PubMed Truncation Rules Explained: Using Wildcard Search (*) and Exact Phrase Searching with Quotes

PubMed is powerful, but it can also be surprisingly strict. Two of the most common reasons a search returns either too many results or misses key papers are (1) not using truncation when you should, and (2) using quotation marks without realizing they change how PubMed interprets your query. This guide breaks down PubMed truncation rules, how wildcard search works with the asterisk (*), and how phrase searching in PubMed behaves when you add quotes—especially the critical detail that quotes can bypass PubMed’s Automatic Term Mapping (ATM).

Why PubMed Search Behavior Matters

PubMed does more than match the words you type. In many cases, it applies Automatic Term Mapping (ATM) to interpret your keywords, map them to MeSH (Medical Subject Headings), and include synonyms or related terms. That can help you find articles you would otherwise miss.

However, two features can dramatically change what PubMed retrieves:

• Truncation (wildcard search) with an asterisk (*) to capture multiple word endings (e.g., child*).
• Phrase searching with quotation marks (e.g., "heart attack"), which can force exact word order and may prevent ATM from expanding your search.

Knowing when to use each approach is a practical skill that can make your searches both broader (to avoid missing studies) and cleaner (to reduce irrelevant hits).

PubMed Truncation Rules: How the Asterisk (*) Works

In PubMed, truncation is typically done using an asterisk (*) at the end of a word root. This is often called a wildcard search. It tells PubMed: “Find any word that starts with this root.”

What Truncation Does (With Examples)

Truncation is best used when you expect multiple variants of a word and you want PubMed to retrieve all of them in one search.

• child* can retrieve: child, children, childhood, etc.
• therap* can retrieve: therapy, therapies, therapeutic, etc.
• immun* can retrieve: immune, immunity, immunology, immunization, etc.

This approach is especially useful during early stages of a literature search, when you’re building a comprehensive set of results and don’t yet know all terminology variations used by different authors.

When to Use Wildcard Search in PubMed

Use truncation when:

• You need to capture singular/plural forms or common suffix changes (e.g., tumor* for tumor/tumors/tumorous).
• You suspect authors use multiple related terms (e.g., genet* for genetic/genetics/genetically).
• You want a broader net for a systematic or scoping search (then refine later).

When Truncation Can Hurt Precision

Wildcard search is powerful, but it can introduce noise. Because truncation can match many unexpected words, it may pull in irrelevant results.

• cat* could match cat, catalyst, catalytic, catheter, etc.
• cell* can explode into many contexts beyond what you intended.

If your root is too short or too common, truncation can quickly reduce precision. In those cases, consider using a longer root, pairing with additional terms, or switching to exact phrase searching when appropriate.

Phrase Searching in PubMed: Using Quotes for Exact Phrases

Adding quotation marks in PubMed turns your query into an exact phrase search in many situations. This means PubMed tries to find the words together, in the same order, as a phrase.

Why Quotes Are “Strict” in PubMed

Phrase searching in PubMed is useful when the word order matters or when a concept is commonly expressed as a fixed phrase. But it also comes with a major tradeoff: quoted searches can be less forgiving than unquoted searches, because PubMed may not apply its normal mapping/expansion behavior.

In practical terms, quotes can:

• Reduce retrieval by requiring the exact wording and order.
• Limit PubMed’s ability to interpret the concept through Automatic Term Mapping (ATM).

ATM and Why Quotes Can Bypass It

PubMed’s Automatic Term Mapping is designed to help users by mapping search terms to standardized vocabulary (such as MeSH) and including variants. When you place a term in quotes, PubMed may treat it as a literal phrase instead of mapping it broadly. That means you might miss relevant articles that use synonyms or a different phrasing.

Example:

• Searching heart attack (no quotes) may allow PubMed to map to related concepts and synonyms.
• Searching "heart attack" may restrict results to articles that literally contain the phrase “heart attack,” potentially missing papers that primarily use myocardial infarction.

This is why phrase searching in PubMed should be used intentionally—not automatically.

When Exact Phrase Searching Is the Right Choice

Use quotes when:

• You are searching a well-known fixed phrase that authors consistently use (e.g., "quality of life").
• You want to avoid ambiguity (e.g., "case control" vs. case and control separately).
• You are narrowing a large set of results and need higher precision.

Truncation vs. Quotes: How to Choose the Best Tool

Both wildcard search and phrase searching are useful, but they solve different problems. Here’s a practical way to decide which to use.

Use Truncation (*) When You Need Word Variants

• Goal: capture multiple endings and related forms
• Best for: broad discovery, terminology variation
• Example: child* to include child/children/childhood

Use Quotes When You Need an Exact Phrase

• Goal: require words to appear together in order
• Best for: precision, fixed expressions
• Example: "quality of life"

A Balanced Strategy for Better PubMed Searches

For many clinical and biomedical topics, a strong approach is:

1. Start broad using unquoted terms (let ATM help you).
2. Add truncation thoughtfully for key concepts with known variations.
3. Use quotes only where exact phrases are essential.
4. Iteratively refine using additional keywords, field tags, or filters as needed.

Common Mistakes to Avoid

• Over-truncating: using very short roots that match many unrelated terms.
• Quoting too early: forcing exact phrases and missing synonyms or MeSH mapping through ATM.
• Assuming quotes improve “accuracy”: quotes improve precision in wording, but may reduce recall in concept coverage.

FAQ: PubMed Truncation and Phrase Searching

What is truncation in PubMed?

Truncation in PubMed uses an asterisk (*) at the end of a word root to retrieve multiple word variants, such as child* matching child, children, and childhood.

What is a wildcard search in PubMed?

A wildcard search generally refers to using symbols like the asterisk (*) to capture variations of a term. In PubMed, this is most commonly done by placing * at the end of a root word.

Do quotes in PubMed force an exact phrase?

Yes, quotes can trigger phrase searching in PubMed, pushing PubMed to look for the words together in the specified order, which can narrow results substantially.

Why can quotes reduce PubMed results?

Because quoted searches can be strict and may bypass Automatic Term Mapping (ATM), potentially excluding articles that use synonyms or different phrasing.

Conclusion: Search Smarter with * and “ ”

Mastering PubMed truncation rules and phrase searching in PubMed can dramatically improve your literature searches. Use the asterisk (*) for a strategic wildcard search when you need to capture word variants (like child*), and use quotation marks when an exact phrase is truly essential. Most importantly, remember that quotes can bypass PubMed’s Automatic Term Mapping—so they should be applied carefully to avoid accidentally missing relevant studies.

With a deliberate mix of truncation, unquoted searching (to benefit from ATM), and selective phrase searching, you can strike the right balance between recall and precision—and spend less time troubleshooting search results.