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Clinical decision support rules clearly play a key role in the overall delivery of CDS. CDS rules follow a typical pattern or structure which becomes evident when rules are broken down into their individual parts. The structure of clinical decision support rules has been modelled in several health information standards and formalisms. The Event-Condition-Action1 model has become more prevalent recently, for example in the HL7 community. The components of this model are similar to other models for CDS rules and have been described below.


A CDS event is the clinical situation in which a decision support rule will be applied. First something must happen before the rule can be utilized. Examples of CDS events include: 

  • A clinician is prescribing a drug to a patient
  • A nursing supervisor is reviewing a list of patients previously diagnosed with cancer
  • A clinician is assessing a patient enrolled in a jurisdictional diabetes monitoring program


A CDS condition defines the question(s) that must be answered to determine the outcome of the rule. Examples of conditions include:

  • Has the patient been prescribed a medication containing a substance to which they are allergic?
  • Have any patients with a suspected cancer diagnosis NOT been referred to a specialist within 14 days of diagnosis?
  • Has the patient with a previous diagnosis of diabetes type II NOT had HBA1C tested within the last 12 months?


The CDS action describes what should be done in the event that the condition evaluates to true. Examples of actions include:

  • Alert the clinician and suggest a safe alternative medication
  • Refer patients to an oncology specialist
  • Order HBA1C test

Diagrammatic Representation

An informal representation or rule template which captures the Event-Condition-Action pattern is shown below. This pattern can be read as "ON event IF condition THEN action".

The following rule is a simple example of the the Event-Condition-Action pattern. This rule can be read as "ON clinical encounter IF diagnosis is asthma THEN display asthma management guidelines".

Rules and External Resources

Rules may reference both instance data in health records and reference data such as terminology to determine whether or not a specific condition is true.

This topic will be explored in more detail the section, Reasoning with SNOMED CT.

Ref Notes


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