsheet observations related to ventilator reported measurements.

  3. There are mechanisms to query for information from systems.

Common Information Model

This IG requires adoption of a national model or implementation guide to ensure that there is a common model for a region. An incomplete list of national models is provided below:

Use of these models ensures consistent use of terminology and approach for recording data within a country that enables broad use of the same Measure across the states, provinces, territories, or other regional divisions.

Localizing Value Sets for Implementation

The mechanisms chosen for automation in this guide use value sets for critical vocabulary used to identify relevant resources for computing measures. This use of value sets enables an implementation to replace a value set defined based on the national implementation model for a region with translations to local codes to simplify localization.

The value sets used to automate the computation of a measure must be available to systems that are computing the measure. These shall be recorded in Library.relatedArtifact in at least one of the library components referenced by the Measure.

Data Access for Computation

Data access is encouraged through one of three mechanisms, all of which rely on core FHIR search capabilities. These mechanisms are described in order from lowest to highest implementation complexity.

  1. FHIR Search is the basic capability supporing automation. Applications can support counting by using FHIR queries to select appropriate data elements for evaluation, and then compute measures based on the responses. This is the most limited and “chatty” of mechanisms supporting integration, as many servers to not provide search capabilities supporting query across resource boundaries using different kinds of joins (e.g., chaining and _has search capabilities). Combining FHIR queries with other FHIR capabilities such as Bulk Data Access or R5 Subscriptions can make this a more efficient process, but integrating with these specifications is beyond the scope of this implementation guide.

  2. FHIRPath is a standalone specification that works with hierarchical models in structured representations (e.g., JSON) originally designed for use with FHIR. It is similar in form and capability to XPath, but has application to more general models than those represented by an XML document. When FHIRPath is used within a FHIR Server context, the language includes capabilities which support automation of queries, and resolution of resources matching the specified path.

  3. Clinical Quality Language is an language designed to automate the computation of measures from FHIR and other data models. It enables localization of models to support variations in organizational workflows and data models. Several open source implementations of CQL interpreters are available, but this technology has not yet reached the maturity of other sorts of systems, and it is not always widely available for users of systems supporting FHIR.

While measures conforming to this guide may use any of the above in expressions, FHIRPath is the preferred format because open source implementations of FHIRPath are generally available for multiple platforms including Java, JavaScript, .Net, and it supports most of the necessary functions to support comparisons for measures, including measures with complex date relationships (e.g., Patients acquiring COVID-19 14 days after admission).

A measure shall use the same language for all population criteria in the measure. A measure should use the same language for all stratifiers in the measure.

Measure Definition Criteria

A Measure is defined by the computable criteria contained in definitions for the Measure population, stratifier and other criteria components within the measure. These criteria elements are defined using the Expression datatype. This datatype requires the language used for the criteria to be defined using one of the following values (cooresponding to mechanisms listed in the list above):

  1. application/x-fhir-query
    Expressions defined using FHIR Search are defined by the URLs allowed by FHIR Search. The URLs provided in the Expression.expression field shall be provided as relative to the base address of the server that would be queried, and shall be expressed in URL syntax according to RFC 3986 with appropriate %-encoding of the URL components.
    Conforming systems may, at their own discretion apply Postel’s law in regard to the formatting of URLs. Such expressions should use search capabilities defined by national models. Extended search parameters are permitted, but these must be defined in a SearchParameter available in one of the libraries referenced by the measure.

  2. text/fhirpath
    Expressions defined in FHIRPath shall conform to FHIRPath and are permitted to use the FHIRPath functions and syntax elements defined specifically for FHIR.

  3. text/cql
    Expressions defines in CQL shall conform to the Clinical Quality Language, and are permitteed to use CQL modules referenced by one of the libraries referenced by the measure.

Implementation and Conformance

The computable criteria found definitions for the Measure in population, stratifier and other criteria components within the measure are deemed to be the “normative” definition of the measure. However, measure developers may wish to provide alternative implementations to support application environments that cannot support FHIRPath, or which have access to a high quality CQL engine. These definitions can be supplied using the Measure Population Alternate Criteria extension. This extension allows alternate criteria to be supplied which can support evaluation on systems not having support for the preferred (and normative) specification for the measure.

Other Measure Definition Content

A computable public health measure may reference ValueSet, ConceptMap, CQL definitions, SearchParameter and other FHIR conformance resources to support automation. The PublicHealthMeasureLibrary defines the constraints relevant to referencing the FHIR resources needed to automate measure computation.

Automated Measure Computation

The process for measure computation relies on several preconditions:

  1. The measure establishes the initial context for evaluation by defining libraries, value sets, code systems or other resources necessary for automating the computation. These can be found in the Library resources defined by the measure.
  2. A measure is broken up into groups, populations and strata which facilitate evaluation.
  3. A group evaluates a measurement associated with a specific kind of resource or event, e.g., a patient, an admission or patient encounter, a lab result, et cetera. This is defined in the groups of the measure as described in the Measure Group Attributes extension.
  4. Populations are evaluated based on queries of resources associated with the group, and may depend on other resources defined within the context of the measure. Populations may be defined by refining other populations within the group or measure, but at least one population (normally the initial population) does not have any unresolved context references. The result of an evaluation is either a collection, in which case the population count is computed based on the number of elements in the collection, OR it is a singular numeric result or quantity, in which case the count is the numeric value.
  5. Once a population has been evaluated, it establishes a element within the measure context that uses a computable name established by the criteria. This element may enable the evaluation of other populations with the measure. This element contains the collection or singular numeric result or quantity returned by the evaluation process.
  6. Having evaluated a population, the strata within the population may be computable, but could reequire evaluation of other populations. Evaluation of strata follow a similar pattern to evaluation of populations.

The general algorithm is:

Given a Measure, find a population or strata within a group of the measure that does not reference an unresolved contextual element. Evaluate that population or strata and update the context with the evaluted result. If the population has strata without an unresolved contextual reference, evaluate its strata.

If there are no more unevaluated populations or strata, then evaluation is complete. There should not be a population or strata that cannot be resolved because of a missing dependency. If an implementation detects a case where a population or stratum cannot be evaluated, it should report an evaluation error. It may populate the unpopulated components of the measure (e.g., count or score) using the data-absent-reason extension, indicating that the value is unavailable due to an error in order to support reporting of partial results while the error condition is being corrected.

Resolving Parameters and Computed Content

Named parameters are essential to support automated measure evaluation. They are used to constrain queries using FHIR Search, FHIRPath or CQL in order to limit the data retrieved to that which is relevant for measure computation. The names of parameters used in Measure resources conforming to this guide shall start with an upper case letter, and may contain lowercase letters and numbers, and may contain a perdiod to must match the regular expression [A-Z][A-Za-z0-9.]+. They should be in PascalCase.

FHIRPath and CQL provide mechanisms to provide named parameters (e.g., reporting period) and collections of FHIR resources during their evaluation.

  • FHIR Search
    To support named parameters in FHIR Queries, this guide uses the notation @Name to indicate a named parameter in a FHIR Query. NOTE: @ is general only found in e-mail addresses, which are not relevant for queries used in this guide.

  • FHIRPath
    Parameters in FHIRPath are supported via FHIRPath Environment Variables. This will be shown as %Name in this guide.

  • CQL
    CQL provides for named parameters. This guide uses the notation Name to indicate a named parameter in a CQL Expression.

Parameter Types

Parameters have a data type, either a FHIR Primitive type such as date, dateTime, or string, or complex types such as Coding, Quantity, Period or Resource and any of its subtypes (e.g., Patient, Encounter). The fields of the parameter are accessible via dot notation.

For example:


Will produce a query that selects those encounters which are either active or which finished during the reporting period.

Parameters Defined in this Guide

The following parameters are predefined by this guide:

Server Details
The Base parameter is of the FHIR string data type and contains the base address of the FHIR Server associated with a Measure Computer. The Measure Computer must supply this information. Note that this base endpoint shall not end with a “/”. This may be used to specify FHIR queries that should be resolved by the FHIRPath engine within the expression. This shall be the same value found in the implementation.url field of the server’s CapabilityStatement.
The Metadata parameter is of the FHIR CapabilityStatement resource type, and contains the Capability statement that would be resolved by the CapabilityStatement resource. The Measure Computer must supply this information. This shall be the equivalent of what is returned by (%Base + “/metadata”).resolve().
Measure Details
The Measure parameter is of the FHIR Measure resource type, and represents the measure being computed.
The MeasureReport parameter is of the FHIR MeasureReport resource type, and represents the current MeasureReport that is being computed.
The PriorReport parameter is of the FHIR MeasureReport resource type, and represents the details of the most recent version (e.g., including any corrections) of the MeasureReport that was computed for the prior reporting period. This simplifies computations for cumulative results, and enables reuse of infrequently changing values (e.g., total ventilators).
The Reporter parameter is of the FHIR Organization resource type and represents the reporter of the MeasureReport. A reference to the reporter can be found in %MeasureReport.reporter.
The Subject parameter is of the FHIR Location resource type and represents the subject of the MeasureReport. A reference to this subject can be found in %MeasureReport.subject.
Date Parameters

Date parameters are essential to support appropriate filters for queries. This enables essential dates such as the date of evaluation and the reporting period to be communicated correctly when evaluating the measure.

The ReportingPeriod parameter is of the FHIR Period data type and contains the starting (inclusive) and ending time (exclusive) of the reporting period. It is determined based on the current reporting period as defined in the ReportingPeriod extension appearing within the Measure. If the reporting period is daily, midnight to midnight, and report is being generated “today”, then the reporting period for it is “yesterday”.
The PriorReportingPeriod parameter is of the FHIR Period data type, and contains the starting (inclusive) and ending time (exclusive) of the immediately prior reporting period. It is determined based on the current reporting period as defined in the ReportingPeriod extension appearing within the Measure. If the reporting period is daily, midnight to midnight, and a report is being generated “today”, then the prior reporting period for it is “the day before yesterday”. The Prior Reporting period is helpful for obtaining data from prior measures, for example, to support reporting of cumulative totals.
This parameter is the “as of” date for the report and is of the FHIR Date data type. This is functionally equivalent to the FHIRPath today() or CQL Today() functions, save that it is the “date of reporting” rather than the current time stamp. It should be used for calculating age of events that are based on the date of reporting. Generally, this is the day after the reporting period ends for daily measures.
This parameter is the day after the “as of date” and is of the FHIR Date data type. It works like the FHIRPath today() function, but returns the day after today() with respect to the date associated with the report.
This parameter is the day before the “as of date” and is of the FHIR Date data type. It works like the FHIRPath today() function, but returns the day before today() with reespeect to the date associated with the report.
Library Resources

Attachments in Library resources referenced by a Measure (through Measure.library) are also available as parameters using the name given to the Attachment in the element (see the notes on Internal Id Scope for Elements in FHIR).

NOTE: The use of to provide a computable name for the component referenced by the measure addresses the issue that while ValueSet and other definition resources include a “computable name” intended to support automation. This name is not appropriate for several reasons:

  1. Value Set developers and publishers do not follow the recommendations for computable names, which results in value sets (and other definition resources) having names that include spaces and other special characters (e.g. “COVID_19 (Organism or Substance in Lab Results)”).
  2. Measure developers may rely on value sets from multiple sources, which can result in value set names which conflict. However measure developers can control the names used in the Library resources they use to support a measure.

This guide chose to use a name specified in the id element of the resource because these names

For a give Measure resource, if Measure.library[0].content references a ValueSet resource, Measure.library[0] is “ReferencedValueSet”, then @ReferencedValueSet is a parameter that can be used in a FHIR Query string. It is available in a FHIRPath expression as %ReferencedValueSet, and in CQL simply as ReferencedValueSet. These parameters are of the type of the referenced resource.

If “VentPatients” is the id given for an attachment referencing the Observations for Patients on a Ventilator value set, then @VentPatients may be used in a FHIR Query as follows:


This query will select all observations whose code in the Value Set referenced by the Attachment whose id is “VentPatients”.

The use of value sets in writing measures for automation is critical to avoid mistakes in entering long lists of codes, but pragmatically, many FHIR implementations will not support the :in or :not-in modifiers. A MeasureComputer implementing queries shall translate these into the appropriate notation using the FHIR notation for combining query parameters for servers which do not understand :in and :not-in.

Servers can declare support for :in and :not-in by fully defining the search capabilities in referenced SearchParameter resources, but in practice, few do. A MeasureComputer should either verify that :in and :not-in modifiers are supported (e.g., by executing test queries), or simply translate for all cases.

Population Parameters

Populations are often computed iteratively. For example, it is common to first identify the initial population via a search, and then the compute the denominator by filtering results from the initial population matching a given criteria, and finally, the numerator by filtering results from the denominator.

These components can be used in the criteria of other components using FHIRPath or CQL (but not when using FHIR Queries). To assign a name to the one of these components, assign an name value to expression defining the population.

Illustrating the Use of Parameters

The example below illustrates the use of three different kinds parameters in evaluation:

           <name value='InitialPop'/>
           <expression value='Encounter.where(...)'/>
           <name value='VentilatedPatients'/>
           <expression value='Observation.where(
                encounter = and
                code.memberOf(%VentPatients.url)).patient and
                effectiveDateTime >= %ReportingPeriod.start and effectiveDateTime < %ReportingPeriod.end'/>


The criteria for the population of the first group of a Measure has the name value of “InitialPop” in the example above. References to %InitialPop in other FHIRPath expressions will resolve to the value returned by execution of criteria.expression where = “InitialPop”. So, the example above:

  1. Selects a particular set of encounters for the initial population, and defines the variable %InitialPop to reference it by.
  2. It defines a second population in terms of the initial population, selecting the distinct Patients that have observations that reference encounters found in %InitialPop identified in the previous population,
  3. including only those Observations having a code in the ValueSet represented by %VentPatients
  4. and which occur during the predefined %ReportingPeriod parameter.
  5. Finally, these patients will now be addressable as %VentilatedPatients in other populations in this measure.

For CQL, this value will be a named parameter in the execution of other CQL Expressions.


  1. Within a measure, values in population.criteria.expression which are expressed in FHIRPath may use:
    1. Variables predefined by FHIRPath, e.g., %ucum, %context
    2. Variables predefined by FHIR in its use of FHIRPath, e.g., %sct, %loinc, %”vs-[name]”, %”ext-[name]”, %resource
    3. Variables explicitely defined by this guide, e.g., %ReportingPeriod, %Today, %Tomorrow, %Yesterday
    4. Variables which have been named in any Libary.content item referenced by the Measure, e.g., %VentPatients
    5. Variables which have been identified in other elements found in the Measure. e.g., %InitialPop, %VentilatedPatients
  2. Any other parmeters found should be reported as a warning, as implementation may agree on additional parmeters.
  3. No two parameters shall be provided in the context of a single measure with the same name having different values. NOTE: The same resource may be referenced by two different libraries. So long as the URL and content of the resource is the same, the name given to it in the referenced measure libaries may be identical.
    1. More specifically, a Library shall not name a resource with any of the names defined by items 1-3 in rule 1 above,
    2. A Measure shall not name a criteria with any of the names defined from items 1-4.
    3. Parameter Names shall not be duplicated within a measure or library.

Implementation Strategies

The MeasureComputer actor is free to use whichever search strategies best fit. Implementers should remember the constraints on Measure Populations to ensure correct evaluation (e.g., an item in the denominator must be present in the initial population).

While expressions are written in a way that supports use of FHIR Search, a MeasureComputer is NOT required to use FHIR Search capabilities depending upon how it is implemented. It may perform queries using FHIR Bulk Data, FHIR Batch mode queries, internal data access mechanisms such as those found FHIR Server implementations, or through integration with CQL or FHIRPath engines supporting internal server access.

Measure Computer implementations should consider strategies for data access that avoid overwhelming a connected FHIR Server with repeated requests for large amounts of data. Depending on the implementation, it may be more efficient to query using easy to evaluate criteria that returns more than what the measure is looking for, and then filter the results after they have been returned to the Measure Computer.

Measure developers should consider writing evaluation criteria in ways that simplify implementation. For example, when using FHIRPath, the first part of the expression can be of the form:

     (%Base + '/Resource?_include=Resource:*'
            + '&status=allowed-status-values'
            + '&date=ge' + %ReportingPeriod.start
            + '&date=lt' + %ReportingPeriod.end
            + 'other search criteria'


This enables the Measure Computer to start evaluations with data automatically extracted via a FHIR Search. The Resource should name the type of resource to query for, and search criteria should be a widely supported search that limits the resources being returned for subsequent filtering by the remainder of thee FHIRPath expression. Consider the use of date and/or _lastUpdated parameters to restrict the data to the time period relevant to the search.

The second part of the expression should be a where() or select() clause which filters out or projects to other relevant content. This where clause can make use of the FHIRPath [iif()] function to support short-circuit evaluation of logical expressions.

Additional retrievals can appear in this clause, but should be done after other filtering that can be performed using available data through FHIRPath. This helps to eliminate excessive queries to the server.


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