Evidence Search Guide

How to Search Grey Literature for a Systematic Review

Find relevant evidence outside conventional journal indexes without turning the search into an undocumented collection of web results.

By Lumina Editorial Team Published Updated Methodology guide
Multi-database literature search builder for selecting scholarly and supplementary sources
A grey-literature search should complement—not silently replace—the bibliographic databases required by the review protocol.

What is grey literature?

Grey literature is research or evidence distributed outside conventional commercial and academic journal publishing. Depending on the review question, it can include government reports, policy papers, regulatory documents, clinical-study reports, theses, conference material, preprints, institutional reports, and repository records.

Important: “Grey” describes how material is published and discovered. It does not establish methodological quality or peer-review status. Apply the same predefined eligibility and appraisal process used for other evidence.

Where should you search?

Select sources from the protocol and review question. Do not treat a generic web search as a complete grey-literature strategy.

Government publications

Research reports, statistics, impact assessments, consultations, policy papers, and official evaluations.

Registries and regulators

Trial registrations, regulatory assessments, safety material, and study reports when applicable.

Repositories and preprints

Institutional repositories, open-research platforms, funder outputs, and disciplinary preprint servers.

Theses and conference material

Dissertations, proceedings, abstracts, and reports that may not have a journal version.

Grey literature search: step by step

  1. 1

    Define eligible publication types

    State which reports, jurisdictions, organizations, languages, dates, and document types are relevant before searching.

  2. 2

    Choose sources deliberately

    Match each source to the evidence types and geography in the protocol. Ask an information specialist to review high-stakes searches.

  3. 3

    Adapt the query per source

    Portal syntax, indexing, languages, and filters differ. Preserve the concepts, but validate how every source interprets Boolean operators and phrases.

  4. 4

    Record the search while running it

    Save the source, URL, complete query, filters, search date, result count, export method, and any screening limitations.

  5. 5

    Combine, label, and deduplicate

    Keep source provenance and literature type attached to each record, then remove duplicate citations before screening.

Minimum search log

FieldRecordWhy it matters
SourcePortal and direct URLMakes the search repeatable
QueryExact submitted stringPrevents undocumented rewriting
LimitsDate, language, typeMakes restrictions visible
Search runDate and result countSupports updates and reporting

Search academic databases and government grey literature together

Lumina provides a supplementary search across scholarly sources and a bundled government-publications source. Government concepts can be expanded across English, Dutch, German, French, and Spanish for review before searching. Results retain source labels and move into the same deduplication and screening workflow.

This does not replace topic-specific databases, registries, regulatory sources, or an information specialist. Use it as a documented supplementary layer alongside the sources required by your protocol.

Explore multi-database and grey-literature search →

Methodology references