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Clinical decision support rules play a key role in the overall delivery of CDS. CDS rules typically follow a common pattern, which has been modeled in several healthcare standards formalisms. The Event-Condition-Action


model, as used in the HL7 community, is 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:

  • Does the usual drug of choice for this patient's condition contain a substance to which the patient is 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 doneif 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

Event-Condition-Action Model

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".

Figure 3.1-1: Event-Condition-Action rule template

Rules may reference both EHR data and reference data such as terminology to determine whether or not a specific condition is true. This topic will be explored in more detail in section 4. Inference Engine.

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