How to request a test: A clinician's guide to the interpretation and evaluation of medical tests

129 Pages·2023·8.866 MB·English

by  Tom Boyles

How to Request a Test

How to Request a Test
A Clinician’s Guide to the Interpretation and
Evaluation of Medical Tests
Tom Boyles
Honorary Associate Professor, London School of Hygiene and
Tropical Medicine, London, United Kingdom
Researcher, Clinical HIV Research Unit, University of the Witwatersrand,
Johannesburg, South Africa
Great Clarendon Street, Oxford, OX2 6DP,
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Library of Congress Control Number: 2022943242
ISBN 978– 0– 19– 286661– 5
DOI: 10.1093/ med/ 9780192866615.001.0001
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Preface
I didn’t write this book because I am an expert on the topic, I wrote it because after
more than two decades of thinking and reading about the subject, I finally feel that
I can convey my knowledge in the way I would like to have learned it from the begin-
ning. In short, it is a book I wish was in my coat pocket as a medical student and junior
doctor.
Testing is ubiquitous in medicine and the task of requesting tests often falls to the
most junior members of the team, despite being vitally important and extremely
costly. In many settings, requesting tests is a simple matter of filling out a form, ticking
any number of boxes, and leaving the rest to phlebotomists, radiographers, and la-
boratory staff. While this has freed up junior doctors’ time, it can lead to a laissez- faire
attitude to testing.
As an intern, it was important to have a range of test results written in the notes as a
post- intake ward round approached. Woe betide anyone who did not have a troponin
result for a patient with chest pain or an amylase for a patient with abdominal pain.
However, I quickly realized that experienced clinicians barely looked at some of my
tests, particularly on follow- up rounds when I thought a new and pristine set of re-
sults was a core part of my job. They saw no value in my multiple C- reactive protein
tests to check that a patient was recovering from pneumonia, for example, instead
being happy with a brief history, examination, and inspection of the vitals chart. At
the time this puzzled me and I began reading about the application of medical tests
and particularly Bayes’ theorem. Gradually I built up an understanding of why and
when we do tests and, importantly, when not to do them.
Now I have gained a degree of experience, particularly within my specialty of infec-
tious diseases, I see the errors I made being repeated. Typically, it is an over- reliance
on tests or an overestimation of their value. Often, when doctors are unsure of the
diagnosis, they request a broad range of tests in the hope that some abnormal result
will lead them in the right direction. While there are some tests that are indicated
for almost all patients, such as a full blood count for a patient admitted under medi-
cine or surgery, there should be no such thing as ‘routine bloods’. As a consultant,
I am frequently presented with pages of blood test results that I have very little if any
interest in reading. To make matters worse, sometimes the single key test I really want
is not there. Sometimes I am asked to review a patient simply because a test result is
‘abnormal’ rather than because there is a specified clinical problem: ‘We measured
the beta- D- glucan [a component of the cell wall of some fungi] and it’s high, please
tell us which fungus the patient has’ is a typical question to infectious diseases. The
oncologist might similarly be faced with ‘We requested a panel of tumour markers
and one is elevated, come and treat this patient’s cancer’. These are typical examples
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of performing tests without thinking through the problem in advance, and without
having a full understanding of the strengths and weaknesses of the tests at hand.
The appropriate steps in evaluating and applying a new medicine is usually taught
well in medical schools and most medical students and junior doctors could tell you
about the four phases of clinical trials and that regulatory approval typically occurs
after phase III trials confirm efficacy and safety. The same cannot be said for the evalu-
ation of tests; this is partly because even experts disagree on the framework for test
evaluation, but also because it is perceived as having less value. After reading this
book, my hope is that medical students and junior doctors will have a greater appre-
ciation of how to use tests efficiently and effectively. The quantity of tests they request
is likely to decrease, which will save time and money, while the quality will increase,
improving patient care.
Acknowledgements
This book would not have been possible without the love and support of my family.
I would like to thank my partner, Lynne, and our two wonderful children, Maya and
Kai, for putting up with me while I wrote it.
The staff at Oxford University Press have been helpful and supportive throughout,
particular thanks go to Nicola Wilson for all the effort she has put in.
A number of people kindly reviewed earlier drafts and gave constructive feedback,
I’d like to thank Druin Burch, Joe Wilson, Evie Rothwell, and Hywel Jones. The latter
introduced me to Bayes’ theorem on a ward round circa 1998, when I was a medical
student— thank you for setting me on this path.