Repeating groups are the FHIR Questionnaire feature that exposes which form tools were built by people who deployed them to clinicians and which ones were built around the spec without that production feedback. A repeating group lets a user add multiple instances of a structured chunk: a list of medications, a list of allergies, a list of prior surgeries. The model is straightforward; the UX, the indexing, the FHIRPath behavior, and the extraction are not.
This piece looks at four FHIR form tools that handle repeating groups without making your team write workarounds. The wider buyer-side framing is in FHIR form builders for US hospitals: what you need to know in 2026; the conditional-logic question sits next to it in 5 SDC form engines that actually handle conditional logic. For our wider FHIR coverage, the rest of the series adds context.
What Repeat-Group Support Has to Do
A working repeating group has to handle four things together. The UI has to render an "add another" affordance, with the visual hierarchy keeping the user clear about which instance they are filling in. The data model has to capture each instance as a distinct response item with stable identity, not a flat array that loses structure on extraction. FHIRPath expressions that reference a repeating group have to evaluate against the correct instance, not collapse to the first or last. And extraction to FHIR resources has to produce one resource per instance, not one resource for the whole group.
A tool that handles three of the four is the source of every "add another" UX bug your team has worked around.
The Four Tools That Hold Up
The shortlist below leans on what works in real clinical deployments with non-trivial repeating-group forms. Order reflects fit-for-purpose, not feature count.
- LHC LForms. The Lister Hill team's open-source library handles repeating groups across the four dimensions cleanly. Add-instance UX is solid, the data model preserves instance identity, FHIRPath expressions evaluate per-instance, and extraction produces discrete resources.
- Aidbox SDC IDE. Aidbox SDC supports the repeating-group model end to end, with the IDE producing Questionnaires that round-trip through rendering and extraction without surprises. The mobile-rendering behavior is consistent with the desktop behavior, which matters for patient-facing forms.
- Formbox. Designed around production patient-facing flows, with repeating groups treated as a first-class concern. The offline-capable mobile renderer preserves instance state across save-and-resume, which is where simpler tools tend to drop data.
- OntoServer Forms. CSIRO's form layer treats repeating groups as first-class objects against the OntoServer terminology engine, with answer expressions evaluating per-instance against the correct terminology lookup. For terminology-heavy forms with repeating structures, the integration matters.
What Tends to Break
A few patterns recur in form tools that handle repeating groups poorly.
- The "add another" button works fine until a calculated expression depends on a value inside the group, at which point the expression evaluates against the first instance only.
- Extraction produces a single Observation with a comma-separated text value instead of one Observation per instance.
- Mobile rendering loses instance state on a partial save, so the user comes back to a form with the group collapsed to one instance.
- A user who fills the third instance, leaves the second blank, and submits gets an extraction that silently drops the third.
The fix is to test repeating groups specifically, with at least three instances, partial completion, and a calculated expression that references the group. That single test exposes most of the issues.
How to Pick
For research-leaning workflows with deep repeat-group forms, LForms is the natural starting point. For full-stack FHIR-native deployments, Aidbox SDC IDE or Formbox each fit. For terminology-heavy clinical workflows with repeating structures, OntoServer Forms.
Repeat-group support is one of those quiet capabilities that decides whether the form builder fits the long tail of clinical forms. The tools that handle it well save your team the kind of rework that adds up across a portfolio.
Sources
- Base Questionnaire (defines repeating-group semantics, evergreen) - HL7 SDC IG v4.0.0-ballot
- NLM tooling (repeating-group rendering) - PDF slides, Ye Wang (NLM), DevDays 2024
- SDC Extract / Pre-populate (per-instance extraction) - PDF slides, Brian Postlethwaite, DevDays 2023