BMJ No 7101 Volume 315
Education and debate Saturday 19 July 1997
How to read a paper
The Medline database
Trisha GreenhalghThis is the first of 11 articles introducing non-experts to finding medical articles and assessing their value
In 1928, in his introduction to Sceptical
Essays, Bertrand Russell wrote: "The extent to which beliefs
are based on evidence is very much less than believers suppose."
Medical beliefs, and the clinical practices that are based on them, are
a case in point. Debate continues as to whether scientific evidence
alone is sufficient to guide medical decision making, but few doctors
would dispute that finding and understanding relevant research based
evidence is increasingly necessary in clinical practice. This article
is the first in a series that introduces the non-expert to searching
the medical li
Articles can be traced in two ways: by any word listed on the
database, including words in the title, abstract, authors' names, and
the institution where the research was done; and by a restricted
thesaurus of medical titles, known as medical subject heading (MeSH)
terms.
To illustrate how Medline works, I have worked through some common
problems in searching. The scenarios have been drawn up using
ovid software.
Problem 1: You are trying to find a known paper
Thus, to find a paper called something like "Confidentiality and
patients' casenotes," which you remember seeing in the
British Journal of General Practice a couple of years
ago,(1) type the following sequence:
1 - confidentiality.ti
You could do all this in one step:
1 - confidentiality.ti and british journal of general
practice.jn
This step illustrates the use
of the boolean operator "and"; it will give you articles common to
both sets. Using "or" will simply add the two sets together.
Note that since 1988 the British Medical Journal is
abbreviated BMJ in ovid software, and Journal of the
American Medical Association is JAMA. Other useful field
suffixes to try when searching for a known article are author (using
the syntax haines-ap.au), institution (for example, manchester.in), or
title (for example, evidence-based medicine.ti).
Problem 2: You want to answer a specific question
1 - anorexia nervosa
You have not typed a field suffix (such as .tw),
so the ovid system will automatically try to "map" your
request to one of its standard medical subject headings (abbreviated
MeSH and colloquially known as "mesh terms"). (Note that not all
Medline software packages will automatically map your suggestion to
MeSH terms. With Silver Platter search software, for example, you need
to enter your heading and click the "suggest" button.) For this
example, the screen offers you either "eating disorders" or
"anorexia nervosa" and asks you to pick the closest one. Choose
"anorexia nervosa" (space bar to highlight the text, then press
"return").
The screen then asks you whether you want to "restrict to focus."
Do you only want articles which are actually about anorexia nervosa, or
do you want any article that mentions anorexia nervosa in passing?
Let's say we do want to restrict to focus. Next, the screen offers us
a choice of subheadings, but we'll ignore these for a moment. Select
"Include all subheadings." We could have got this far using a
single line command:
2 - *anorexia nervosa/
The * shows that the term is a major focus of
the article, and the / represents a MeSH term. You should have about
750 articles in this set.
Similarly, to get articles on osteoporosis (which is also a MeSH term),
use the following single line command:
3 - osteoporosis/
You should get about 2,200 articles. Note that in
ovid, if you know that the subject you want is an official
MeSH term, you can shortcut the mapping process by typing a slash (/)
after the word. Note also that we have not used an asterisk here,
because osteoporosis may not be the focus of the article we are looking
for.
Finally, put in the term "oral contraceptives" (without an asterisk
and without a slash) to see what the MeSH term here is. You will be
offered "contraceptives, oral," and if you had known this you could
have used the following command:
4 - contraceptives, oral/
This set should contain around 1,200
articles. You can combine these three sets, either by using their set
numbers 1 and 2 and 3 or by typing the single line command:
5 - *anorexia nervosa/ and osteoporosis/ and contraceptives,
oral/
With this you will have searched over 4000 articles and
struck a single bull's eye.(2) (If you don't find it,
check the syntax of your search carefully, then try running the same
search through the previous five year database using the Alt-B
command.)
Problem 3: You want to get general information quickly about a
well defined topic
The option "AIM journals" denotes all journals listed in the
Abridged Index Medicus - that is, the "mainstream" medical journals.
Alternatively, if you want articles relating to nursing, rather than
medical care, you could limit the set to "Nursing journals." This
is often a better way of limiting a large set than asking for local
holdings. If you are not interested in seeing anything in a foreign
language (even though the abstract may be in English), select this
option, again bearing in mind that it is a non-systematic (indeed, a
very biased) way of excluding articles from your set.(3)
Note that instead of using the "limit set" function key you can use
direct single line commands such as:
9 limit 4 to local holdings
Problem 4: Your search gives irrelevant articles
1 - (surrogate not mother$).tw
Deciding to use the "not"
operator is a good example of how you can (and should) refine your
search as you go along - much easier than producing the perfect search
off the top of your head. I used the truncation symbol
$ to find all words from a single stem, such as
mother, mothers, motherhood, and so on.
Another way of getting rid of irrelevant articles is to narrow your
textword search to adjacent words using the "adj" operator. For
example, the term "home help" includes two very common words linked
in a specific context. Link them as follows:
1 - home adj help.tw
Problem 5: The search gives no articles, or too few
Another important strategy for preventing incomplete searches is to use
the powerful "explode" command. The MeSH terms are like the
branches of a tree with, for example, "asthma" subdividing into
"asthma in children," "occupational asthma," and so on. Medline
indexers are instructed to index items by using the most specific MeSH
terms they can. If you just ask for articles on "asthma" you will
miss all the articles indexed under "asthma in children" unless you
"explode" the term using the following syntax:
1 - exp asthma/
Problem 6: You don't know where to start
searching
1 - ptx stress
The screen shows many options, including
post-traumatic stress disorders, stress fracture, oxidative stress,
stress incontinence, and so on.
The command "ptx" is useful when the term might be found in
several subject areas. If your subject is a discrete MeSH term, use the
tree command. For example:
2 - tree epilepsy
will show where epilepsy is placed in the MeSH
index - as a branch of "brain diseases," which itself branches into
generalised epilepsy, partial epilepsy, post-traumatic epilepsy, and so
on.
Problem 7: Limiting a set loses important articles but does not
exclude those of low methodological quality
These EBQFs (evidence based quality filters), which are listed in
Appendix 1, are complex search strategies developed by some of the
world's most experienced medical information experts. You can copy
them into your personal computer and save them as strategies to be
added to your subject searches. Other search strategies that will
identify cohort studies, case-control studies, and so on will soon be
available from the UK Cochrane Centre, Summertown Pavillion, Middle
Way, Oxford OX2 7LG (general@cochrane.co.uk).
Problem 8: Medline hasn't helped
If you wish to broaden your search to other electronic databases, ask
your local librarian where you could access the following: AIDSLINE - Covers AIDS and HIV back to
1980.
Allied and Alternative Medicine - Covers
complementary and alternative medicine.
American Medical Association
Journals - Provides the full text of JAMA plus 10 specialty
journals produced by the American Medical Association; from 1982.
ASSIA - An applied social sciences database
covering psychology, sociology, politics, and economics since 1987. All
documents have abstracts.
Cancer-CD - A compilation by Silver Platter
of cancerlit and Embase cancer related records from 1984.
The CD ROM version is updated quarterly.
CINAHL - The nursing and allied health
database covering all aspects of nursing, health education,
occupational therapy, social services in health care, and other related
disciplines from 1983. The CD ROM version is updated monthly.
Cochrane Library - The Cochrane
Controlled Trials Register (cctr), Cochrane Database of
Systematic Reviews (cdsr), Database of Abstracts of Reviews
of Effectiveness (dare), and Cochrane Review
Methodology Database (crmd) are updated quarterly;
authors of systematic reviews on cdsr undertake to update
their own contributions periodically.(4)
Current Contents Search - Indexes journal
issues on or before their publication date. It is useful when
checking for the very latest output on a subject. Updated weekly; from
1990.
Current Research in Britain - The British
national research database of trials in progress.
DHData (formerly DHSS-Data) - The
database of the UK's Department of Health indexes articles covering
health service and hospital administration; from 1983.
Embase - Focuses on drugs and pharmacology
but also includes other biomedical specialties. It is more up to date
than Medline and has better European coverage. The CD ROM version is
updated monthly.
HELMIS - The Health Management Information
Service at the Nuffield Institute of Health, Leeds, UK, indexes
articles on health service management.
Psychlit - Produced by the American
Psychological Association as the computer searchable version of
Psychological Abstracts; covers psychology, psychiatry, and related
subjects; journals are included from 1974 and books from 1987 (English
language only).
Science Citation Index - Indexes references
cited in articles as well as the usual author, title, abstract, and
citation of articles themselves. Useful for finding follow up work done
on a key article and for tracking down addresses of authors.
SHARE - Based at the King's Fund library in
London; published and ongoing research into the health of, and health
services for, black and minority ethnic groups.
Toxline - Information on toxicological
effects of chemicals and drugs on living systems; from 1981.
UNICORN - The main database of the King's
Fund, London. Covers health, health management, health economics,
and social sciences. Particularly strong on primary health care and the
health of Londoners.
In these examples, upper case denotes
controlled vocabulary and lower case denotes free text terms.
Search statements 8, 9, 19, and 27 could be omitted if your
search takes too long a time to run
Search statements 9 and 10
could be omitted if your search seems to be taking a long time to run.
Thanks to Mr Reinhard Wentz, Ms Jane Rowlands, Ms
Carol Lefebvre, and Ms Valerie Wildridge for advice on this chapter. I
am grateful to Carol Lefebvre of the UK Cochrane Centre for permission
to reproduce the EBQFs in Appendix 1.
Unit for Evidence-Based Practice and Policy,
References
1 Caman D, Britten N. Confidentiality and medical records:
the patient's perspective. Br J Gen Prac 1995;45:485-8.
2 Seeman E, Szmukler G I, Formica C, Tsalamandris C, Mestrovic R.
Osteoporosis in anorexia nervosa: the influence of peak bone
density, bone loss, oral contraceptive use, and exercise. J Bone
Mineral Res 1992;7:1467-74.
3 Moher D, Fortin P, Jadad A R, Juni P, Klassen T, Le Lorier J,
et al. Completeness of reporting of trials published in languages other
than English: implications for conduct and reporting of systematic
reviews. Lancet 1996;347:363-6.
4 Bero L, Rennie D. The Cochrane Collaboration: preparing,
maintaining, and disseminating systematic reviews of the effects of
health care. JAMA 1995;274:1935-8.
Summary points
Not all medical articles are indexed on Medline, and many that
are have been misclassified Searching by textword can supplement a search by MeSH headings To increase the sensitivity of a search, use the "explode" command
and avoid using subheadings Scan titles on screen rather than relying on the software to find the
most valid or relevant ones The Medline database
Over 10 million medical articles exist on library shelves.
About a third are indexed in the huge Medline database, compiled by the
National Library of Medicine of the United States. The Medline database
is exactly the same, whichever company is selling it, but the
commands differ according to the software. Vendors of Medline
online and on CD ROM include Ovid Technologies (OVID)
and Silver Platter Information (WinSPIRS). 
Useful search field suffixes (OVID) Syntax Meaning Example
.ab Word
in abstract epilepsy.ab
.au Author smith-r.au
.jn Journal lancet.jn
.me Single word, wherever it may
appear
as a MeSH term ulcer.me
.ti Word in title epilepy.ti
.tw Word in title or abstract epilepsy.tw
.ui Unique
identifier 91574637.ui
.yr Year of
publication 87.yr
Useful subheadings (ovid) Syntax Meaning Example
/ae Adverse
effects thalidomide/ae
/co Complications measles/co
/ct Contraindications (of
drug) propranolol/ct
/di Diagnosis glioma/di
/dt Drug
therapy depression/dt
/ed Education asthma/ed
/ep Epidemiology poliomyelitis/ep
/hi History mastectomy/hi
/nu Nursing cerebral
palsy/nu
/og Organisation/administration health
service/og
/pc Prevention and control influenza/pc
/px Psychology diabetes/px
/th Therapy hypertension/th
/tu Therapeutic use (of
drug) aspirin/tu
Solution: Search the database by field suffix (title,
author, journal, institution, etc) or by textwords.
First, get into the part of the database which covers the approximate
year of the paper's publication. If you are already in the main
Medline menu, select "database" (Alt-B). If you know the
approximate title of the paper and perhaps the journal where it was
published, you can use the title and journal search keys or (this is
quicker) the .ti and .jn field
suffixes. The box shows some other useful field suffixes.
2 - british journal of general practice.jn
3 - 1 and 2
Solution: Construct a focused (specific) search by combining
two or more broad (sensitive) searches.
I was recently asked by the mother of a young girl with anorexia
nervosa whose periods had ceased to prescribe oral contraceptives for
her so as to stop her bones thinning. This seemed a reasonable request,
though there were ethical problems to consider. But is there any
evidence that taking oral contraceptives in these circumstances really
prevents long term bone loss? I decided to explore the subject using
Medline. To answer this question, you need to search very broadly under
"anorexia nervosa," "osteoporosis," and "oral
contraceptives." The search described below involves articles from
1992; when replicating it, make sure the database you are searching
goes back that far. Type:
Solution: Use subheadings and/or the "limit set"
options.
Subheadings are the fine tuning of the Medline indexing system; they
classify articles on a particular MeSH topic into aetiology,
prevention, therapy, and so on. The most useful ones are listed in the
box. I try not to use subheadings unless I have unearthed an
unmanageable set of articles, since an estimated 50% of articles in
Medline are inadequately or incorrectly classified by subheading. It
actually doesn't take long to browse through 50 or so articles on the
screen. It is better to do this than to rely on the "limit set"
command (see box) to give you the best of the
bunch.
Useful "limit set" options
AIM journals Abstracts
Nursing journals Local holdings
Dental
journals English language
Cancer journals Male
Review articles Human
Editorials Publication year
10 limit 5 to human
Solution: Refine your search as you go along in the light of
interim results.
Often, a search uncovers dozens of articles which are irrelevant to
your question. The boolean operator "not" can help here. I recently
undertook a search to identify articles on surrogate endpoints in
clinical pharmacology research. My search revealed hundreds of articles
I didn't want - all on surrogate motherhood. The syntax to exclude the
unwanted articles is:
Solution: Firstly, don't overuse subheadings or the
"limit set" options. Secondly, search under textwords as well as
MeSH terms. Thirdly, learn about the "explode" command, and use it
routinely.
Many important articles are missed not because we constructed a flawed
search strategy but because we relied too heavily on a flawed indexing
system. For this reason, you should adopt a "belt and braces"
approach and search under textwords as well as by MeSH terms. After
all, it is difficult to write an article on the psychology of
diabetes without mentioning the words "diabetes," "diabetic,"
"psychology," or "psychological," so the truncation stems
"diabet$.tw." and "psychol$.tw." would supplement a search
under the MeSH term "diabetes mellitus" and the subheading
"/px" (psychology).
Solution: Use the "permuted index" option.
Let's take the term "stress." It comes up often, but searching for
particular types of stress would be laborious and searching
"stress" as a textword would be too unfocused. We need to know
where in the MeSH index the various types of stress lie, and when we
see that, we can choose the sort of stress we want to look at. For
this, we use the command ptx ("permuted index"):
Solution: Apply an EBQF (evidence based quality
filter).
If your closely focused search still gives you several hundred
articles, and if applying subheadings or limit set functions seems to
lose valuable (and valid) papers, you should insert a quality string
designed to limit your set to therapeutic interventions, aetiology,
diagnostic procedures, or epidemiology. Alternatively, you could apply
search strings to identify the publication type, such as randomised
controlled trial, systematic review, or meta-analysis.
Solution: Explore other medical and paramedical
databases.
Entry of articles onto the Medline database is open to human error,
both from authors and editors who select key words for indexing, and
from the librarians who group articles under subheadings and type in
the abstracts. In addition, some sections of indexed journals are not
available on Medline (for example, the News section of the
BMJ). According to one estimate, 40% of material which
should be listed on Medline can, in reality, only be accessed by
looking through all the journals again, by hand. Furthermore, a number
of important medical and paramedical journals are not covered by
Medline at all. It is said that Medline lacks comprehensive references
in the fields of psychology, medical sociology, and non-clinical
pharmacology.
Appendix 1: Evidence based quality filters for everyday use
(a) Therapeutic
interventions (What works?)
1 exp clinical trials 2
exp research design
3 randomized controlled trial.pt.
4
clinical trial.pt.
5 (single or double or treble or triple).tw.
6 (mask$ or blind$).tw.
7 5 and 6
8 placebos/ or
placebo.tw.
9 1 or 2 or 3 or 4 or 7 or 8
(b) Aetiology
(What causes it? What are the risk factors?)
1 exp
causality
2 exp cohort studies
3 exp risk
4 1 or 2 or 3
(c) Diagnostic procedures
1 exp "sensitivity and
specificity"
2 exp diagnostic errors
3 exp mass screening
4 1 or 2 or 3
(d) Epidemiology
1
sn.xs
(This would find all articles indexed under any MeSH term
with any of "statistics," "epidemiology," "ethnology,"
or "mortality" as subheadings.)
Appendix 2: Maximally sensitive search
strings
(to be used mainly for research)
(a) Maximally sensitive
qualifying string for randomised controlled trials
1 RANDOMIZED CONTROLLED TRIAL.pt.
2
CONTROLLED CLINICAL TRIAL.pt.
3 RANDOMIZED
CONTROLLED TRIALS.sh.
4 RANDOM
ALLOCATION.sh.
5 DOUBLE-BLIND METHOD.sh.
6 SINGLE-BLIND METHOD.sh.
7 or/1-6
8
ANIMAL.sh. not HUMAN.sh.
9 7 not 8
10
CLINICAL TRIAL.pt.
11 EXP CLINICAL
TRIALS
12 (clin$ adj25 trial$).ti,ab.
13 ((single or
double or treble or triple) adj25 (blind$ or mas$)).ti,ab.
14
PLACEBOS.sh.
15 placebo$.ti,ab.
16 random$.ti,ab.
17
RESEARCH DESIGN.sh.
18 or/10-17
19 18 not 8
20
19 not 9
21 COMPARATIVE STUDY.sh.
22 exp EVALUATION STUDIES/
23 FOLLOW UP STUDIES.sh.
24 PROSPECTIVE
STUDIES.sh.
25 (control$ or prospectiv$ or
volunteer$).ti,ab.
26 or/21-25
27 26 not 8
28 26 not (9 or
20)
29 9 or 20 or 28
(b) Maximally sensitive
qualifying string for identifying systematic reviews
1 REVIEW, ACADEMIC.pt.
2 REVIEW,
TUTORIAL.pt.
3 META-ANALYSIS.pt.
4
META-ANALYSIS.sh.
5 systematic$ adj25 review$
6
systematic$ adj25 overview$
7 meta-analy$ or metaanaly$ or (meta
analy$)
8 or/1-7
9 ANIMAL.sh. not
HUMAN.sh.
10 8 not 9
The articles in this series are
excerpts from How to read a paper: the basics of evidence based
medicine. The book can be ordered from the BMJ Bookshop: tel
0171 383 6185/6245; fax 0171 383 6662. Price £13.95 UK members,
£14.95 non-members. Or order from the BMJ Publishing Group Web site.
Department of Primary Care and Population Sciences,
University College
London Medical School/Royal Free Hospital School of Medicine,
Whittington Hospital,
London N19 5NF
Trisha
Greenhalgh,
senior
lecturer
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