Custom-built subsets: Reference sets in Snow Owl


SNOMED CT is big: it's the most comprehensive clinical terminology in the world. It's also been around the block: today's SNOMED CT has evolved from terminologies first created in the mid-1960s. Growth has come both organically over time, but also by acquisition through mergers of other terminologies. The result is that some areas of SNOMED CT have inconsistent levels of detail. (A favorite example is Atomic power plant malfunction in watercraft, occupant of small unpowered boat injured.)

Therefore it's often necessary to extract a smaller collection of concepts from SNOMED CT. This is where reference sets come in handy. They are purpose-built mini-versions of the terminology that contain only the terms that are needed in a specific context, for example all terms needed in an emergency department or for cardiologists.

Snow Owl allows you to create and maintain different kinds of reference sets. They are named from the SNOMED CT Release Format 2 specifications, which are described in detail in the IHTSDO's Technical Implementation Guide .

When working with reference sets you will primarily use the

Reference sets view

Existing reference sets are displayed in the Reference Sets view on the right side of your screen. The shop icon will make it easy for you to distinguish the Reference set view from other views, for example the SNOMED CT view or the Parent view.

If you want to check out a few reference sets (e.g. mental health, neurology), go to the B2i examples folder in the Reference Sets view.

Reference Sets view

Reference set editor

Double-clicking an reference set will launch the reference set editor (in the middle of your screen). The icon in the header indicates the type of reference set (e.g. the icon for a simple type reference set).

Reference set editor with referenced components on the left side and information about selected component on the right side

The reference set editor is pretty easy to understand since it contains components from the SNOMED CT concepts view and the SNOMED CT concepts editor that you are already familiar with:

Did you know? Rather than a flat list of all reference sets for a particular type, reference sets are grouped by their parent. This is useful to create a reference set to group several child reference sets for easier navigation.

Different types of reference sets for different purposes

There are three types of reference sets used for creating subsets. They can be easily recognized by their dedicated icons:

Simple type

The simple type reference set is a plain grouping of concepts by user preferences. We will show you how to create one later in this chapter.

Simple type reference set editor

Attribute value type

An attribute value type reference set allows associating a value concept with the referenced component. It can be used to extend the ontology with custom properties on the concept.

Attribute value type reference set with descriptions as members

Query type

The members of a query type reference set are determined based on a semantic query.

Query specification type reference set editor

Did you know: Query type reference sets and the generated simple type reference sets can be automatically updated. This is useful when the underlying terminology changes, for example when a new version of SNOMED CT is released. The action can be applied either on an individual intensional reference set or on the entire set of query type reference sets and will refresh all the generated simple type reference sets.

Update reference set to current ontology action

Creating and maintaining reference sets

Almost magic: Creating new reference sets with Snow Owl


The reference set wizard will guide you through the process of creating a reference set. To launch it, pick one of the icons on the main tool bar. Each type of reference set has a different icon.

Alternatively, you can right click on any of the main type categories in the Reference Set view. This will will also open the reference set wizard of the appropriate type. It can also be invoked with File > New.

New reference set wizard

Creating a new simple type reference set: Step by step

As an example, we want to you show you how to create a new simple type reference set.

1. Click the icon on the main tool bar to launch the wizard.

2. Type a reference set description (here: Shoulder Reference Set) and select the referenced component type (here: SNOMED CT Concept). The wizard automatically creates a reference set identifier concept with the title as a description. Click "Finish" to proceed.

New simple type reference set wizard

3. You may enter a commit comment in the subsequent dialog, then click "OK" to open the Reference set editor.

4. Your new reference set will appear in the reference set editor. The reference set description is displayed in the title. The list of reference set members is empty since there were no referenced components added yet.

Reference set editor

5. To add referenced components go to the SNOMED CT View and choose a concept from the hierarchy (here: Finding of shoulder region). Right-click the concept and choose Add concept and descendents to the active reference set from the list. This will add the concept and all of its children to the reference set. If you want to add only this particular concept use Add concept to the active reference set.

Adding a concept and its children to a reference set

6. The concept is now displayed in the reference set editor. The " matching members" indicates the number of components in the reference set. The small * asterisk in front of the editor title indicates unsaved changes.

Reference set with newly added components

7. To save your reference set, use the icon in the main tool bar. The new reference set will also be incorporated in the hierarchy of the reference set view as a "simple type reference set".

Other ways of adding new members to a reference set