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What the All of Us FHIR-vs-OMOP Study Actually Measured

· 9 min read
Joel Montavon
FHIR4DS

A 2026 study from the All of Us Research Program compared the same 29,512 participants' data as OMOP tables and as FHIR R4 payloads — Patterson et al., JAMIA 33(9):1593–1604 (doi:10.1093/jamia/ocag095). The headline numbers are stark:

Median events per personOMOPFHIR
Measurement846832
Condition25811
Procedure724
Participants with visits/encounters97%1.5%

The conclusion many people draw — OMOP is richer, FHIR is not ready for research — deserves a harder look, because the study itself tells you exactly what it measured. It is careful, honest work — and with AMIA ahead, it is likely to shape how people talk about FHIR analytics. Here is what it did and did not show, and the concrete answer to the barrier it identified.

What the study actually compared​

The two data streams reached the researchers through very different pipelines:

  • OMOP arrived through institution-level ETL, built per site by engineers who map concepts from anywhere in the EHR into the standard vocabulary, with an iterative submission-and-feedback loop until the data passed quality review.
  • FHIR arrived as participants connected to FHIR providers through digital participant portals, which "extract data via API from clinical datasets in the FHIR format"; a data aggregator then "moderately standardizes the data and submits it to AoURP" — with, per the authors, "no feedback cycle," and FHIR limited to "standard mappings used in its default bulk export functionality," which are determined by the EHR vendor.

The authors say this themselves: "clinical data documented in an unexpected area of the EHR are often excluded from FHIR exports" because vendor defaults decide what leaves the building. But the underlying challenge is not FHIR-specific. Clinical data that is never properly mapped and encoded — documented in free text instead of coded into structured entries — is hard to use in either standard; OMOP confronts exactly the same problem and solves it by throwing institution-funded ETL engineers at the source. That is a curation investment, not a model capability. FHIR itself is very rich in the information it can hold and handle — when data is properly encoded into its resources, nothing in the Condition or Procedure domains is beyond it. The Condition 258-vs-11 gap is therefore a statement about acquisition pipelines and encoding discipline, not about the expressive power of the two data models.

The study also quantified a real and durable tradeoff: FHIR's blocking, ingestion-time validation produced structurally cleaner data (unique ids, resolvable references), while OMOP's non-blocking post-hoc checks traded conformance for yield. Both findings are worth keeping.

Where we agree​

OMOP is the right tool for observational research today. It is purpose-built for it: flat, regular tables, standardized concept ids, and a decade of methods and tooling that assume its shape. If your institution has already paid for an OMOP ETL, use it. We are not arguing against OMOP — FHIR4DS (the open-source engine behind this blog) ships an OMOP CDM mapping library, and the FHIR → OMOP walkthrough exists precisely because OMOP is often the destination downstream teams need.

The gaps the study found are real — but they live in acquisition and encoding, not in FHIR. Data that is never properly mapped and coded — documented in text rather than structured entries — yields poor research data in either standard; OMOP overcomes it only through funded ETL curation, and default FHIR exports simply haven't had that curation. The encounter gap is the clearest case: the study's window (January 2020 – March 2024) sat entirely inside the USCDI v1 regulatory baseline, which had no Encounter data class at all — encounters only entered USCDI in v2 (2021), and only as a voluntary update; the mandatory certification baseline (USCDI v3, per the HTI-1 rule) took effect January 1, 2026. The finding that just 1.5% of participants had FHIR Encounters (versus 97% with OMOP visits) measures a v1-era export floor, not a FHIR limitation — and the mandate the authors said would fix it had already taken effect by the time the paper published.

Where the framing needs a correction​

The headline many people take away — "OMOP has 20× more clinical data than FHIR" — conflates the data model with the pipeline that filled it. The fair statement is:

Institutionally-funded ETL currently yields more research-ready data than zero-configuration vendor exports, regardless of the destination model.

Three observations from the study itself point at what is actually missing — and each one is answered by a standard that already exists. The relevant body of work is SQL-on-FHIR v2, an HL7-community specification — maintained under the FHIR project, with an official conformance test suite — that makes FHIR resources queryable as tables. Its artifacts, plus one plain-FHIR resource for vocabulary, are worth knowing by name because they turn "extracting data from FHIR payloads" from a per-institution engineering project into a set of small, versioned, shareable files. FHIR4DS is a DuckDB-based engine that runs all of them — plus CQL and FHIRPath — directly over FHIR payloads, in a notebook or in the browser.

1. The FHIR analyses were only possible after a third party added a terminology-mapping layer. Raw FHIR payloads were not directly analyzable by the research team without it. That is a correct diagnosis of the tooling, not the data — and the vocabulary step is a solved problem in the specification itself. A ConceptMap is a standard FHIR resource whose entire job is "this code in this system means that concept" — vocabulary translation as data rather than as a bespoke mapping program:

{
  "resourceType": "ConceptMap",
  "group": [{
    "source": "http://hl7.org/fhir/administrative-gender",
    "target": "urn:omop:concept",
    "element": [
      { "code": "male",   "target": [{ "code": "8507", "equivalence": "equivalent" }] },
      { "code": "female", "target": [{ "code": "8532", "equivalence": "equivalent" }] }
    ]
  }]
}

Flatten it once with a ViewDefinition (next paragraph) and it is an ordinary lookup table — source_system + code → concept_id — joined in a query. No third party required: the mapping ships with the pipeline, and the OMOP CDM mapping library carries one covering 108,683 codes.

2. The stated obstacle was "extracting data from FHIR payloads" — a data-engineering problem, not a clinical-content problem. And the gap stops being expensive when the extraction becomes a shareable standard artifact. The ViewDefinition is SQL-on-FHIR v2's reshaping artifact: a small JSON file that declares, per resource type, which elements become which columns, with FHIRPath expressions for paths — declarative and precise rather than procedural code. Where an OMOP ETL is months of local engineering, the same reshape is one of these, runnable by any conformant engine against any FHIR export:

{
  "name": "omop_person_source",
  "resource": "Patient",
  "select": [
    { "column": [
      { "name": "person_source_value", "path": "id", "type": "id" },
      { "name": "year_of_birth", "path": "birthDate.substring(0, 4)", "type": "string" },
      { "name": "gender_source_value", "path": "gender", "type": "code" }
    ] }
  ]
}

A SQLQuery then composes the views: a standard FHIR Library resource whose body is ordinary SQL over the view names — joins, unions, and derived columns live there, portable across engines. Write the mapping once, version it, publish it, cite it — a mapping you review and share, not one you rebuild at every institution. The FHIR → OMOP walkthrough builds a complete person table this way, live in the browser, and every other OMOP table is the same recipe.

3. FHIR captured what OMOP could not. 6,300 participants had multi-provider FHIR data versus 20 in OMOP, and 18.4% of timeline segments existed only in FHIR — care delivered at outside institutions the institutional ETL never saw. The study recommends FHIR as a supplement; we agree, and would add that a supplement you cannot query is a supplement you cannot use. Native FHIR querying — ViewDefinitions, CQL, FHIRPath, straight over the payloads — makes the multi-provider slice analyzable the day it lands, with no second ETL project. The two models compose rather than compete: query FHIR natively for what only FHIR has, and project into OMOP where the research ecosystem demands it, from the same standard artifacts.

Standards conformance, not vendor claims​

Skeptics are right to ask what "spec compliant" means. Each of the specifications FHIR4DS implements ships an HL7-community-built reference implementation and an official test suite designed to validate other implementations. FHIR4DS passes all of them — 2,785 official conformance tests at 100% in release 0.0.21:

SuiteTestsPassing
CQL — official HL7 cql-tests corpus1,706100%
FHIRPath — official reference suite935100%
SQL-on-FHIR v2 — official spec_tests144100%
CMS eCQM measures (QI-Core 2025) — evaluated end-to-end47100%

The OMOP CDM v5.5 mapping library carries its own example suite (38 expected-output tests — OMOP has no official test suite), kept green in the same runner.

The conformance runner ships in the repository — anyone can reproduce the numbers.

The bottom line​

The All of Us study is a valuable census of what today's default acquisition paths deliver, and its recommendation of FHIR as a supplement to OMOP is sound for programs that already have both. But its own evidence points past its framing: the cost it identified was not FHIR's ability to represent clinical reality — it was the absence of native analysis tooling for FHIR payloads. That absence is what FHIR4DS removes. As bulk exports fatten under the USCDI v3 mandate (in force since January 2026) and standard mapping artifacts accumulate, the "OMOP richer, FHIR raw" tradeoff narrows from both ends — and the multi-provider data only FHIR captures becomes queryable the day it arrives.

Want to see the extraction barrier dissolved end to end? Run the FHIR → OMOP walkthrough — it executes live in your browser, standard artifacts only.