How to read research papers? Papers that report diagnostic or screening tests
Ten_men_in_the_dock
If you are new to the
concept of validating diagnostic tests, the following example may help you. Ten
men are awaiting trial for murder. Only three of them actually committed a
murder; the seven others are innocent of any crime. A jury hears each case and
finds six of the men guilty of murder. Two of the convicted are true murderers.
Four men are wrongly imprisoned. One murderer walks free.
View larger version:
PETER BROWN
This information can be expressed in what is known as a two by
two table (table 1). Note that the “truth” (whether or not the
men really committed a murder) is expressed along the horizontal title row,
whereas the jury's verdict (which may or may not reflect the truth) is
expressed down the vertical row.
View this table:
Table 1
Two by two table showing outcome of trial for
10 men accused of murder
These figures, if they
are typical, reflect several features of this particular jury:
·
the jury correctly identifies two in every three true murderers;
·
it correctly acquits three out of every seven innocent people;
·
if this jury has found a person guilty, there is still only a
one in three chance that they are actually a murderer;
·
if this jury found a person innocent, he or she has a three in
four chance of actually being innocent; and
·
in five cases out of every 10 the jury gets it right.
These five features
constitute, respectively, the sensitivity, specificity, positive predictive
value, negative predictive value, and accuracy of this jury's performance. The
rest of this article considers these five features applied to diagnostic (or
screening) tests when compared with a “true” diagnosis or gold standard. A
sixth feature—the likelihood ratio—is introduced at the end of the article.
Validating tests against a gold standard
Our window cleaner
told me that he had been feeling thirsty recently and had asked his general practitioner
to be tested for diabetes, which runs in his family. The nurse in his surgery
had asked him to produce a urine specimen and dipped a stick in it. The stick
stayed green, which meant, apparently, that there was no sugar in his urine.
This, the nurse had said, meant that he did not have diabetes.
Summary points
New tests should be
validated by comparison against an established gold standard in an appropriate
spectrum of subjects
Diagnostic tests are
seldom 100% accurate (false positives and false negatives will occur)
A test is valid if it
detects most people with the target disorder (high sensitivity) and excludes
most people without the disorder (high specificity), and if a positive test
usually indicates that the disorder is present (high positive predictive value)
The best measure of the usefulness of a test
is probably the likelihood ratio—how much more likely a positive test is to be
found in someone with, as opposed to without, the disorder
I had trouble explaining that the result did not necessarily
mean this, any more than a guilty verdict necessarily makes someone a murderer.
The definition of diabetes, according to the World Health Organisation, is a
blood glucose level above 8 mmol/l in the fasting state, or above 11 mmol/l two
hours after a 100 g oral glucose load, on one occasion if the patient has
symptoms and on two occasions if he or she does not.1 These stringent criteria can be termed the
gold standard for diagnosing diabetes (although purists have challenged this
notion2).
The dipstick test, however, has some distinct practical
advantages over the fullblown glucose tolerance test. To assess objectively
just how useful the dipstick test for diabetes is, we would need to select a
sample of people (say 100) and do two tests on each of them: the urine test
(screening test) and a standard glucose tolerance test (gold standard). We
could then see, for each person, whether the result of the screening test
matched the gold standard (see table 2). Such an exercise is known as a validation
study.
View this table:
Table 2
Two by two table notation for expressing the
results of validation study for diagnostic or screening test
The validity of urine testing for glucose in diagnosing diabetes
has been looked at by Andersson and colleagues,3 whose data I have adapted for use (expressed
as a proportion of 1000 subjects tested) in table 3.
View this table:
Table 3
Two by two table showing results of validation
study of urine glucose testing for diabetes against gold standard3
From the calculations
of important features of the urine dipstick test for diabetes (box), you can
see why I did not share the window cleaner's assurance that he did not have
diabetes. A positive urine glucose test is only 22% sensitive, which means that
the test misses nearly four fifths of people who have diabetes. In the presence
of classical symptoms and a family history, the window cleaner's baseline
chances (pretest likelihood) of having the condition are pretty high and is
reduced to only about four fifths of this (the negative likelihood ratio, 0.78;
see below) after a single negative urine test. This man clearly needs to
undergo a more definitive test.
View this table:
Features of diagnostic test that can be
calculated by comparison with gold standard in validation study
Does the paper validate the test?
The 10 questions below can be asked about a paper that claims to
validate a diagnostic or screening test. In preparing these tips, I have drawn
on several sources.4 5 6 7 8
Question 1: Is this test potentially relevant to my practice?
Sackett and colleagues call this the utility of the test.6 Even if this test were 100% valid, accurate,
and reliable, would it help me? Would it identify a treatable disorder? If so,
would I use it in preference to the test I use now? Could I (or my patients or
the taxpayer) afford it? Would my patients consent to it? Would it change the
probabilities for competing diagnoses sufficiently for me to alter my treatment
plan?
Question 2: Has the test been compared with a true gold
standard?
You need to ask,
firstly, whether the test has been compared with anything at all. Assuming that
a “gold standard” test has been used, you should verify that it merits the
description, perhaps by using the questions listed in question 1. For many
conditions, there is no gold standard diagnostic test. Unsurprisingly, these
tend to be the conditions for which new tests are most actively sought. Hence,
the authors of such papers may need to develop and justify a combination of
criteria against which the new test is to be assessed. One specific point to
check is that the test being validated in the paper is not being used to define
the gold standard.
Question 3: Did this validation study include an appropriate
spectrum of subjects?
Although few investigators would be naive enough to select only,
say, healthy male medical students for their validation study, only 27% of
published studies explicitly define the spectrum of subjects tested in terms of
age, sex, symptoms or disease severity, and specific eligibility criteria.7 Importantly, the test should be verified on a
population which includes mild and severe disease, treated and untreated
subjects, and those with different but commonly confused conditions.6
View this table:
Calculating the important features of
screening test
Although the
sensitivity and specificity of a test are virtually constant whatever the
prevalence of the condition, the positive and negative predictive values depend
crucially on prevalence. This is why general practitioners are sceptical of the
utility of tests developed exclusively in a secondary care population, and why
a good diagnostic test is not necessarily a good screening test.
Question 4: Has workup bias been avoided?
This is easy to check. It simply means, “Did everyone who got
the new diagnostic test also get the gold standard, and vice versa?” There is
clearly a potential bias in studies where the gold standard test is performed
only on people who have already tested positive for the test being validated.7
Question 5: Has expectation bias been avoided?
Expectation bias
occurs when pathologists and others who interpret diagnostic specimens are
subconsciously influenced by the knowledge of the particular features of the
case—for example, the presence of chest pain when interpreting an
electrocardiogram. In the context of validating diagnostic tests against a gold
standard, all such assessments should be “blind.”
Question 6: Was the test shown to be reproducible?
If the same observer performs the same test on two occasions on
a subject whose characteristics have not changed, they will get different
results in a proportion of cases. Similarly, it is important to confirm that
reproducibility between different observers is at an acceptable level.9
Question 7: What are the features of the test as derived from
this validation study?
All the above standards could have been met, but the test might
still be worthless because the sensitivity, specificity, and other crucial
features of the test are too low—that is, the test is not valid. What counts as
acceptable depends on the condition being screened for. Few of us would quibble
about a test for colour blindness that was 95% sensitive and 80% specific, but
nobody ever died of colour blindness. The Guthrie heel-prick screening test for
congenital hypothyroidism, performed on all babies in Britain soon after birth,
is over 99% sensitive but has a positive predictive value of only 6% (it picks
up almost all babies with the condition at the expense of a high false positive
rate),10 and rightly so. It is more important to pick
up every baby with this treatable condition who would otherwise develop severe
mental handicap than to save hundreds the minor stress of a repeat blood test.
Question 8: Were confidence intervals given?
A confidence interval, which can be calculated for virtually
every numerical aspect of a set of results, expresses the possible range of
results within which the true value will probably lie. If the jury in the first
example had found just one more murderer not guilty, the sensitivity of its
verdict would have gone down from 67% to 33%, and the positive predictive value
of the verdict from 33% to 20%. This enormous (and quite unacceptable)
sensitivity to a single case decision is, of course, because we validated the
jury's performance on only 10 cases. The larger the sample, the narrower the
confidence interval, so it is particularly important to look for confidence
intervals if the paper you are reading reports a study on a relatively small
sample.11
Question 9: Has a sensible “normal range” been derived?
If the test gives
non-dichotomous (continuous) results—that is, if it gives a numerical value
rather than a yes/no result—someone will have to say what values count as
abnormal. Defining relative and absolute danger zones for a continuous variable
(such as blood pressure) is a complex science, which should take into account
the actual likelihood of the adverse outcome which the proposed treatment aims
to prevent. This process is made considerably more objective by the use of
likelihood ratios (see below).
Question 10: Has this test been placed in the context of other
potential tests in the diagnostic sequence?
In general, we treat
high blood pressure simply on the basis of a series of resting blood pressure
readings. Compare this with the sequence we use to diagnose coronary artery
stenosis. Firstly, we select patients with a typical history of effort angina.
Next, we usually do a resting electrocardiogram, an exercise electrocardiogram,
and, in some cases, a radionuclide scan of the heart. Most patients come to a
coronary angiogram only after they have produced an abnormal result on these
preliminary tests.
If you sent 100
ordinary people for a coronary angiogram, the test might show very different
positive and negative predictive values (and even different sensitivity and
specificity) than it did in the ill population on which it was originally
validated. This means that the various aspects of validity of the coronary
angiogram as a diagnostic test are virtually meaningless unless these figures
are expressed in terms of what they contribute to the overall diagnostic work
up.
A note on likelihood ratios
Question 9 above described the problem of defining a normal
range for a continuous variable. In such circumstances, it can be preferable to
express the test result not as “normal” or “abnormal” but in terms of the
actual chances of a patient having the target disorder if the test result
reaches a particular level. Take, for example, the use of the prostate specific
antigen (PSA) test to screen for prostate cancer. Most men will have some
detectable antigen in their blood (say, 0.5 ng/ml), and most of those with
advanced prostate cancer will have high concentrations (above about 20 ng/ml).
But a concentration of, say, 7.4 ng/ml may be found either in a perfectly
normal man or in someone with early cancer. There simply is not a clean cutoff
between normal and abnormal.12
We can, however, use the results of a validation study of this
test against a gold standard for prostate cancer (say a biopsy of the prostate
gland) to draw up a whole series of two by two tables. Each table would use a
different definition of an abnormal test result to classify patients as
“normal” or “abnormal.” From these tables, we could generate different
likelihood ratios associated with an antigen concentration above each different
cutoff point. When faced with a test result in the “grey zone” we would at
least be able to say, “This test has not proved that the patient has prostate
cancer, but it has increased [or decreased] the odds of that diagnosis by a
factor of x.”
The likelihood ratio thus has enormous practical value, and it
is becoming the preferred way of expressing and comparing the usefulness of
different tests.6 For example, if a person enters my consulting
room with no symptoms at all, I know that they have a 5% chance of having iron
deficiency anaemia, since I know that one person in 20 in the population has
this condition (in the language of diagnostic tests, the pretest probability of
anaemia is 0.05).13
View larger version:
Fig 1
Now, if I do a diagnostic test for anaemia, the serum ferritin
concentration, the result will usually make the diagnosis of anaemia either
more or less likely. A moderately reduced serum ferritin concentration (between
18 and 45 μg/l) has a likelihood ratio of 3, so the chances of a patient with
this result having iron deficiency anaemia is 0.05x3—or 0.15 (15%). This value
is known as the post-test probability of the serum ferritin test. The
likelihood ratio of a very low serum ferritin concentration (below 18 μg/l) is
41, making the chances of iron deficiency anaemia in a patient with this result
greater than unity. On the other hand, a very high concentration (above 100
μg/l; likelihood ratio 0.13) would reduce the chances of the patient being
anaemic from 5% to less than 1%.13
Figure 1 shows a nomogram, adapted by Sackett and
colleagues from an original paper by Fagan,14 for working out post-test probabilities when
the pretest probability (prevalence) and likelihood ratio for the test are
known. The lines A, B, and C, drawn from a pretest probability of 25% (the
prevalence of smoking among British adults), are the trajectories through
likelihood ratios of 15, 100, and 0.015, respectively—three different tests for
detecting whether someone is a smoker.15Actually, test C detects whether the person is
a non-smoker, since a positive result in this test leads to a post-test
probability of only 0.5%.
The articles in this series are excerpts from How to read a
paper: the basics of evidence based medicine. The book includes chapters on searching the
literature and implementing evidence based findings. It can be ordered from the
BMJ Publishing Group: tel 0171 383 6185/6245; fax 0171 383 6662. Price £13.95
UK members, £14.95 non-members.
