The Value Set Authority Center (VSAC) is considered the source of truth for high-quality value sets for use across various programs, such as Meaningful Use, CMS electronic Clinical Quality Measures (eCQMs), and Consolidated Clinical Document Architecture (C-CDA), among others. These value sets use terminology standards such as RxNorm, SNOMED CT, and LOINC which frequently update their code systems. When terminology standards publish new code system versions, VSAC consumes the new code system versions and generates updated code membership lists (expansions) of the value sets based on the latest rules (definitions) that each author used to define each value set.

Authors define the rules of the value set membership. VSAC refers to this as the value set definition. For extensional value sets, the definition identifies codes or concepts from a given code system.

VSAC refers to the list of codes or concepts that belong to a value set as the Expanded Code List of the value set, or simply expansion. The term 'expansion' refers to the process of applying a value set's definition against a specific code system version. Based on the value set's definition, the resulting expanded code list may change as the underlying code system updates its codes. For example, a drug company could develop a new drug that results in a new RxNorm code. Conversely, an existing drug could go off the market, and its corresponding code will become inactive in the next version of RxNorm, and thus excluded from the value set in the subsequent automatic update by VSAC. Codes that become inactive or retired due to a code system update may map one or more codes, as determined by the source terminology. Sometimes retired, inactive codes may have no corresponding remapped codes offered by the source terminology. Such events lead to code membership change in the value set, and result in a new value set expansion. For grouping value sets, a new expansion version reflects changes in member value sets due to code membership or definition updates.

VSAC consumes new code system versions as they become available through their corresponding sources. For example, RxNorm is inserted monthly. When VSAC receives the new version of the code system, it generates new expansions of the value sets. We refer to these expansions as 'Automatic updates by VSAC', and these are essentially recalculations of the value set code membership list based on the current definition.

Automatic VSAC expansions may significantly differ from the expansions intended for consumption by a specific program or measure, for example by the Meaningful Use program. Therefore, such updates are intended mainly as a snapshot of the dynamic code member changes in a value set and are not endorsed by respective value set Stewards for use within a certain program. Depending on the rules that govern a given program, Authors and Stewards should periodically update their value sets to bring them in line with changes that may have occurred in the underlying terminologies. As a result of this revision process, Authors and Stewards create new value set definition versions they approve for specific program use.

In the VSAC Authoring Tool, when Authors want to update a value set, they create a new definition version of the value set. This new definition version retains the value set OID, but assumes Draft status. While the value set is in Draft status, Authors should regularly check if a new code system version becomes available through the provided VSAC Authoring Tool interface. Authors should select the most recent available code system version to re-expand the value set code list to capture the most recent values in the terminology. VSAC creates a new definition version and a new expansion version for value sets only after Stewards approve and publish the Authors' revised and submitted value sets.

In the VSAC search interface, Authors can view value set expansion versions that have been automatically generated by VSAC, as well as value set expansion versions that have been Steward-approved for use by a program. Both types of value set expansion versions are identified by corresponding selections in the Available Updates pull-down menu as 'Approved by Steward' and 'Automatic updates by VSAC' (see snapshots below).

The selections in the secondary 'Expansion Versions' pull-down menu include official program-published value set expansion versions by the program's official release. The default Value Set Details view shows the most recent Approved by Steward expansion version.

Automatic updates by VSAC display as expansion versions formatted as the date of the update, e.g. 20150720 for July 20, 2015 (see the snapshot below).