Using new D-dimer tests to rule out venous thromboembolism
Clinicians and laboratory scientists have begun in recent
years to get a clearer picture of the value of the D-dimer assay
in assessing patients with suspected venous thromboembolism. Venous
thromboembolism produces a large mass or burden of clot within the
circulation. When a clot breaks down, it forms fibrin degradation
products, including D-dimer molecules. As a result, patients with
deep venous thrombosis or pulmonary embolism are expected to have
excessive numbers of D-dimer molecules in their blood.
The problem is that many other conditions activate clotting in
the veins and raise D-dimer levels, such as infection, noninfective
inflammatory diseases, malignancies, trauma, and even the normal
process of recovery after surgery.
Then there has been the lack of a quick and accurate D-dimer assay
that could be used in an acute-care setting. The fibrin split products
(FSP) or alternative fibrin degradation products teststhe
latex agglutination procedures that have been used in the past to
evaluate disseminated intravascular coagulationare not sufficiently
sensitive or reliable to use on patients with suspected venous thromboembolism.
Enzyme-linked immunosorbent assay determinations of the D-dimer
concentration have high sensitivity, but they must be performed
in batches. Standard ELISA tests therefore have a slow turnaround
time; a clinician could wait a day or two before getting test results
and, in the meantime, miss the opportunity to institute definitive
therapy for potentially life-threatening venous thromboembolism.
Newer ELISA-based or whole blood agglutination assays are sensitive
and rapidyielding test results in minutes or a few hours.
But they can be confusing to use, so as these assays are being introduced
in medical centers around the country, clinicians and pathologists
are working together to iron out the kinks.
"They are integrating the test into the whole gestalt of the situation
with patients, the experience of the clinicians in identifying signs
and symptoms of venous thromboembolism, and in performing other
tests in addition to the D-dimer. They are determining how valuable
the D-dimer is by itself and how it fits into the whole process
of evaluating patients with suspected venous thromboembolism," says
Douglas Triplett, MD, professor of pathology at Indiana University
School of Medicine and director of Midwest Hemostasis and Thrombosis
Laboratories, Muncie, Ind.
New D-dimer assays
Venous thromboembolism is one of the most common venous disorders
clinicians see, but only about one quarter of the patients who have
signs and symptoms of a clot in the vein actually have a thrombosis
and need treatment. And the disease can be difficult to diagnose.
Clinical signs and symptoms are notoriously vagueshortness
of breath with or without leg swelling, pain, edema, or discoloration.
As a result, clinicians cannot diagnose venous thromboembolism on
the basis of clinical findings alone. Although clinicians routinely
order a number of different types of imaging studies, these noninvasive
tests may have a low cost-benefit return. While venous ultrasound,
in combination with clinical findings, is accurate for venous thromboembolism,
it is costly because large numbers of patients with suspicious signs
and symptoms do not have the disease and many of these patients
require a second ultrasound examination after one week to detect
small clots, usually in the calf, which have extended during this
period. Venography, which is still the gold standard for identifying
venous thromboembolism, is invasive, costly, and itself poses the
risk of causing deep venous thrombosis or other complications.
The D-dimer increasingly is being seen as a valuable tool for
ruling out venous thromboembolism and sparing low-risk patients
from further workup.
The new D-dimer assays are not the same as the conventional latex
agglutination or ELISA tests for measuring D-dimer in patients who
are being screened for disseminated intravascular coagulation. Latex
agglutination assays, which detect elevated D-dimer levels on the
basis of the presence of agglutinates in blood samples, provide
rapid results at low cost. These tests are semiquantitative at best,
however, says John T. Brandt, MD, senior clinical research pathologist
at Eli Lilly & Company, Indianapolis. In one typical study of 98
patients, D-dimer levels derived through latex agglutination were
significantly higher in individuals who had angiographic evidence
of pulmonary embolism. However, an abnormal D-dimer test (>250
ng/mL) had a sensitivity of only 73.3 percent and a negative predictive
value of 83 percent. The authors concluded that D-dimer values obtained
by latex agglutination should not be used to assess patients with
suspected PE (Arch Intern Med. 1999;159:1567-1572).
Although standard ELISA assays can quantitate D-dimer, they are
not practical for quick evaluation of patients for DVT or PE. "An
ELISA format with its 96 wells can take two to four hours at a minimum
to complete, which is not particularly well suited for diagnosing
single patients in the emergency room 24 hours a day," Dr. Brandt
New D-dimer assays are quantitative and quick. The Instant IA
D-dimer from Diagnostica Stago and the Nycocard D-dimer from Nycomed
Pharm take only about 10 minutes to produce results. The Vidas D-dimer
from bioMérieux takes 35 minutes. The whole blood agglutination
SimpliRED assay from Agen Biomedical is the fastestyielding
results in only two minutes. It is done with a drop of whole blood,
so it doesn’t require centrifuging blood to separate out plasma.
All these D-dimer assays have a high sensitivity for deep venous
thrombosis and pulmonary embolism. The rapid ELISA assays Nyco-card
and Instant IA achieved a sensitivity of 94.3 percent in two studies
with a total of 213 patients with clinically suspected DVT and a
sensitivity of 96 percent in three studies of 592 patients with
suspected PE. SimpliRED had a sensitivity of 89 percent in one study
of 214 patients with signs of DVT, and, in two studies including
1,263 patients, SimpliRED had a sensitivity of 89 percent for pulmonary
embolism (Thromb Haemost. 1999; 688-692).
Ruling out VTE
A series of clinical investigations in recent years have shown that
a normal D-dimer test, with some assays, has a high negative predictive
value for venous thromboembolism. In a study of more than 400 patients,
a whole blood agglutination D-dimer assay had a negative predictive
value of 97.1 percent, and in combination with impedance plethysmography,
D-dimer had a negative predictive value of 98.5 percent (Arch
Intern Med. 1997; 157:1077-1081).
A rapid ELISA D-dimer assay had a negative predictive value of
99.8 percent in another study that followed patients who had both
a normal D-dimer and a normal proximal venous ultrasound for three
months after their initial evaluation for DVT. In this study only
one of 598 such patients who received no further testing or anticoagulant
therapy ended up developing venous thromboembolism (British Medical
A sequential diagnostic strategy involving D-dimer, lower limb
venous compression ultrasonography, and lung scanning yielded a
definitive diagnosis in 94 percent of 918 patients assessed for
suspected deep vein thrombosis or pulmonary embolism at Geneva University
Hospital, Geneva, Switzerland, and Hopital Saint-Luc, Montreal.
A rapid D-dimer ELISA ruled out venous thromboembolism in 286 patients
(31 percent); ultrasound established the diagnosis of venous thromboembolism
in 157 (17 percent); and lung scanning was diagnostic in 80 (nine
percent). This diagnostic scheme prevented invasive testing; angiography
was required in only 50 (five percent of) patients. It also effectively
identified the patients who needed anticoagulation; the three-month
risk of thromboembolism was 1.8 percent among patients who had not
been treated on the basis of the three-tier D-dimer-based diagnostic
protocol (Lancet. 1999;353:190-195).
"The use of the D-dimer for the management of patients with suspected
venous thromboembolism has been validated by prospective studies.
So we have strong scientific evidence that a normal D-dimer test
with certain assays is a safe way to manage patients and exclude
the diagnosis of venous thromboembolism," says Clive Kearon, MD,
PhD, associate professor of medicine at McMaster University and
head of the Clinical Thrombosis Service at Henderson Hospital, Hamilton
Health Science Corp., Hamilton, Ontario.
Understanding the fine points
Use of the new tests can be confusing, however. The Nycocard assay
is calibrated on the basis of the mass of purified D-dimer; Instant
IA D-dimer and Vidas are calibrated in terms of fibrinogen equivalent
units, which is the mass of fibrinogen that gives rise to the amount
of D-dimer present. Although both types of these ELISA-based tests
report similar cut-off values for normal D-dimer (500 ng/mL), the
actual numbers of D-dimer molecules will be different, depending on
the calibration scheme, Dr. Brandt points out.
"There is about a twofold difference in the numbers of D-dimer;
if you had 1 ng/mL on a mass D-dimer scale and then measured it
with a kit calibrated in fibrinogen equivalent units, you would
get 2 ng/mL. So you wouldn’t want somebody randomly taking a cut-off
value derived with a mass D-dimer unit and applying that number
to a kit that measures fibrinogen equivalent units to arrive at
a threshold between abnormal and normal," he says.
According to Dr. Brandt, some of the articles published on the
use of D-dimer to rule out DVT and PE have not clearly explained
whether they used mass D-dimer or fibrinogen equivalent units to
distinguish normal from abnormal values. "If someone didn’t read
the literature critically and took a cut-off value for D-dimer from
some of the published articles, he might end up with very much the
wrong value," Dr. Brandt notes.
It is unlikely that laboratories will repeat the studies that
have been published to find the appropriate normal threshold for
the new ELISA-based D-dimer tests. However, Dr. Brandt believes
laboratories should assess a reasonable number of normal patients
to find out what the normal range is for their assay. Laboratories
also should get some experience with patients who have DVT or PE
to find out where their D-dimer levels fall. "The manufacturer of
each test will probably provide a suggested cut-off. What the laboratories
need to do is verify that in their hands this cut-off value actually
separates normal patients from patients who have disease," he says.
It’s not as if a laboratory has to conduct a formal statistical
validation of the threshold for the rapid ELISA-based D-dimer tests.
"But laboratories should more or less confirm the cut-off value
from the manufacturer or the literature is appropriate for them.
And nobody has come up with how a laboratory ought to do that in
practical terms and without doing a major study," Dr. Brandt adds.
The whole blood agglutination SimpliRED assay is not plagued with
problems associated with cut-off values; it simply provides a positive
or a negative result. The assay therefore can be done at the point
of care. However, there has been interobserver bias in reading test
results. If this interobserver variation could be minimized, the
assay may prove to be more reliable, Dr. Brandt says.
"Performing the test in a central laboratory is one way to minimize
the variation. Limiting the number of personnel who perform the
bedside testing is another," he explains.
Developing clinical algorithms
In addition to understanding the vagaries of the rapid D-dimer assays,
pathologists also need to help emergency department physicians identify
the patients who are suitable for testing. When St. Louis University
Hospital, St. Louis, Mo., introduced the SimpliRED assay about four
months ago, J. Heinrich Joist, MD, PhD, professor of pathology and
internal medicine and director of St. Louis University Coagulation
Consultants, began educating clinicians about the differences between
the D-dimer assays that are available for diagnosing disseminated
intravascular coagulation and the tests that can rule out venous
Dr. Joist is in the process of creating with clinicians diagnostic
algorithms for the SimpliRED test . One question the physicians
are trying to address is whether they can postpone ultrasound solely
on the basis of the D-dimer for patients who come into the emergency
department during off-hours. "When a patient comes in at 2:00 in
the morning with a swollen, pain-ful leg or chest pain with shortness
of breath and has a negative SimpliRED D-dimer test, can that patient
be told to come back the next morning during regular hours to get
an ultrasound examination or ventilation-perfusion lung scan? To
what extent does individual patient clinical pretest probability
of DVT or PE have to be taken into account? Those issues are still
being discussed," Dr. Joist says.
Dr. Kearon, who has been testing the SimpliRED assay for the last
few years, still considers the test to be investigational and cautions
against applying it indiscriminately to any patient who has signs
and symptoms of DVT or PE. "A real concern would be that people
at clinical centers might latch onto using the D-dimer alone for
excluding venous thromboembolism and performing the test on every
patient who comes in with suspected DVT or PE, because that will
lead to missed diagnosis," he says.
Dr. Kearon also worries about the possibility that D-dimer testing
will be overused. "It is common for patients who do not have venous
thromboembolism to have an abnormal test. So if we start doing this
test when it is not truly indicated or start using it in a way that
is different from the original published studies, there is the potential
to get a large number of false positive results, which will precipitate
further investigation," he adds.
Dr. Brandt agrees. "A negative test is useful in ruling out venous
thromboembolism. A positive test does not rule in venous thromboembolism;
the diagnosis must be established by other means," he emphasizes.
In this light, he continues, "The critical parameter of assay performance
is negative predictive value, not sensitivity," where sensitivity
is the percentage of patients with the disease having a positive
Physicians at Dr. Kearon’s hospital apply a rapid D-dimer test
in a carefully designed protocol that first clinically stratifies
patients as having high, moderate, or low probability of PE. High-risk
patients are those who have clinical features that are typical of
venous thromboembolism, risk factors for venous thromboembolism,
and no alternative diagnosis. Typical clinical features include
swelling of the legs, particularly if it is accompanied by pain
and tenderness that is confined to areas where deep veins are present.
Risk factors are recent immobilization, underlying malignancy, and
hereditary predisposition to thrombosis.
On the other end of the spectrum are low-risk patients who have
atypical, usually mild symptoms and signs, no risk factors for venous
thromboembolism, and an alternative diagnosis that may account for
the patient’s presentation.
Dr. Kearon feels reasonably confident that if a patient has a
low clinical suspicion or probability for venous thromboembolism
and a negative D-dimer test, no further testing needs to be performed.
(The group at McMaster University, of which he is part, is testing
this in ongoing studies.) Other patients need followup investigations.
Dr. Kearon’s group has published findings that show if a patient
has a negative impedance plethysmograph and a negative D-dimer,
the prevalence of DVT is extremely low, on the order of one percent
or two percent. "So we’re prepared to exclude the diagnosis of DVT
on that basis." He also would exclude the diagnosis of DVT in patients
who have a normal D-dimer and normal compression ultrasound of the
The rapid D-dimer assays are part of an emerging pattern of care
that is changing the management of venous thromboembolism across
the country, says Dr. Triplett. Because of these assays and the
use of low molecular weight heparin, patients with uncomplicated
DVT can be identified quickly and economically and their care can
be managed as outpatients.
The D-dimers nevertheless are not being used extensively, and
rightfully so, Dr. Triplett says. Many clinicians are not aware
of the difference between the standard and the new rapid D-dimer
tests. Clinicians may not have a complete understanding of the patient
populations that will benefit from D-dimer screening for venous
thromboembolism. As Dr. Brandt points out, D-dimer levels gradually
increase with age; thus, the utility of the test as a diagnostic
screen diminishes in elderly patients.
The level of D-dimer changes also during the course of development
of venous thromboembolism. "D-dimer is highest when the thrombotic
episode first starts. If a patient comes in within a day or two
of the episode, the D-dimer will likely be positive. If the patient
waits for seven days, the level of D-dimer may drop below the threshold
and give a false negative result," Dr. Brandt says.
These factors lead Dr. Triplett to caution that "we need to be
careful with these tests. We need to know why we’re doing them,
how to interpret them, and when to use them with other studies."
"Medical centers are right to be slow to pick up this technology,"
Dr. Kearon says. "It is a technology in evolution, and I would be
concerned about clinical centers selecting a D-dimer test and using
it carelessly or without adequate precautions. Clinical centers
that are introducing the D-dimer test need to know its limitations
and realize that it can do more harm than good if it’s applied in
a noncritical fashion."
Karen Sandrick is a freelance writer in Chicago.