This article focuses on building and refining a comprehensive literature search strategy to identify relevant evidence. It covers how to combine search terms, validate the completed strategy, and translate the strategy across multiple databases. If you are still working on earlier steps in the process, see From Idea to Research Question to Search for guidance on clarifying the underlying problem and developing a focused, answerable question. For guidance on translating that question into concrete search terms, see Identifying Search Terms for Your Literature Review.  

What is a comprehensive search strategy?

A comprehensive literature search strategy is a structured approach to identifying relevant literature that combines keywords, subject headings, database-specific search techniques, and, when appropriate, supplementary search methods.  

For projects such as systematic reviews and scoping reviews, the goal is to maximize sensitivity (the ability to retrieve relevant studies) while maintaining reasonable precision (the proportion of retrieved studies that are relevant).

A strong search strategy should be:

• Comprehensive: captures relevant studies despite differences in terminology.
• Reproducible: allows another researcher to repeat the search.
• Transparent: clearly documents search decisions and methods.
• Appropriate: aligns with the purpose and scope of the review.

Comprehensiveness and appropriateness are addressed primarily during development, as the strategy is built and refined. Validation evaluates whether the completed strategy retrieves known relevant literature and identifies potential errors or gaps. Reproducibility and transparency depend primarily on documenting and reporting the search process clearly.

Selecting databases

Before building a search strategy, identify the databases most appropriate for the research question and review type. Database selection should be guided by the research question, review type, and relevant reporting guidance. Because individual databases may vary in coverage and indexing practices, searching multiple appropriate databases often improves retrieval of relevant evidence and reduces the risk of missing eligible studies.  

Consider the following:

• Subject coverage: Does the database cover the disciplines and topics relevant to the research question?  
• Indexing practices: Does the database use controlled vocabulary or indexing systems that support retrieval of relevant concepts?  
• Database coverage: Will searching multiple databases identify unique records and improve coverage of the literature?
• Disciplinary relevance: Are the database’s journals and publications aligned with the population, intervention, context, or topic being studied?  

The selected databases determine the terminology, controlled vocabulary, search fields, and syntax used when developing and translating the search strategy.

Supplementary search methods

A comprehensive search may include more than database searching alone. Depending on the review question and methodology, additional approaches may include searching gray literature sources, clinical trial registries, guideline repositories, and preprint servers. Supplementary methods such as handsearching key journals, which involves manually reviewing the contents of relevant journals, and citation searching, which involves reviewing the reference lists of relevant articles or identifying studies that have cited them, may also help identify relevant evidence that is not retrieved through database searches. These approaches should be considered alongside database searching when appropriate for the review.

Although these steps are presented in sequence, search development is iterative. Findings from validation, peer review, and translation may require returning to earlier steps and revising the strategy.

Step 1: Build the search strategy

Before building a database search, organize the terms identified during search term development into concept groups. Each concept group represents a key element of the research question and becomes a separate search segment that will be combined into the complete strategy.

Not every element of the research question will become a search concept. Including unnecessary concepts may reduce retrieval of relevant studies, particularly when outcomes or other concepts are inconsistently reported in titles and abstracts. The decision to include or exclude a concept should be guided by the purpose and scope of the review, the terminology used in the literature, and the balance between sensitivity and precision.

Within each concept, combine the related terms using the Boolean operator OR. OR broadens the search by retrieving records containing any of the included terms.  

Include both controlled vocabulary and keywords. Controlled vocabulary retrieves articles indexed under standardized concepts, while keywords capture newer terminology, variations in wording, and articles not yet indexed. Field tags specify where databases should search for the term. For example, in PubMed, [tiab] searches titles and abstracts, while [MeSH] searches Medical Subject Headings assigned during indexing. Using appropriate field tags is important because they can affect which search results are retrieved. Searching in the wrong field may unintentionally exclude relevant records or retrieve irrelevant results.

Run each search concept separately to review:

• The number of results retrieved.
• Whether relevant articles are appearing.
• Whether additional terms should be added.
• Whether the search contains any syntax or formatting errors.
• Reviewing concepts individually helps identify problems before combining the complete strategy.

After each concept has been reviewed, combine the search concepts using the Boolean operator AND. AND narrows the search by requiring results to contain terms from multiple concepts. The complete strategy should represent the combination of concepts needed to retrieve literature relevant to the research question.

For example, consider the research question:

In postoperative patients, does early mobilization improve recovery outcomes?

The key search concepts are postoperative patients and early mobilization. Each concept is developed as a separate search segment using the keywords and controlled vocabulary that represent that concept in the database.

Concept 1: Postoperative patients  

("postoperative period"[MeSH] OR postop[tiab] OR "postoperative patient*"[tiab])

AND

Concept 2: Early mobilization  

("Early Ambulation"[MeSH] OR "early mobilization"[tiab] OR "early ambulation"[tiab] OR "postoperative walking"[tiab])

The final search strategy represents the combination of concepts necessary to retrieve literature relevant to the research question.

Step 2: Refine the search strategy

After creating the initial search strategy, check whether the retrieved results reflect the scope of the research question and are appropriately comprehensive and relevant. The focus now shifts to the combined search strategy as a whole, rather than individual terms considered in isolation. Refinement is an expected part of this process, not a sign that the initial strategy was flawed.

Consider:

• Are relevant articles being retrieved?
• Are important concepts represented?
• Are there too many irrelevant results?
• Are key terms missing?
• Are any search limits or filters justified and appropriate for the review question? Filters should be applied cautiously because they may exclude eligible studies.

Refinement may include:

• Adding missing keywords or subject headings
• Removing terms that retrieve unrelated results
• Adjusting search fields
• Revising search combinations
• Reviewing newly retrieved articles for additional terminology

The goal is not simply to retrieve the greatest number of records, but to identify the most relevant evidence while minimizing missed studies. How much refinement is enough will also depend on the project's timeline and the resources available to screen results.

The strategy may now retrieve relevant results, but refinement alone does not establish that it captures known relevant literature or is free from important errors. The following steps help evaluate and document the completed strategy before it is translated and reported.

Step 3: Validate results using key articles

A useful method for validating a search strategy is to compare retrieval against a set of known relevant articles. During search term development, key articles help identify terminology; during validation, they are used to determine whether the completed search strategy retrieves known relevant studies. These articles should represent studies that are clearly eligible or highly relevant to the review question.

Key articles may be identified through:  

• Preliminary investigation
• Expert recommendations
• Reference lists
• Citation searching

In PubMed, key articles can be cross-checked using PubMed Identifiers (PMIDs), unique identification numbers assigned to records indexed in PubMed.

To perform this check:

• Create a search string containing the PMIDs of key articles.
• Run the comprehensive search strategy separately.
• Combine the comprehensive search strategy AND the PMID set.  
• Review whether the known relevant articles appear in the results.

If the known relevant articles are retrieved, this provides evidence that the strategy can identify those articles and increases confidence that the search incorporates terminology and concepts relevant to the review question. However, retrieving all known articles does not establish that the strategy will identify every eligible study.

If known relevant articles are not retrieved, first confirm that they meet the review’s eligibility criteria. If they do, review them to determine why they were missed.  

Possible reasons include:

• Missing search terms
• Different terminology used by authors
• Alternative spellings
• Missing subject headings
• Incorrect search syntax
• Indexing limitations (e.g., indexing delays or inconsistent subject headings)

Failure to retrieve a known article does not automatically mean that the search strategy is inadequate. The article may fall outside the review’s eligibility criteria, may not be indexed consistently, or may use terminology not represented in the search strategy. The missed retrieval may indicate that refinement of keywords, subject headings, or search syntax is needed. Even well-validated strategies may remain subject to database limitations and other factors that influence retrieval, such as indexing delays, inconsistent subject headings, and potential language or publication biases.

Step 4: Peer review the search strategy

Cross-checking against key articles confirms that a strategy retrieves known relevant literature, but it does not catch every kind of error. Peer review adds an independent check, using a structured tool such as the Peer Review of Electronic Search Strategies (PRESS) checklist.

PRESS can be used two ways:  

• As a self-assessment, where the original searcher reviews their own strategy against the checklist
• As a formal peer review, where a second specialist who did not build the strategy evaluates it independently.  

The checklist prompts reviewers to assess whether the strategy aligns with the research question, uses appropriate search approaches, and contains potential errors or omissions that should be addressed.

Peer review of the search strategy is increasingly recommended for systematic reviews and is considered good practice for scoping reviews and other comprehensive reviews intended for publication.

Step 5: Document the search strategy

Documenting the complete search process allows the strategy to be reported accurately, evaluated by others, and reproduced or updated in the future.

Document:

• Database name and search platform
• Date searched
• Complete search strategy
• Number of results retrieved
• All limits or filters applied (e.g., language, publication date, study design, population)
• Search revisions
• Rationale for applying limits, filters, or other major search decisions

Save:

• Database search histories
• Exported search strategies
• Search documentation records (e.g., search logs or spreadsheets)

A structured search log or spreadsheet can help organize search details across databases and support transparent reporting in accordance with guidance such as PRISMA and JBI.

Step 6: Translate the search strategy

When searching multiple databases, translate the strategy rather than copying it directly.

Each database has different:

• Controlled vocabulary
• Search syntax and field tags
• Truncation and phrase searching conventions

Adapt the strategy to the requirements of each database while maintaining the same core concepts. Once translated, test each version in its respective database to confirm that it retrieves the expected results.

Developing and validating together

Developing and validating a search strategy are complementary components of a rigorous literature search. Development helps ensure that the strategy is comprehensive and appropriate for the research question, while validation helps identify potential errors or gaps and confirm that the strategy retrieves known relevant literature. Clear documentation and reporting support the reproducibility and transparency of the search process.