Spotlight on Test Utilization: Should the Qualitative Serum Pregnancy Test Be Considered Obsolete?


dr. Grenache would you put this study
into context by discussing the background issues that led to the idea
for this study a few years ago I was a co-author on a paper that evaluated or
discussed some of the tests that we believe to be antiquated in laboratory
medicine and one of those tests that we considered to be obsolete was the
qualitative serum pregnancy test we commonly call them pregnancy tests
although they really detect the presence of a hormone that’s only produced during
pregnancy called human chorionic gonadotropin and tests for HCG are
usually done very rapidly in order to quickly assess a woman’s pregnancy
status there are two flavors of pregnancy tests or HCG tests they can
either be done on urine samples or on serum samples when urine is the sample
of choice the test can be performed rapidly and very close to the patient at
the point of care because urine doesn’t require any special processing however a
qualitative serum pregnancy test really can’t be done at the point of care it
has to be done the laboratory and that’s because the blood sample has to be
centrifuged to obtain the serum and typically centrifugation is only done in
the laboratory so we thought that qualitative serum pregnancy tests were
obsolete for really four reasons first they can’t be performed at the point of
care second laboratories usually have a way to perform a quantitative serum hcg
test in a relatively short amount of time as well third it’s conventional
wisdom that qualitative HCG tests are not as analytically sensitive as the
quantitative tests and forth it could be argued since the only clinical use for a
qualitative serum pregnancy test is to rule out or rule in pregnancy we could
argue that in that situation the most sensitive test the quantitative test
should be used unfortunately there were no data published in the literature that
helped address that question so I decided to do a study our hypothesis was
that physicians ordered qualitative serum pregnancy tests because they believed
they got the results back from the laboratory more quickly than had they
ordered a quantitative serum pregnancy test so one objective was to compare the
turnaround times for the qualitative and the quantitative serum HCG tests and a
second objective was to examine the analytical sensitivities of the two
types of tests dr. Furtado we hypothesized that
physicians ordered qualitative serum pregnancy tests instead of quantitative
serum pregnancy tests because they believed they would get the results back
more quickly from the qualitative test to help answer that question we did a
survey of physicians at the University of Utah so could you tell me what those
survey results showed sure so the survey showed that most doctors use the urine
HCG test instead of serum which was not that surprising however of those who
used serum half preferred to use qualitative tests instead of
quantitative tests despite the fact that they believe that quantitative test have
better accuracy so as we suspected the reason they prefer to use qualitative
tests is because it’s that impression that they receive the results more
rapidly than the quantitative tests and our data our turnaround time data
didn’t support this fact so there are a few definitions off turnaround time for
clinicians turnaround time is the time that it takes for them to get a result
after a test has been ordered while for laboratorians the definition of turn around
time is the time that it takes for a result to be produced after they receive
the sample in the lab so for in the lab if we use the laboratorias our
definition of turnaround time the qualitative tests are performed more
quickly than the quantitative tests although by the doctors definition of
turn around time there are no difference between the quantitative and qualitative
tests and that’s because the time that it takes for a sample to make it to the
lab for the sample to be transported to the lab is one of the main factors in
the delay of a total test processing so although physicians believe that they do
get results faster with the qualitative test our data show that it doesn’t seem
to be the case one of the objectives of this study was to compare the analytical
sensitivity in the performance characteristics of the qualitative and
quantitative serum HCG tests how did you do that and what were your results
in order to compare the analytical sensitivity of the qualitative tests
against the quantitative test we had to have samples that for which the
qualitative serum test had been ordered and that was easy we took samples that
came to the laboratory for which a physician had ordered a qualitative
serum pregnancy test and we simply measured the concentration of HCG using
our quantitative assay now the cutoff for the qualitative test is claimed by
the manufacturer for the device that we used was 25 iu’s per liter meaning that
a serum sample with an HCG concentration that was greater than 25 should guild a
positive result and technically one that was less than 25 should yield a negative
result but we know that there’s that a cut-off isn’t always absolute and we
were curious to know if the qualitative test was able to detect HCG at a
concentration that was really lower than the cutoff and actually that’s what we
did find about twenty percent of the samples that were interpreted as
positive by the qualitative test actually had an HCG concentration that
was less than 25 indicating that they that the qualitative tests that we used
was actually more analytically sensitive than it was claimed by the manufacturer
and that’s a good thing determining the performance characteristics of the
qualitative test was a little bit more challenging because the performance
characteristics are going to be based upon what you compare it to what we
would call the gold standard and so we had to think about what gold standard we
would use to compare the qualitative test against and we decided to use
multiple criteria multiple gold standards for example when we we did
chart review on the patient samples to determine actual pregnancy status which
makes sense this is a test that’s being used to identify pregnancy or not so
when we compared the performance of the qualitative tests against actual
pregnancy status we found that the qualitative test devices performed as
well as the quantitative test meaning that they were they were both very
sensitive and specific that means that there are very few false positive or
false negative results a particular performance characteristic
that’s very important is the negative predictive value that is if a result is
interpreted as negative how reliable is it the negative predictive value of both
the qualitative and the quantitative test when compared against the actual
pregnancy status was ninety-nine point nine percent that tells us that a
negative result from the qualitative test is very believable and rules out
the possibility of pregnancy I think it’s important for laboratories
to understand possible causes of false positive and false negative HCG test
results could you explain those so first it’s important to determine what a false
result really is because although we see the HCG tests as pregnancy tests they
really aren’t they are tests that are designed to detect and measure HCG and
there are occasions in other than pregnancy in which HCG can be present in
measure by the devices that are used for test for example if a pregnancy is
spontaneously lost very early after fertilization that’s the so-called
biochemical pregnancy if an HCG test is performed at that time it’s going to
detect HCG in that it can be considered a false positive result for a pregnancy
that really isn’t a false positive result in terms of measuring HCG another
conditions in which HCG can be present without in the absence of a pregnancy is
in certain types of tumors such as those derived from placental trophoblastic
tissue or some germ cell tumors in which HCG can also be elevated and there is
also the pituitary gland that can produce HCG especially in those women
who have gone through menopause and there are also the issue of a —— HCG that can be used for fertility treatments or in the HCG diet in all of
these different situations in which HCG is produced if we test with the hcg test
device we will detect HCG despite of pregnancy so you’ve told us how HCG tests could be
positive in the absence of pregnancy but they’re positive because HCG is actually
present are there any situations in which you could get a positive HCG test
yet HCG is not present yes and in fact there are two main reasons why we can
have a true false positive results with HCG testing and one of them is the
presence of an interfering antibody in the patient sample that can mimic HCG
this happens more commonly when we are testing serum rather than urine because
those antibodies they are not present in urine another situation is when a HCG test device is misinterpret by the person who is doing the test because
those tests are actually analyzed by visual inspection and sometimes the
person who is reading the test might make mistakes and report them
incorrectly can you tell me about some causes of
false-negative HCG test results yeah so the false negative test results are not
as common as the false positive results and the most common cause is when the
test is performed too early after fertilization occurs because in order
for HCG to be produced they there is a need for the implementation of the
fertilized egg and it might take a couple of days but for the HCG to reach
a level in which it can be measured it’s important to note here that no
laboratory tests to be perfect and the importance is that if a physician notice
that there is a discrepancy between the HCG test result in a clinical
presentation of the patient to contact the laboratory so actions can be taken
towards troubleshooting a potential problem so dr. Grenache based on the results of
this study should the qualitative serum HCG test be considered
obsolete that’s a good question I think we’ve learned three important points
from our study first that physicians do order qualitative serum pregnancy tests
or serum HCG tests because they believe they can get the results back from the
laboratory more quickly than if they ordered a quantitative test what we
discovered with our study was that at least here at the University of Utah
that is not true because the turnaround times for the qualitative and the
quantitative serum hcg test were not significantly different from each other
the second thing I think we learned was that the qualitative serum HCG test has
a greater or is more analytically sensitive than is claimed by the
manufacturer and the third point that we learned was that when compared against
actual pregnancy status the reason the test is being done both the qualitative
and the quantitative serum HCG test had similar performance characteristics
particularly they both had a negative predictive value that was ninety-nine
point nine percent effectively ruling out pregnancy if a negative result was
obtained so in light of that I’d say yes the qualitative serum pregnancy test
could be replaced by a quantitative test but there doesn’t seem to be any
compelling reason to do it are there any limitations of this study
yes there were a few first it’s important to emphasize that we performed
this study just here at the University of Utah and so the turnaround time data
that we collected may not apply to other health care systems secondly I think
it’s important to emphasize that our study was focused on the comparison of
serum qualitative and quantitative HCG test we didn’t look at urine tests urine
qualitative HCG test and they’re very different and so I think it’s important
for people to appreciate that what we’ve been talking about today is in regards
to serum tests and not urine HCG tests if a laboratory is considering abandoning
performing the qualitative serum HCG test I would caution that they evaluate
their turnaround time data before they do so

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