The VSAC versus custom ValueSet decision is one US health systems quietly fight about every year. The Value Set Authority Center publishes the canonical CMS-driven value sets for quality measures, ONC reporting, and increasingly for USCDI conformance. Custom ValueSets are the ones your own clinical and analytics team builds and maintains for the long tail of internal workflows. Picking which one to use for which workflow is a small decision that has a long shadow.
This walkthrough lays out the trade-offs concretely. For our digital health coverage, the homepage covers more of the terminology landscape.
What Each Source Actually Provides
VSAC is the NLM-operated authority that publishes value sets tied to CMS quality measures, ONC certification criteria, and a growing set of USCDI-related artifacts. The value sets are versioned, the codes inside them are anchored to specific releases of LOINC, SNOMED CT, RxNorm, ICD-10, and the other major US code systems, and the publication cadence is tied to CMS reporting cycles.
Custom ValueSets are the FHIR ValueSet resources your own team defines. They cover the local rollups, the internal clinical groupings, the network-specific service lines, and the analytics buckets that no national authority is going to publish on your behalf.
For the broader picture, the FHIR terminology servers buyer's guide covers the underlying capability set.
Where VSAC Wins
VSAC has three structural advantages for US healthcare workflows. The value sets carry CMS authority, which is the bar that matters for quality measure reporting and for regulatory conformance. The versioning is rigorous, which is the bar that matters for effective-dated lookups. And the maintenance is somebody else's job, which is the bar that matters for any team that has more clinical priorities than terminology priorities.
The cost shows up in flexibility. VSAC value sets are what they are; if a clinical leader at your network wants to define rheumatology service line membership differently from the CMS definition, the VSAC value set is the wrong tool for that job.
Where Custom ValueSets Win
Custom ValueSets win whenever the rollup is specific to your network. Internal service line definitions, locally-defined risk stratification cohorts, network-specific quality programs, and analytics buckets that do not match a CMS measure all need custom ValueSets. The maintenance is your team's job, but the value sets reflect the clinical reality at your network rather than the regulatory reality at CMS.
The honest weakness is the maintenance burden. Each custom ValueSet has to be reviewed against new code system releases, the membership has to be revisited as clinical practice changes, and the audit trail has to be defensible against an internal review.
How to Decide
A handful of questions usually settle the decision.
- Is the value set tied to a CMS or ONC reporting requirement? If yes, VSAC is the right choice; do not duplicate work the authority has already done.
- Is the value set used by an internal clinical or analytics workflow that no national authority addresses? If yes, custom is the right choice; do not force-fit a VSAC value set.
- Is the value set part of a quality measure that will be audited externally? If yes, VSAC sourcing makes the audit easier.
- Does the value set need to track a specific code system release that VSAC has not yet published? If yes, custom is the right choice for the interim.
Most US health networks end up running both: VSAC sourcing for reporting-bound value sets, custom for everything else. The shared FHIR ValueSet format makes the dual-source pattern cleaner than it sounds.
For the operational side, the Top 6 terminology tools for CMS quality measure reporting guide covers the tools that handle the VSAC ingestion and the audit logging together. Most US health networks find that the right answer is sourcing what the authority has already published and reserving custom work for the parts the authority is unlikely to ever address.
Sources
- Value Set Authority Center (VSAC) home - HTML portal, NLM/ASTP/CMS
- VSAC overview - HTML tool page, ONC eCQI Resource Center, 2025
- US Core Terminology page - HTML IG section, HL7 US Realm SC, 2025