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Plasma D-Dimers in the Diagnosis of Venous Thromboembolism
James Kelly, BSC, MRCP;
Anthony Rudd, FRCP;
Roger R. Lewis, MD, FRCP;
Beverley J. Hunt, MD, FRCP, FRCPath
Arch Intern Med. 2002;162:747-756.
ABSTRACT
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Clinical suspicion for venous thromboembolism (VTE) mandates objective
testing to confirm or exclude the diagnosis. However, current imaging modalities
are imperfect because of a small but important risk of complications with
invasive techniques or limited sensitivity with noninvasive ones. A diagnostic
tool for VTE is needed that is noninvasive and highly accurate, allowing immediate
treatment decisions to be made in most cases. Plasma D-dimers (D-ds), specific
cross-linked fibrin derivatives, partially fulfill these criteria in that
they are sensitive markers for thrombosis but lack specificity. They therefore
cannot be used to make a positive diagnosis of VTE; however, they generally
have high negative predictive value and are useful as an exclusionary test,
a potentially important role given that VTE is eventually ruled out in most
patients investigated. Clinical management studies are clarifying the role
of D-ds in the diagnostic paradigm of VTE: negative ultrasound and D-d findings
obviate the need for serial imaging in suspected deep vein thrombosis, and
anticoagulant therapy can be safely withheld in patients with non–high
clinical suspicion for pulmonary embolism and non–high probability ventilation
perfusion scan if D-d test results are negative. More recently, the combination
of a negative SimpliRED (AGEN Biomedical Ltd, Brisbane, Australia) D-d result
and low clinical suspicion derived using a formal scoring system has been
shown to exclude deep vein thrombosis and pulmonary embolism and to obviate
the need for imaging. Several different D-d assays are now available, and
clinicians should be aware of the performance characteristics of the test
used before incorporation into diagnostic algorithms as these will differ
between assays, and the results of clinical management studies cannot necessarily
be safely extrapolated to assays other than those specifically evaluated.
If alternative assays are to be substituted, these should consistently have
been shown to possess equivalent or greater sensitivity.
INTRODUCTION
The incidence of venous thromboembolism (VTE) rises progressively with
age,1-2 with a cumulative event
rate greater than 10% by age 80 years,2 and
is associated with substantial morbidity and mortality in the absence of treatment.3-4 The clinical signs and symptoms are
not sufficiently specific to establish or exclude the diagnosis,5
and objective testing is required in the presence of clinical suspicion to
aid management decisions,6 although all imaging
techniques currently used have clinical or practical limitations.
Contrast venography (CV) and pulmonary angiography remain the gold standard
diagnostic tests for deep vein thrombosis (DVT) and pulmonary embolism (PE),
respectively, but they are invasive and are associated with a small but significant
risk of complications.7 These tests are therefore
no longer widely used as first-line investigations. Noninvasive diagnostic
strategies have been developed, usually using ultrasound, the most accurate
noninvasive test for suspected DVT,8 and ventilation-perfusion
(VQ) scanning for suspected PE. However, VTE can only be diagnosed or excluded
with reasonable confidence using a 1-step process in a few patients. Three
fourths or more of the patients with suspected DVT have negative ultrasound
findings8-9 and require repeated
imaging to identify the further 2% to 6% in whom occlusive proximal DVT becomes
apparent in a week.8, 10 In addition,
70% of patients with suspected PE have a nondiagnostic VQ scan11
(low or intermediate probability) and require further imaging using either
pulmonary angiography or serial noninvasive imaging with ultrasonography to
identify residual proximal DVT, a valid approach in patients with adequate
cardiopulmonary reserve.12-14
Given these shortcomings, a simple but reliable noninvasive test for
VTE is highly desirable and should ideally have a sensitivity and negative
predictive value of 100% as the consequences of nondiagnosis are potentially
life threatening.3-4 Plasma D-dimers
(D-ds) have proved to be the most useful blood markers of intravascular fibrinolysis15 and are of interest as an adjunctive exclusionary
test in suspected VTE, potentially increasing the number of patients who can
be satisfactorily treated without recourse to a second level of investigation.16 We review D-ds in relation to VTE and their incorporation
into diagnostic strategies.
WHAT ARE D-ds?
Plasma D-ds are generated when the endogenous fibrinolytic system degrades
fibrin, as in VTE, and they consist of 2 identical subunits derived from 2
fibrin molecules. Unlike fibrinogen degradation products, which are derived
from fibrinogen and fibrin, D-ds are a specific cross-linked fibrin derivative.17-18 Because 2% to 3% of plasma fibrinogen
is degraded to fibrin, small amounts are detectable in the plasma of healthy
individuals. The half-life is approximately 8 hours, with plasma clearance
via urinary excretion and the action of the reticuloendothelial system.19
RELATIONSHIP BETWEEN VTE AND D-d LEVELS
D-dimer levels are increased by any condition in which fibrin is formed
and degraded by plasmin19 and are the best
currently available laboratory marker of activation of coagulation.20 D-dimer levels are elevated approximately 8-fold
after VTE compared with controls, with levels falling to approximately one
quarter of the initial value between weeks 1 and 221;
they are significantly higher in patients with extensive proximal DVT than
in those with below-the-knee DVT,22 with peak
levels corresponding to the extent of thrombosis.23
D-dimer levels may be particularly useful in the diagnosis of recurrent DVT,
a subgroup in which conventional imaging has important shortfalls.18 Using direct thrombus magnetic resonance imaging
(MRI), Fraser et al24 recently showed that
D-d levels correlate with clot volume and surface area. Clot surface area
seemed to be the more important determinant, supporting the concept that D-d
generation, release, or both occur primarily at the surface of the thrombus.
After a thrombotic event, D-d levels may normalize within 15 to 20 days20, 23 and are probably most useful for
diagnosis within 11 days of symptom onset.21
Although initiation of heparin calcium therapy causes a sharp decline in levels,
absolute values remain increased compared with those of controls, and the
test remains useful in patients awaiting investigation in whom treatment has
already been started.20, 25-26
OTHER CONDITIONS ASSOCIATED WITH RAISED D-d LEVELS
Levels of D-ds are rarely elevated in healthy individuals22
but may be increased in any condition involving the formation and degradation
of fibrin, such as infections, cancer, surgery, cardiac or renal failure,
acute coronary syndromes, acute nonlacunar stroke, pregnancy, and sickle cell
crises.19, 27-30
Furthermore, many of these conditions are also risk factors for VTE and may
have initial symptoms or signs similar to PE.29
D-dimer levels are therefore less likely to be useful in patients with suspected
VTE and 1 or more of these diagnoses because, for example, increased values
occur in 80% to 90% of those with infections or malignancy.31
MEASUREMENT OF D-d LEVELS
Measurement of D-d levels has been enabled by the development of monoclonal
antibodies that bind to epitopes on D-d fragments that are absent on fibrin,
fibrinogen, and non–cross-linked fragments of fibrin, with detection
of resulting complexes by enzyme-linked immunosorbent assay (ELISA) or agglutination
techniques.29, 32-33
The classic microplate ELISA technique is considered the gold standard,34 but it is not useful as a routine emergency test
as it is suitable for batch analysis and is labor intensive. However, the
recently developed VIDAS test (bioMérieux SA, Marcy-Étoile,
France), which combines the ELISA method with a final detection in fluorescence,35 is fully automated and provides a result within 35
minutes and can therefore be used for single-sample testing.36
Two immunofiltration (membrane ELISA) techniques have also been introduced
that have sensitivities similar to those of conventional ELISAs but higher
specificities: the Instant IA D-d assay (Diagnostica Stago, Inc, Parsippany,
NJ) gives a result in less than 8 minutes but is performed manually and is
qualitative (positive or negative),37 and the
NycoCard D-d assay (Nycomed Pharma AS, Asker, Norway) is semiquantitative
and provides a result in less than 2 minutes.37-38
Other techniques involve agglutination of latex beads or red blood cells
and give a qualitative or semiquantitative result within a few minutes. For
example, the SimpliRED test (AGEN Biomedical Ltd, Brisbane, Australia) is
a red blood cell agglutination assay designed for use with fresh capillary
or venous whole blood. It provides a result in less than 5 minutes and is
therefore suitable for near-patient testing.39
More recently, immunoturbidimetric techniques have been developed that allow
a quantitative estimation and represent a second generation in latex agglutination
technology (eg, TinaQuant assay [Roche Diagnostics, F. Hoffmann-La Roche Ltd,
Basel, Switzerland], Liatest assay [Diagnostica Stago, Inc], and MDA D-d [Organon
Teknika B.V., Boxtel, the Netherlands]). Immunofiltration and immunoturbidimetric
techniques may therefore combine the advantageous properties of the ELISA
with the speed and simplicity of the latex tests. The properties of the main
D-d detection techniques are given in Table
1.
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Table 1. Characteristics of the Different Classes of D-Dimer Detection
Techniques*
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ACCURACY OF TECHNIQUES
Pooled data from 20 studies of more than 2000 outpatients with clinically
suspected VTE using 3 different classic microplate ELISA assays (Dimertest
[American Diagnostica Inc, Greenwich, Conn], Asserachrom Ddi [Diagnostica
Stago, Inc], and Fibrinostika FbDP [Organon Teknika B.V.]) have shown a diagnostic
sensitivity of 97% at a cutoff value of 500 ng/mL,40
with false-negatives presumably explained on the basis of a small thrombus
mass (sensitivity may be lower in patients with isolated below-the-knee DVT22, 41), a long time lag between thrombus
formation and D-d testing,20 and, possibly,
an impaired pathophysiological fibrinolytic response in the occasional patient.42 Specificity in these studies was approximately 35%
to 45%, and D-d levels are currently regarded as useful only as exclusionary
tests for VTE and are less useful in inpatient populations because of lower
specificity consequent on comorbidity.31 More
recent data21, 36-38,40-41,43-53
also show high sensitivities for the 3 novel rapid ELISA-based assays; indeed,
several studies36, 41, 43-46
evaluating the VIDAS assay have reported a sensitivity of 100%.
Whereas conventional latex agglutination assays are not regarded as
sufficiently sensitive to be of clinical value,40, 54-55
second-generation kits using either whole blood agglutination (SimpliRED)
or immunoturbidimetric techniques (eg, TinaQuant, Liatest, and MDA D-d) have
emerged with higher sensitivities and are clinically useful.41, 55-61
Van der Graaf et al41 recently assessed
the diagnostic performance of 10 novel rapid tests based on ELISA or latex
agglutination technology and 3 conventional ELISAs in 99 outpatients with
suspected DVT who underwent CV (Table 2). Correlation between different assays is poor,20, 54
and it is not currently possible to standardize D-d results from different
assays, making it difficult to extrapolate results from one setting to another.
Also, important interobserver variation may occur with semiquantitative tests.36 Studies58, 62-63
have also shown that combining an additional variable, such as a clinical
probability assessment, respiratory rate, arterial blood gas estimation, or
measurement of alveolar dead space, augments the negative predictive value
of a normal D-d value. For example, in a study62
of patients undergoing evaluation for suspected PE, the combination of a negative
SimpliRED assay result and a PaO2 greater than 10.7 kPa had a negative
predictive value of 100%.
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Table 2. Performance Characteristics of 13 D-Dimer (D-d) Assays in
Suspected Deep Vein Thrombosis*
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The VIDAS and SimpliRED assays are the most extensively studied and
widely used in the diagnosis of VTE. Pooled data indicate that although the
VIDAS assay is the more sensitive of the two (90%-100% [generally 98%-100%]),
specificity (5%-55% [generally, 40%]) is relatively poor so that a normal
result has high negative predictive value but occurs in a small proportion
of patients, limiting its diagnostic utility (only data from studies using
a threshold of 500 ng/mL were included).36, 41, 43-53
By contrast, the SimpliRED assay is somewhat less sensitive (61%-100% [generally,
85%]) but more specific (20%-94% [generally, 70%]) so that the negative predictive
value is lower but the test is likely to be diagnostically useful in a greater
proportion of patients.41, 43, 56, 59-60,64-71
The utility of these two tests in clinical management studies are reviewed
herein. Note that specificity of assays vary depending on the population studied
and will be highest in outpatient populations with a low prevalence of comorbidity.
D-d LEVELS IN THE ELDERLY
D-dimer levels increase linearly with age, particularly in the presence
of coexisting functional impairment,72 because
of a combination of factors, including reduced renal clearance, increased
levels of plasma fibrinogen, and the presence of occult disease.19, 73
In one study19 of healthy individuals, average
levels in the highest age quartile (71-90 years) were approximately 4 times
greater than those in the lowest quartile (11-30 years), hence specificity
and therefore diagnostic utility for VTE is lower in older patients (Table 347, 74-75),
although a negative result retains the same clinical value as in younger patients.47, 72, 74-75 In
one study53 evaluating the optimal discriminatory
threshold of the VIDAS assay in elderly inpatients (average age, 86 years)
with suspected DVT, specificity improved at a cutoff value of 750 ng/mL compared
with 500 ng/mL without a decrease in sensitivity, but it was still poor at
only 20%.
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Table 3. Performance Characteristics of the Asserachrom Ddi ELISA (Diagnostica
Stago, Inc, Parsippany, NJ) in Different Age Strata at a Cutoff Value of 500
ng/mL in 671 Outpatients Investigated for Suspected Pulmonary Embolism*
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CAN D-d LEVELS EVER BE USED TO MAKE A POSITIVE DIAGNOSIS OF VTE?
The positive predictive value for VTE rises as D-d levels increase progressively
above the diagnostic threshold,76 and in a
study74 evaluating 671 outpatients with suspected
PE, the specificity of D-d (Asserachrom Ddi ELISA) was 93% when levels exceeded
4000 ng/mL. This raises the possibility that, depending on the pretest probability,
in certain situations a high D-d level might be sufficient grounds to initiate
treatment.74, 77 However, this
issue requires further study and, in the absence of further data, D-d levels
should be used in an exclusionary capacity only at this stage.
MANAGEMENT STUDIES OF D-d IN SUSPECTED VTE
Data demonstrating the high sensitivity of certain D-d assays for the
diagnosis of symptomatic VTE under the optimal conditions of performance studies
suggest a putative role as an exclusionary test. However, outcome studies
demonstrating that treatment can be safely withheld in suspected VTE using
diagnostic approaches incorporating D-d assays under routine conditions are
required, as similar performance cannot be assumed in the less predictable
domain of clinical practice, and even a small margin of error may not be acceptable
with a potentially lethal disease.78 For example,
previous studies have demonstrated that outcome is excellent when treatment
is withheld in suspected DVT on the basis of negative CV79
or serial ultrasound8 findings and in suspected
PE with negative pulmonary angiographic findings80
or normal VQ scintigraphy results.81 Without
similar data, routine clinical use of D-d assays would be premature and could
not be recommended.77
Several management studies have now evaluated D-ds in diagnostic algorithms
for VTE incorporating noninvasive imaging55
and/or information from an a priori clinical probability assessment, the complimentary
role of which in diagnosis has been previously demonstrated.82-85
Bernardi et al86 investigated 946 outpatients
with suspected DVT. Treatment was withheld in patients with negative initial
ultrasound findings and normal D-d levels (Instant IA ELISA: <500 ng/mL
regarded as normal) and the risk of VTE was less than 0.2% at 3 months in
this subgroup. D-dimer results were negative in almost 90% of patients with
negative initial ultrasound findings so that management decisions could be
made in most patients the day of presentation. In a similar study, Kraaijenhagen
et al87 also found that the combination of
negative initial ultrasound findings and normal D-d levels (SimpliRED assay)
in 552 outpatients with suspected DVT was associated with a risk of clinically
apparent VTE of only 0.4% at 3 months in the absence of treatment. The results
of these 2 studies show that serial testing is obviated in the presence of
normal D-d levels, and a recent decision analysis model has shown that this
strategy is likely to be cost-effective.88
In a study by de Groot et al59 evaluating
245 patients for suspected PE, anticoagulant therapy was withheld in the presence
of a nondiagnostic VQ scan, non–high clinical probability, and negative
SimpliRED test results. Only 1 patient (1.5%) in this subgroup experienced
a possible VTE event at 3-month follow-up, an incidence similar to that in
those with normal VQ scan findings, suggesting that this approach was safe.
Similarly, Perrier et al89 evaluated a diagnostic
protocol for PE in 308 patients combining clinical probability, D-d assay
(Asserachrom Ddi ELISA), and ultrasonography in patients with nondiagnostic
VQ scans, with pulmonary angiography reserved for cases in which the noninvasive
workup was inconclusive. Treatment was withheld in 53 patients with nondiagnostic
VQ scans and intermediate clinical probability of PE in whom D-d levels were
less than 500 ng/mL, none of whom developed VTE during the following 6 months.
An additional 363 patients were subsequently recruited, including another
74 with an intermediate clinical probability of PE, nondiagnostic VQ scans,
and negative D-d test results. Again, no cases of VTE occurred in this subgroup
during another 3 months of follow-up.74
Three trials have assessed the safety of obviating imaging in patients
with suspected VTE on the basis of D-d level alone or in combination with
a clinical probability assessment. Perrier et al90
evaluated a noninvasive diagnostic algorithm in 918 patients with suspected
DVT or PE using D-d estimation (VIDAS assay) as the first step. Levels were
less than 500 ng/mL in 159 patients (36%) with suspected PE. These patients
were not investigated further, irrespective of clinical suspicion, and none
developed VTE during the next 3 months. Patients with elevated D-d levels
underwent VQ scanning or ultrasonography, whereas those with suspected DVT
underwent US irrespective of D-d levels. By incorporating a clinical probability
assessment, noninvasive diagnosis was possible in 94% of the entire cohort,
and the risk of VTE at 3 months in those not given anticoagulants using this
algorithm was 1.8%; only 2 of the 12 patients in this subgroup had normal
D-d levels at presentation.
Kearon et al91 evaluated 445 outpatients
with a suspected first episode of DVT who underwent a standardized assessment
of clinical probability (Table 483) and D-d assay (SimpliRED); 40% had a low clinical
probability for DVT and negative D-d findings, and this subgroup was not investigated
further. This strategy was safe, with only 1 of 177 patients developing VTE
during the subsequent 3 months, giving a negative predictive value for this
combination of 99.4%. Finally, Wells et al92
evaluated a predominantly noninvasive diagnostic strategy in 930 outpatients
with suspected PE in which the initial step comprised a standard assessment
of pretest clinical probability93 (Table 5) and D-d assay (SimpliRED), as
in the previous study. Those with low suspicion and negative D-d findings
did not undergo further investigation, with all other patients undergoing
VQ scanning. If the VQ scan was nondiagnostic, bilateral, lower-limb ultrasonography
was performed, with further testing guided by the results of the clinical
assessment, D-d assay, and VQ. Forty-seven percent of the entire group were
considered not to have PE on the basis of low suspicion and negative D-d findings,
and only 1 of these 437 patients developed VTE during the next 3 months, giving
a negative predictive value of 99.5% for this combination. The overall frequency
of VTE during follow-up in the cohort in whom PE was thought to have been
initially excluded and in whom the protocol was followed correctly was 0.1%,
and pulmonary angiography was required in only 1% of the total.
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Table 4. Clinical Probability Score Used in Patients With Suspected
Deep Vein Thrombosis*
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Table 5. Clinical Probability Score Used in Patients With Suspected
PE*
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These studies provide an evidence base for the use of D-d tests in noninvasive
diagnostic algorithms for suspected VTE. In particular, the studies of Kearon
et al91 and Wells et al92
demonstrate that imaging can safely be obviated in up to half of patients
with suspected VTE using a formal clinical probability assessment in combination
with a SimpliRED D-d test, and invasive testing is rarely required in the
remainder. However, 3 caveats should be borne in mind. First, these results
apply to a specific D-d test and method of clinical probability assessment.
Substitution of an alternative assay would be safe only if its sensitivity
equaled or exceeded that of SimpliRED, though diagnostic utility might be
inferior if specificity were lower. Less sensitive assays should not be used.
Second, the overall prevalences of VTE in these cohorts were less than 15%
so that the frequency of thromboses in the low-probability groups was only
1% to 2%. The negative predictive value of a combined low probability and
negative D-d finding will diminish as disease prevalence rises so that the
safety of this approach cannot be assumed if a significantly higher prevalence
of VTE is anticipated. Third, VTE was diagnosed in up to 20% of patients with
a negative D-d result and high clinical suspicion, demonstrating that the
SimpliRED assay cannot be used in isolation and reinforcing the need for a
careful clinical assessment. In contradistinction, exclusion of PE on the
basis of a negative VIDAS assay finding seemed safe in one study.90 Such an approach would not be acceptable unless sensitivity
of the assay used approximated to 100%, although in the absence of further
confirmatory data, it would seem prudent to exercise caution when the results
of D-d testing using a highly sensitive assay and clinical suspicion are at
odds.
D-d AS A SCREENING TEST FOR ASYMPTOMATIC VTE
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