The most visible hierarchical construct in SNOMED CT is the subtype hierarchy. This is constructed using a set of logical rules.
Figure 5.5.1-1: Matches shown in the subtype hierarchy
The Concept "Laparoscopic emergency appendectomy" can be reliably located by subtype navigation from any of its supertypes: "appendectomy," "laparoscopic appendectomy" or "emergency appendectomy."
It is possible to start at the top of hierarchy and navigate from parent to child in order to find a Concept or term in SNOMED CT. A more efficient approach, however, is to use the hierarchy to supplement a keyword search by enabling the user to look at related Concepts in order to consider them as alternative matches, or to check the context of a search result. The following examples illustrate these two uses of the SNOMED CT hierarchy.
A user wishes to find a Description that relates to the condition of a patient who is hypersensitive to an allergen. The user performs a search on the keyword "Hypersensitivity" and finds an exact match. Before the user selects the Description for inclusion in the patient record, they check the Fully Specified Name, which is | Sensitivity (finding) |. The user then checks the hierarchy and discovers that the selected Concept has | Psychological finding | as an ancestor, which indicates that this is not the correct Description to use in this context.
A user wishes to find a Description that relates to the condition of a patient who is hypersensitive to an allergen. The user searches for the keyword "allergy," and finds one Concept having a Description that is an exact match. The user then looks at the children of the Concept (i.e. those Concepts immediately below it in the hierarchy). One of the children has the preferred Description| Contact Hypersensitivity | which matches the user's intended meaning. The user selects this Concept for inclusion in the patient record.