A universally unique identifier (UUID) is a 128-bit label used for information in computer systems. The term globally unique identifier (GUID) is also used.

OIDs that need to be referenced from other files like a Study OID could be represented with an UUID or other mechanism that renders a unique ID (e.g., ClinicalTrials.gov Identifier of the study “ClinicalTrials.gov.NCT12345678”; NASDAQ’s #: (“nasdaq<#>/STUDYXYZ”). 

Uniform Resource Identifiers (URI) or Internationalized Resource Identifiers (IRI) are also useful mechanisms for referring to external entities; some examples are:

• https://clinicaltrials.gov/ct2/show/NCT12345678 (referring to a study registry reference)
• http://snomed.info/ns/concept/24762387 (referring to an external concept in the SNOMED vocabulary)
• https://loinc.org/78200-3/ (referring to an external concept in the LOINC vocabulary)

The preference is to use persistent URIs for these contexts (i.e. URIs that should exist so any future references can be followed and understood).

NOTE: when using these concepts in an evaluable context, the recommended approach is to use the Coding Element.