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This section presents some examples of standards used to represent CDS rules. Please note that this list is not exhaustive, and other established and emerging standards for rule representation do exist.

Expression Constraint Language

The SNOMED CT expression constraint language (ECL) provides a computable way of intensionally defining a set of clinical meanings represented in SNOMED CT. For example, the expression constraint below represents the set of lung disorders that have an associated morphology that is a type of edema.

When executed against a specific SNOMED CT edition, an expression constraint will return the set of concepts that match the given constraint. Expression constraints can also be used to query over precoordinated and postcoordinated expressions recorded in EHRs.

SNOMED CT expression constraints provide a standard way of referring to intensionally defined sets of SNOMED CT concepts (or expressions) that are required to test CDS rule criterion. Examples of CDS rules that use SNOMED CT expression constraints can be found in  3.1.3. Rule Examples.

For more information about expression constraints, please refer to  Expression Constraint Language - Specification and Guide.

Arden Syntax 

The Arden Syntax is a widely-used and mature markup language for representing, sharing, and processing clinical knowledge, which makes it suitable in the application of expressing rules for use in decision support. The syntax has a long history, but is currently maintained by HL7 International. One of the advantages of the ARDEN syntax is improved human readability which is achieved by its resemblance to natural language. This in turn makes ARDEN code easier for non-technical audiences to interpret.

When used in CDS, Arden code can be embedded in independent files called medical logic modules (MLMs).


The improved readability of Arden syntax makes it easier for a clinician to validate the clinical accuracy of any given MLM. MLMs have been widely used and libraries of these modules are available. It is also worth noting that the Arden syntax does not define how it should be integrated within an electronic health record or how an application should use it.

For more information on the Arden Syntax, please refer to the HL7 Implementation Guide for Arden Syntax, Release 1.

FHIR CDS Resource

PlanDefinition is a general FHIR resource which can be used to represent a range of CDS artifacts such as rules, order sets, and protocols.


According to the HL7 FHIR specification, a resource contains a set of structured data items that conform to the definition of the resource type and can be used to exchange and/or store data to satisfy a wide range of clinical and administrative healthcare information needs.


PlanDefinition is currently defined as a draft resource within FHIR's Clinical Reasoning module. The Clinical Reasoning module is a draft of the Clinical Quality Framework Implementation Guide (or FHIR-Based Clinical Quality Framework). The guidance in this module is prepared as a Universal Realm Specification, which means it is designed to be used Internationally.

The PlanDefinition resource can be used to represent a rule using the Event-Condition-Action pattern. This pattern is defined within the actionDefinition element of the PlanDefinition resource. "A single, top-level actionDefinition represents the overall rule, with the triggerDefinition element used to specify the triggering event(s), the condition element used to specify the applicable condition for the rule, and the actionDefinition itself describing the action to be performed."


The PlanDefinition resource is used to describe series, sequences, or groups of actions to be taken, while the ActivityDefinition resource is used to define each specific step or activity to be performed. An example of an XML instance of a PlanDefinition resource that encapsulates a Chlamydia Screening rule is shown below.

Figure 3.1.4-1: Example PlanDefinition XML instance

For more information on this FHIR resource, please refer to or

Ref Notes

The Arden Syntax standard for clinical decision support: Experiences and directions, Samwald et. al




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