A fundamental need throughout a CDA document is the ability to use unique identifiers.

The preferred form of identifier in all HL7 standards is something called an Object Identifier, which is commonly abbreviated OID. In CDA, OIDs are used for code systems, template identifiers, and any identifier created by HL7.

OID Format
OIDs are written as a sequence of numbers separated by periods, such as: 2.16.840.1.113883.

OIDs can use any number in each of its “number components” (separated by periods) and can have any number of those components – although there is an implementation limitation to keep the total OID display representation under 64 characters. These “number components” of an OID are often termed “arcs”.

OID Management
OIDs are unique by virtue of the mechanism for distributing ownership of the responsibility for managing OIDs.

Subsets of a complete OID that start with the first “arc” on the left, can be thought of as “OID prefixes”. An “OID prefix” starts on the far left and includes some number of “arcs” from left to right. Each “OID prefix” is itself an OID. Whenever an OID is extended with another arc on the right, the original OID becomes a “prefix” of the new OID.

The first “arc” on the left in an OID is the top-level prefix. Top-level prefixes are assigned to global standards organizations such as ISO.

When an organization is assigned an OID, it is allowed to extend that OID any way it wants. In other words, using the OID it was assinged as an “OID prefix”, the organization that was assigned the OID is allowed to create as many new OIDs as it wants that begin with that prefix.

It’s up to the organization that was assigned the OID to decide what conventions it wants to use for any new OIDs it creates (including those it assigns to others).

For example, HL7 was assigned the OID 2.16.840.1.11383. It has created thousands of OIDs that start with that prefix. Two examples:

  • 2.16.840.1.113883. is the OID for the “Progress Note” CDA document template
  • 2.16.840.1.113883.5.1076 is the code system for religious affiliation used by HL7

More generally, HL7 sets its own conventions for managing its OIDs. For example:

  • 2.16.840.1.11383.5 is the prefix for HL7 code systems
  • 2.16.840.1.11383.10 is the prefix for HL7 templates

OID Assignment
As noted, the organization assigned an OID can create a new OID that extends the one it was assigned, and then assign that new OID to another organization. The result is an iterative process in which an organization is assigned ownership of an OID, uses that OID as a prefix to create new OIDs, optionally assigns some of the new OIDs it creates to yet another organizations, and those organization then repeat the same process…

For example, HL7 took the OID that it was assigned (2.16.840.1.11383) and decided to extended it to create a new OID 2.16.840.1.11383.3, that serves as a prefix for additional OIDs that HL7 creates and assigns to others. HL7 assigned the OID 2.16.840.1.113883.3.3208 to CDA PRO. CDA PRO now uses that OID as a prefix for OIDs that it creates and, optionally, that it may assign to others.

OID Management Tools
Once an OID is assigned by one organization to another, the assigning organization can no longer track which additional OIDs are created by the assigned organization, or what new OIDs the assigned organization will further assign to other organizations.

There is no single central management of all OIDs.

At http://www.oid-info.com you can learn more about OIDs and explore the assignment of top-level OIDs (which in turn are OID prefixes for all other OIDs).

At http://www.hl7.org/oid/index.cfm you will find the HL7 OID registry which catalogs OIDs that HL7 created with its assigned OID used as a prefix, including OIDs it created as extensions of its OID, and assigned to others. Anyone can receive an OID from HL7 using that web site.

OIDs in CDA Documents
Refer to the article Identifiers in CDA, for information and examples about how OIDs are used in CDA identifiers.

Refer to the articles GUIDs and UUIDs? and GUID/UUID or OID?, for a discussion of the other common form of identifier in CDA documents (GUID/UUID) and its advantages and disadvantages relative to OIDs.

Refer to the article How should we manage our OIDs?, for some recommendations on how organizations should manage the OIDs that they create using their assigned OID as a prefix.

Other CDA PRO Know Articles Referenced In This Article