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 CDS condition defines the question(s) that must be answered to determine the outcome of the rule. Examples of conditions include:
The CDS action describes what should be done in the event that the condition evaluates to true. Examples of actions include:
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 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.