VSAC vs Locally Authored ValueSets for US Quality Reporting

VSAC, the NLM-hosted Value Set Authority Center, is the canonical source for value sets used in US clinical quality measures. Locally authored value sets are the ones an organization writes itself when VSAC content does not quite fit a specific reporting need. Both have a place in a US quality reporting stack in 2026, and the choice between them is rarely either-or. Knowing where each fits saves time and audit trouble down the road.

This comparison walks through the VSAC versus locally authored ValueSet tradeoff for US quality reporting. The cornerstone FHIR terminology for US value-based care: a 2026 buyer's guide frames the broader context. For our FHIR coverage, the rest of the topic threads on this site go deeper.

What VSAC Gets Right

VSAC value sets have one big advantage: they are canonical. CMS quality measures reference VSAC value sets by OID and version, NCQA HEDIS digital measures pull from VSAC, and many payer-specific measures use VSAC content as a starting point. Using VSAC values directly means your reporting logic aligns with the reference implementations CMS and other regulators expect.

The price is rigidity. VSAC content changes on its own schedule, value set definitions sometimes shift in ways that affect measure logic, and the version pinning that quality reporting requires has to be managed carefully. An organization that loads VSAC content fresh each reporting period without version control is going to produce different results than a reference run, and that gap is where audit findings come from.

What Locally Authored Value Sets Get Right

Locally authored value sets give an organization full control of the content. A health system can author a value set that captures exactly the clinical concepts its internal quality measure logic requires. A specialty practice can author a value set that reflects the specific coding conventions its clinicians use. And a payer can author a value set that aligns with its specific medical policy.

The price is responsibility. Local value sets have to be versioned, audited, and defended in front of any external reviewer who asks how a measure was calculated. When the value set diverges from VSAC content, the divergence has to be deliberate and documented.

Where the Decision Usually Lands

Three factors dominate the VSAC versus local value set decision for US quality reporting.

Measure portfolio. CMS-published measures and NCQA HEDIS measures use VSAC. Internal quality improvement measures often use local value sets.
Audit exposure. Organizations under heavy payer or regulatory audit benefit from staying as close to VSAC as possible.
Engineering capacity. Local value set authoring requires both clinical and terminology expertise, which not every organization has on staff.

A typical US quality reporting stack ends up using VSAC for regulated measures, locally authored value sets for internal improvement work, and a clear governance process that decides which is which.

Common Failure Modes Worth Knowing

A few patterns recur in VSAC-versus-local missteps.

VSAC content gets reloaded without version pinning, and a measure that was performing well in November suddenly produces different results in January. Locally authored value sets diverge from VSAC content over time without anyone noticing, until an audit asks why. A measure mixes VSAC and local content without a clear rule about which wins when the two overlap.

The fix in each case is governance, not technology. Whichever the choice, document the version, capture the rationale, and review when CMS or NCQA publishes new content.

Where to Go From Here

For a related architectural decision often paired with this one, the FHIR terminology servers vs EHR vocabularies for Da Vinci workflows comparison walks through where terminology should live in payer-provider exchanges. The right answer is rarely all VSAC or all local; it is a clear policy about which fits where.