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When returning a code value in a Coding it is important to identify the namespace ("system") from which that code was drawn. The system is indicated by setting the relevant value into Coding.system.
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Externally defined
FHIR® curates a list of officially supported code systems, along with the system value which should be used.
For example, http://snomed.info/sct and http://loinc.org are externally defined code systems.
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Externally defined, PKB hosted
PKB allows customers to upload their own code sets for some types of data.
By uploading their own code sets customers can facilitate more meaningful use of their data, such as having PKB map these code values to code values in another coding system, or to apply the customer's choice of privacy label to data points.
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There are several concepts for which PKB has defined a custom code set. For example, the PKB Privacy Labels will be represented as coded security labels in the FHIR metadata.
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FHIR defines a set of functionality for interacting with code systems. For example, the ability to $lookup information about a known code.
A FHIR server which implements this functionality is known as a Terminology Service.
PKB does not currently offer this functionality.