1. Are you sure you need an extension?
Maybe your use case is already handled by resources or elements you’re unfamiliar with.
2. Does an extension already exist that meets your requirements?
An official extension, an unofficial extension, or a full-on Implementation Guide.
3. Is the use case for the extension likely to expand over time?
This may determine whether you create a single value extension or a nested extension.
4. Is there a requirement to run search queries on the extension values?
Custom search parameters need to be created — often more than one if the extension is complicated.
5. Will you want to validate the contents of the extension?
FHIR Profiles will have to be created for the resources that contain the extension.
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