Subfields

A subfield is the third level of OpenAlex’s four-level classification hierarchy: 4 domains → 26 fields → 252 subfields → 4,516 topics. Subfields are groupings like “Health Informatics,” “Geophysics,” or “Oncology” — broader than a topic, narrower than a field. Each subfield sits under exactly one field and gathers a set of related topics beneath it. A subfield’s OpenAlex ID is a bare number, e.g. 2718; fetch one at api.openalex.org/subfields/2718. Subfields are the level OpenAlex uses to normalize citation impact (FWCI) and are one of several aboutness signals — see that page to pick the right granularity for your question.

How we build it

Subfields aren’t classified on their own — they’re a level of the same hierarchy topics are built from. Each of the ~4,500 topics is mapped to a single subfield using Scopus’s ASJC categories, and a work “belongs to” a subfield through the topics assigned to it. The subfield labels are standardized (the 252 ASJC-derived groupings), but a work lands in one only because a machine-learning model inferred its topic from the work’s text — which is why subfields, like the rest of the hierarchy, live under Aboutness rather than Vocabulary. See Topics → How we build it for the full assignment pipeline, including the deliberate choice to give every work a single primary subfield from its own text (not from its journal).

A work’s primary subfield is the subfield of its primary_topic. To find works in a subfield, filter Works by topics.subfield.id (any assigned topic’s subfield) or primary_topic.subfield.id (the primary only). One level up is a field; one level down is a set of topics.

Fields

This is the canonical dictionary of every field on a subfield object. Fields shared with other entities (id, ids, display_name, works_count, cited_by_count, created_date, updated_date) are documented once on Common fields; subfield-specific notes are below.

id

String. The OpenAlex ID for this subfield, e.g. https://openalex.org/subfields/2718. Subfields use a bare numeric ID (four digits), not the letter-prefixed scheme. See Common fields.

ids

Object. External identifiers for this subfield, as URIs: openalex, plus wikidata and wikipedia where a matching article exists.

display_name

String. The subfield’s name, e.g. “Health Informatics.” See Common fields.

display_name_alternatives

List. Other names the subfield is known by (e.g. “medical informatics,” “clinical informatics” for Health Informatics), for matching and search.

description

String. A short description of what the subfield covers.

field

Object. The field this subfield belongs to (id, display_name) — the level directly above it in the hierarchy.

domain

Object. The domain this subfield rolls up into (id, display_name) — the top of the hierarchy.

topics

List. The topics that belong to this subfield (id, display_name) — the level directly below it.

siblings

List. The other subfields (id, display_name), for navigating laterally across the classification.

works_count

Integer. How many works fall in this subfield (through their assigned topics). See Common fields.

cited_by_count

Integer. Total citations across all works in this subfield. See Common fields.

works_api_url

String. A ready-made Works API URL for every work in this subfield, e.g. https://api.openalex.org/works?filter=topics.subfield.id:2718. A convenience link; work IDs aren’t stored on the subfield object.

created_date

String. The date this subfield was added to OpenAlex (YYYY-MM-DD). See Common fields.

updated_date

String. The ISO 8601 UTC timestamp of the last change to the subfield object. See Common fields.

In the API

The Subfields endpoint is at api.openalex.org/subfields. Fetch a single subfield by ID — /subfields/2718 — or a list, and filter, search, sort, group, and page over it.

You can filter and group subfields by their place in the hierarchy — domain.id, field.id, and topics.id — and by works_count, cited_by_count, id, and display_name, all of which also sort. Full-text matching uses the search parameter (the older display_name.search filter is deprecated). To find works in a subfield, use the Works endpoint: filter=primary_topic.subfield.id:2718 (primary only) or filter=topics.subfield.id:2718 (any assigned topic). See the Subfields API reference and the endpoints index.