David J Woods
Drug Information Consultant
Senior Teaching Fellow
School of Pharmacy
University of Otago, Dunedin
Enhanced access to clinical information has many obvious advantages, but the sheer volume of data and its diverse forms also present significant obstacles to its optimal use. This article contains some basic tips for effective searching.
For online tutorials in relation to this article, please visit the website: www.pharminfotech.co.nz/hpe.htm
Analyse and frame the question
Question analysis is important to ensure that you have interpreted the context correctly and gathered sufficient background information to help you search and then give an answer. The next step is to break the question down into concepts and decide on the correct terms to use, and the required sensitivity and specificity.
For example, you are asked if amlodipine can cause a dystonic reaction. Standard texts give insufficient information, so a MEDLINE search is the next step. Compare the searches in Figure 1. For more explanation, refer to the online tutorial.
Choose the most appropriate resource
In the context of a drug information question, MEDLINE is used to supplement information (or lack of it) found in standard reference texts and other resources. The Cochrane database is used to find systematic reviews. An internet search engine is often useful to find very specific information when little has been published, or to track down a specialist centre or expert. This may often be the case with experimental or preliminary investigations. For clinical practice or evidence-based guidelines, numerous specialised websites exist.
Other databases are important
In addition to MEDLINE there are other important databases to consider searching. EMBASE covers a different set of journals to MEDLINE. In fact, the common coverage is only about 40%. If you want a more comprehensive search then it is advisable to use both databases. This could be the case if looking for rare case reports or preparing a systematic review. International Pharmaceutical Abstracts (IPA) covers the pharmaceutical literature more extensively that MEDLINE.
Which version of MEDLINE?
Different software and search engines are used to access the same information. PubMed is free and is the most popular, but other free versions include Biomednet and Medscape. Institutions may subscribe to other platforms such as OVID or Silver Platter. The search engines are different, and if you have access to both free and subscribed databases it becomes a matter of personal preference as to which one is used. It is important to be aware of major differences if you use both free and subscribed versions. For example, in PubMed the index headings (MeSH) are automatically exploded, whereas in OVID you have to actively select this option. If you are mobile and cannot always access OVID, you can save your searches in the PubMed “cubby” or add the URLs of your searches to your personal web page.
MEDLINE search tips
Each record in MEDLINE is assigned a number of fields that can be searched singly or in combination. For example, there are TITLE, ABSTRACT, AUTHOR, JOURNAL, PUBLICATION TYPE, MeSH and TEXTWORD fields. A MeSH (or Medical Subject Heading) is an index term assigned to a record as a keyword or concept that can be searched. A TEXTWORD is an actual word that appears in the title, abstract or both. The syntax used to identify the fields that you wish to search are not consistent across all versions of MEDLINE, so initially you can familiarise yourself with one version by going to the help function.
Search terms are combined using Boolean operators. For example, combining using AND will search for records containing both terms, whereas OR will search for records containing either term.
When constructing a search strategy, identify the most important concepts of your subject. If necessary break down the subject into separate components and then decide on the most appropriate terms to use. Search engines will usually map your term to a MeSH, but it is advisable to check whether the scope of the term fits your requirements or there are any other terms that are more suitable or should be used in addition.
Use the index or MeSH browser to correctly define your term. Use your core knowledge to think around the term and select more than one if necessary. For example, if looking for information about a drug’s safety in pregnancy it is tempting to just use the term ”pregnancy”. However, more specific terms such as “teratogens” and “birth defects” should also be included.
Use a combination of MeSH and textwords. Most MEDLINE search engines now default to search a mapped MeSH along with a textword. You should always be aware of how your search terms have been interpreted. In PubMed this is done by clicking on “details”.
Start with a broad search, then narrow the focus if necessary. Avoid the initial use of the “restrict to focus” option or subheadings. These can always be introduced later should you need to refine your search.
Consider exploding the term. When a MeSH is exploded, that term is searched plus all the related terms below it in the “tree” structure of the literature. For example, if you are looking for information on antidepressant-induced hyponatraemia, exploding the broad MeSH is obviously preferable and quicker than using each individual drug name. Remember, you can always narrow down and refine your search if you retrieve too much information. PubMed automatically explodes all MeSHs unless you override the default.
Think of the “ideal title” of the paper you are looking for and then break down the title into concept components. This can sometimes be a useful lead to a more structured search.
If you have a copy of a related article or if you locate a reference in a tertiary source, find the article in MEDLINE and check the MeSHs that were used. You can then search using these terms. Most versions (eg, PubMed) have a very useful “related links” feature, which may retrieve further articles of interest or others that you have missed in your initial search.
Sensitivity and specificity
A literature search needs to be sensitive to detect the important citations that will help you solve a clinical problem, but must also be specific to avoid scrolling through numerous irrelevant “hits”. This applies whether doing a MEDLINE search or using an internet search engine. Most search engines have help functions that explain how sensitivity or specificity can be increased.
To improve specificity in MEDLINE consider adding another concept, apply subheadings, use the limit command to select the category (eg, age group) of interest, or use the “restrict to focus” option with one or more of the MeSHs. You can also consider excluding (using the “NOT” operator) if you find numerous papers on an irrelevant subject.
For an increase in sensitivity, identify a more general MeSH term and explode it. Find related MeSH terms and combine using “OR”. The indexing process has an element of subjectivity, so it is possible that the assigned MeSH has been taken slightly out of context. For textword searching, consider any probable spelling variations.
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