The Role of D-Dimer in the Diagnosis of Venous Thromboembolism

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1 science [chemistry coagulation and hematology] The Role of D-Dimer in the Diagnosis of Venous Thromboembolism Donald H. Schreiber MD, CM FRCPC FACEP Division of Emergency Medicine, Stanford University School of Medicine, Palo Alto, CA 136 D-dimers, the fibrinolytic degradation products of cross-linked fibrin, have emerged as the most useful of the procoagulant activity and ongoing fibrinolysis markers. D-dimer measurements are very sensitive in excluding a diagnosis of pulmonary embolism in the setting of normal values, a low clinical suspicion, and non-diagnostic lung scans. Deep venous thrombosis (DVT) and its sequel, pulmonary embolism (PE), collectively termed venous thromboembolism (VTE), are associated with significant morbidity and mortality. 1,2 Clinical diagnosis is inherently insensitive and inaccurate because the common signs and symptoms of the disease overlap considerably with other conditions. 3 It is unacceptable to diagnose DVT or PE clinically and commit patients to the risks of anticoagulation without confirmatory objective testing. Various diagnostic algorithms that emphasize non-invasive and cost effective strategies have been evaluated. 4-6 The discovery of a single laboratory marker that would either confirm the diagnosis or rule out the disease can be considered the holy grail of clinical medicine. Pathophysiology DVT of the lower extremity usually begins in the deep veins of the calf around the valve cusps or within the soleal plexus. 7 A minority of lower extremity DVT arise in the ileofemoral system as a result of direct vessel wall injury, as seen with hip surgery or catheter-induced DVT. The vast majority of calf vein thrombi dissolve completely without therapy. Approximately 20% propagate proximally. Propagation usually occurs before embolisation. The process of adherence and organization of the venous thrombus does not begin until 5 to 10 days after thrombus formation. Until this process has been fully established, the non-adherent, disorganized thrombus may propagate and/or embolize. Not all venous thrombi pose equal embolic risk. Studies have shown that isolated calf vein thrombi carry a limited risk of pulmonary embolism. 8 Furthermore, studies have suggested that isolated calf vein thrombi are smaller and do not cause significant morbidity or mortality if they embolize. Contradictory evidence from several other studies has indicated that isolated calf vein thrombi do embolize and that propagation proximally may occur rapidly leading to fatal pulmonary embolism. 9,10 The current diagnostic and therapeutic management of DVT is influenced strongly by the different risks assigned to proximal and calf vein thrombi. Most clinicians do not anticoagulate patients with isolated calf vein thrombosis unless objective evidence of proximal extension is found on serial evaluation. Exceptions may be made for highrisk patients. Pulmonary embolism is best viewed as a complication of DVT. The two conditions often co-exist. Asymptomatic DVT is found in 40% to 50% of patients with PE. Forty percent of patients with DVT have clinically silent PE based on objective studies. 11 D-Dimer Overview Venous thrombosis activates the coagulation and fibrinolytic systems, and results in elevated levels of serum markers collectively called fibrin spit products. During thrombus formation, fibrinogen is converted to fibrin monomers that are extensively cross-linked into a polymer network. This cross-linking of fibrin takes place in the region of the polymer termed the D-domain. Adjacent D-domains are covalently linked and constitute a fibrin specific feature of a thrombus, not found in fibrinogen or non-cross-linked fibrin degradation products. 12 Fibrin polymers are degraded by plasmin in the fibrinolytic process. One of the terminal products of fibrinolysis is the covalently linked D-Domain called the D-Dimer fibrin fragment. Monoclonal antibodies to D-Dimer have been developed that can differentiate fibrin specific clot from noncross-linked fibrin as well as fibrinogen. It is important to recognize that these antibodies are specific for both freshly formed fibrin clots as well as the products of fibrinolysis. This specific characteristic of D-Dimer antibodies explains its high sensitivity for venous thromboembolism. As opposed to other markers that only detect products of acute coagulation, D-Dimer assays expand the diagnostic window. 13 D-Dimer antigen levels are elevated in the acute phase of clot formation as would occur in acute deep venous thrombosis, and also the fibrinolytic stage that would occur in the setting of acute pulmonary embolism. D-Dimers have a half-life of 4 to 6 hours. Continued fibrinolysis that occurs in DVT and pulmonary embolism causes the D- Dimer to remain elevated for about 7 days. Patients who present late in their course once clot organization and adherence begins, may have low levels of D-Dimers. 14 Patients with isolated calf vein thrombosis may have a small clot burden that results in low levels that fall below the detection limits of the various assays. This explains the finding of false negative D-Dimer assay results in the setting of confirmed calf vein DVT. Fortunately, routine venipuncture and peripheral intravenous catheters that are associated with small clots and hematoma formation do not usually cause elevation of D-Dimer levels. 15 D-Dimer levels correlate with the presence of fibrin clots without regard to location. In a variety of other medical

2 Current D-Dimer assays D-Dimer Assay Manufacturer Method T1 SimpliRED Agen M RBC Agglutination, Qualitative Vidas DD BioMerieux M Rapid ELISA, Quantitative D-DI Test Diagnostica Stago M Latex Agglutination slide, Semi Quantitative MDA D-Dimer Organon Teknika M Micro-Latex Immunoassay, Quantitative IL-Test D Dimer Beckman Coulter M Immunoturbidimetric, Quantitative Accuclot Sigma Diagnostics M Latex Agglutination, Semi-quantitative AUTO D Dimer Sigma Diagnostics Automated latex agglutination, quantitative Dimertest Latex Dade Behring M Latex Agglutination, qualitative Asserachrom Diagnostica Stago ELISA, quantitative Fibrinosticon D Dimer Organon Teknica ELISA, quantitative Minutex Biopool M Latex Agglutination, qualitative Liatest D Di Diagnostica Stago Micro-latex immunoassay,quantitative Advanced D- Dimer Dade Behring Immunoturbidimetric, quantitative Miniquant Biopool Immunoturbidimetric, quantitative conditions such as trauma, recent surgery, hemorrhage, cancer, and sepsis that are associated with activation of the coagulation system and formation of fibrin clots, D- Dimer levels are elevated. 16 This explains the low specificity of the D-Dimer assay for venous thromboembolism. D-Dimer Assays There are many different D-Dimer assays currently on the market [T1]. They may be grouped by laboratory method with advantages and disadvantages of each type. Extensive discussion of the merits of individual assays is beyond the scope of this article. The classic enzyme-linked immunosorbent assay (ELISA) is highly sensitive but is also time consuming, labor intensive, and impractical for use as an emergency test. 17 Slide latex agglutination assays have also been evaluated. Results are qualitative (or semiquantitative if serial dilutions are used). Although rapid, their sensitivities are not high enough to reliably exclude venous thrombosis. They should not be used in any diagnostic algorithm for DVT or PE. 18 Whole blood RBC agglutination tests are based on a hybrid monoclonal antibody that binds the D-Dimer antigen and RBCs, leading to agglutination. The Simplired D-Dimer assay (Agen Biomedical Limited M, Brisbane, Australia) is rapid and can be performed at the bedside with only a few drops of capillary blood. The result is qualitative. Sensitivities ranging from 61% to 100% have been reported. 19 Interobserver variability with this assay has been noted because the test result must be interpreted visually. Experienced laboratory personnel performed best. Immunofiltration assays are based on the binding of gold labeled monoclonal antibodies to D-Dimer antigen fixed to a filtration membrane. A qualitative result is interpreted visually or semi-quantitatively using a reflectometer. These assays are rapid and have the added advantage of availability for bedside point of care testing. Sensitivities range from 81% to 100%. 17,19 Automated rapid ELISA assays have recently been introduced with turnaround times of 1 hour. These assays have all the benefits of the classical ELISA assay but rapid availability of results facilitates their use as an emergency test. The Vidas D- Dimer (Biomerieux M, Marcy L etoile, France) is the only rapid ELISA assay available with reported sensitivities ranging from 94% to 100%. 12,17,19 Latex enhanced photometric immunoassays are quantitative, turbidimetric or colorimetric assays that use latex particles coated with human monoclonal antibody to the D-Dimer antigen. These assays can be performed either in the laboratory or at the bedside. Turnaround times are rapid, facilitating their use as an emergency test. In limited studies the Tinaquant D-Dimer BM, STA LIAtest D-Di (both of Roche Diagnostics M, Indianapolis, IN), Turbiquant D-Dimer (Merck M, Whitehouse, NJ), and IL-D-Dimer (Beckman Coulter M, Fullerton, CA) performed best with sensitivities approaching that of the rapid ELISA tests, 92% to 100%. 12,17,19 Characteristics of the ideal D-Dimer assay are listed in T2. The current assays use different monoclonal antibodies and there is an overall lack of standardization. Identical antibodies may react differently in different assay formats. The calibrators used in one assay may give a different result when used in a different assay. The units also vary with some reported in ng/ml of D-Dimer units and others in fibrinogen equivalent units (FEU). Results from one assay cannot necessarily be extrapolated to another. 12,13 Statistics and the D-Dimer Assays in Venous Thromboembolism Different D-Dimer assays have been evaluated in various diagnostic algorithms for DVT to improve cost-effectiveness. The 137

3 138 Characteristics of the Ideal D-Dimer Assay High sensitivity Rapid lab turnaround Availability for 24 hours a day/7 days a week Point of care Low interobserver variability Low coefficient of variation T2 introduction of the D-Dimer assay to the clinical diagnostic algorithm for DVT or PE has generated considerable interest, but the clinician must be aware of their limitations. D-Dimer assays are sensitive but relatively non-specific. The potential value of this test rests only on a negative result to rule out the diagnosis. The relation between a diagnostic test and its clinical usefulness is usually expressed in terms of its sensitivity and specificity. From a clinical point of view, the primary concern is the probability of disease after the test result (post-test probability). Bayes original theorem states that the interpretation of a test depends on both the pretest probability of the disease and the sensitivity and specificity of the test. For example, if the sensitivity of a test is 80% and the specificity is 90%, the predictive value of a negative test result varies with the pretest probability of the disease. If the pretest probability is low, eg, 10%, then the negative predictive value is 97.6%. This means that 2.4% of patients with a negative test result have the disease. 20 However, if the pretest probability is 90%, the negative predictive value is only 33.3% and 66.6% of this patient group that have the disease will have a negative result. As the pre-test probability of a test increases, the negative predictive value of the test falls if the sensitivity remains unchanged. 20 The sensitivity of the test must approach 100% in order to effectively rule out the disease in high-risk patients (ie, to achieve a post test probability of the disease of less than 1%). In low-risk groups with a pretest probability of less than 5%, a lower test sensitivity approaching only 80% may still rule out the disease by achieving a negative predictive value of 98%. 19 These basic principles of statistics must be remembered when interpreting test results. The overall sensitivity and specificity of a test has very different implications depending on the pretest probability of the disease. In high-risk patients with suspected DVT or PE, a negative D-Dimer result does not rule out the disease unless the test sensitivity is almost 100%. When the pretest probability of the disease falls into the low or moderate range, lower degrees of test sensitivity can still yield negative predictive values in the 99% range and preserve the usefulness of the test. When evaluating studies, attention must be directed to the patient population. Clinical studies that analyze results across specific risk groups are best. Other investigations that combine all risk groups into a single cohort are difficult to interpret. D-Dimer in the Diagnosis of DVT The gold standard for the diagnosis of DVT has been contrast venography. 21,22 For many reasons including allergic reactions, contrast-induced DVT, 23 technical difficulties, inadequate studies, interobserver variability, and lack of availability, venography is either contraindicated or non-diagnostic in as many as 20% to 25% of patients. Noninvasive studies have essentially replaced venography as the initial diagnostic test of choice Duplex ultrasound has been shown to be sensitive and specific for DVT with sensitivity for proximal vein thrombosis of 97% The major limitation of duplex ultrasound is its reduced sensitivity to calf vein thrombosis. Venous thrombosis proximal to the inguinal ligament is also difficult to visualize. Nonoccluding thrombi may also be hard to detect. In patients with suspected recurrent DVT, ultrasound may not be able to differentiate between old and new clots. Current diagnostic strategies have recommended the use of duplex ultrasound (US) as the initial test of choice. The majority of patients with clinically suspected DVT ultimately rule out for the disease. Wells has prospectively validated a simple 9-point DVT score to determine the pre-test probability of DVT in symptomatic outpatients Incorporating the score into the diagnostic algorithm for DVT has facilitated patient management. In the validation study, the prevalence of DVT was 5%, 33%, and 85% in the low, moderate, and high risk categories, respectively. Incorporation of a D-Dimer assay into the diagnostic algorithm for DVT has been evaluated in a number of different studies. 34 Ginsberg evaluated the SimpliRED RBC agglutination D Dimer assay in 398 patients with clinically suspected DVT using the Wells DVT score. In low-risk patients with a negative SimpliRED result, only 1/178 patients with a low pretest probability and a negative D-Dimer result had a DVT. The NPV was 99.4% (95% CI = 96.9% to 100.0%). Overall, the negative predictive value of the test was 97.2%. 35 Anderson and associates evaluated 214 emergency department patients with clinically suspected DVT. All patients underwent D-Dimer testing using the SimpliRED assay, ultrasound evaluation, and 3-month follow-up. Overall the sensitivity of this D- Dimer assay was only 82.1%; but in the subgroup of patients with low pretest probability, the sensitivity was 100%. The negative predictive value was 100% (96.3% to 100.0%) In the moderate- and high-risk subgroups, the negative predictive values were 94.1% (83.8% to 98.8%) and 86.7% (59.4% to 98.3%), respectively. The authors concluded that in patients with low probability of the disease, a negative SimpliRed D-Dimer result rules out DVT. 36 Similar results were reported by Aschwanden in 343 patients with clinically suspected DVT. The likelihood ratio for low risk patients with a negative D-Dimer (SimpliRED) was.05 for proximal DVT but 0.20 for calf vein DVT. 37 Van der Graaf evaluated 13 different D-Dimer assays on 99 emergency department patients with clinically suspected DVT. Deep venous thrombosis was evaluated using venography as the gold standard. 19 Fifty percent of the 99 patients ultimately had DVT, a much higher prevalence than most outpatient studies. Only 2 D-Dimer assays achieved 100% sensitivity and NPV: the VIDAS DD and the Tinaquant D-Dimer assays. The latter is an automated immunoturbidometric assay. The author noted that an NPV of at least 98% is required to safely exclude DVT in outpatients. They concluded that these assays

4 may be used to reliably exclude DVT in all patients but no analysis based on the pretest probability of DVT was performed. Dempfle reviewed D-Dimer testing for DVT and concluded that overall the Vidas rapid ELISA assay performed best, achieving excellent sensitivity, low inter-assay, and inter-laboratory variation. 12 The feasibility of a single negative D- Dimer test to exclude DVT was evaluated in a recent Franco-Canadian study involving 474 outpatients with suspected DVT. 38 Ultrasound was performed in all patients. The Vidas D-Dimer assay achieved a NPV of 98.4%. Only 2/129 had false negative D- Dimer results and both occurred in patients with calf vein DVT and a moderate pretest probability of DVT. Only 1.6% of all patients with DVT would be missed with a negative Vidas D-Dimer assay and no patients with a proximal vein DVT would have been missed. All the remaining 127 patients had a negative US study and no evidence of DVT/PE on 3 month follow-up. Bates 39 retrospectively evaluated a slide immunoagglutination assay, MDA D- Dimer (Organon Teknika Inc M, Durham NC), in a series of 595 patients. The sensitivity and the NPV of the assay in the low probability group were both 100%. In the moderate risk group, they were 96% and 98% respectively. Combining patients with low and moderate risk of DVT, the NPV was 99%. As 80% of patients fell into the low or moderate risk groups and 40% had a negative D-Dimer assay, the potential for a single blood test to eliminate the need for further diagnostic investigation proved to be clinically valuable. D-Dimer as an Adjunct to Ultrasound The risk of DVT and PE is approximately 2%-4% in patients with moderate probability and an initial negative US. At the present time, patients with moderate or high pretest probability for DVT and negative initial ultrasound studies are followed with a repeat ultrasound evaluation at 1 week to detect proximal extension of an undetected calf vein DVT. 16,40-43 Alternatively, these patients may be referred for venography instead of serial US evaluation. Both of these strategies are inconvenient and relatively expensive. This has prompted a continued search for more cost-effective approaches. Serial ultrasound evaluation is not cost effective because the positive yield of the second ultrasound, required in as many as 76% of patients, is only 1%. 42 A number of studies have evaluated the benefit of sequentially combining a D-Dimer assay with ultrasonographic examination in moderate probability patients. Bernardi 44 evaluated 946 patients with suspected DVT using ultrasound and a rapid qualitative ELISA D-Dimer assay (Instant IA D Dimer, Stago M, Asnietres, France). Only 1/598 (0.2%) with negative US and DD developed VTE during followup. Patients with discordant results received serial US at 1 week. Van der Graaf found that the sensitivity of this assay for DVT was only 91%. 19 Lennox 45 evaluated 200 patients at a vascular lab for suspected DVT with US and the SimpliRED D-Dimer assay. They concluded that patients with low clinical risk and a negative D-Dimer may be ruled out for DVT because only 1/78 patients had a calf vein DVT on US. Perrier et al showed that 0/236 patients with either moderate or low pretest probability, a negative Vidas DD, and a negative US had DVT. Two patients with discordant evaluations high clinical pretest probability but negative DD and US studies, who were referred for venography ultimately had calf vein DVT confirmed. T3 summarizes results in studies on D- Dimer in the diagnostic algorithm for DVT. D-Dimer in Pulmonary Embolism Simple cost-effective management strategies for pulmonary embolism remain elusive. The diagnostic gold standard, pulmonary angiography is invasive, expensive, not readily available, and carries a small but defined risk of significant morbidity/mortality. 47 In addition, 1.6% of patients with a normal pulmonary angiogram develop pulmonary embolism during 1 year follow up. 47 The PIOPED study revealed that ventilation-perfusion lung scans are only diagnostic in a minority of patients (17%). 48 A high probability scan has a high predictive value for PE unless the patient has a low pretest probability. A normal scan rules out PE. A low probability scan combined with low pretest probability reduces the likelihood of PE to 4%. In more than 50% of patients, the ventilation-perfusion scan is non-diagnostic, yet the prevalence of PE in this subgroup is as high as 25%. In order to reduce the need for pulmonary angiography, alternative management strategies have been proposed and evaluated Clinical probability estimates have been developed by Wells and Perrier. 52,53 Wells score is based solely on clinical criteria but requires a subjective evaluation that another clinical diagnosis is as or more likely than PE. Perrier s probability model incorporates arterial blood gas analysis on room air which is not always feasible. Nevertheless, clinical evaluation to determine risk is an essential component of current diagnostic strategies. 51,54 Hull introduced the concept of using the diagnosis of a DVT as a surrogate for PE. 49 Management strategies that incorporate serial ultrasound of the lower extremities have been shown to be cost effective. However these strategies excluded patients with hemodynamic instability. Serial evaluations are difficult and inconvenient if patient follow-up is not assured. Contrast enhanced spiral CT scanning has been evaluated in a number of studies The sensitivity and specificity improves as collimation width decreases. Other technological advances such as 2D reformation and electron beam CT appear to further increase sensitivity. Limitations are interobserver variability and lack of sensitivity for sub-segmental PE. Current diagnostic strategies combine clinical probability estimates with ventilationperfusion scanning, ultrasound of the lower extremities, spiral CT where available, and pulmonary angiography. The use of D-Dimer assays in the diagnosis of PE has been considered for many years. Only the introduction of more sensitive and rapid tests has permitted its effective use. Ginsberg evaluated the use of the SimpliRED assay incorporated into a management strategy in 1177 patients. 59 Overall this D-Dimer assay showed a sensitivity of 84.8%. However in the low pretest probability subgroup of 703 patients, only 5/521 patients with a negative D-Dimer had a PE. The NPV was 99%. Forty-four percent of the entire study population could be ruled out for PE without additional testing based solely 139

5 Summary Results in D Dimer studies in DVT Author Assay N NPV T3 Ginsberg 35 SimpliRED % in low prob. patients Anderson 36 SimpliRED % in low prob. patients Aschwanden 37 SimpliRED 343 LR- = 0.05 in low prob. patients Van der Graaf 19 Vidas DD % Tinaquant % SimpliRED 99 82% Perrier 38 Vidas DD % for proximal DVT Bernardi 44 Instant IA % for -DD + -US Lennox 45 SimpliRED % for proximal DVT, 96.9% calf vein DVT Bates 39 MDA DD % in low+mod risk patients Sadouk 46 Vidas DD % Tinaquant % Turbiquant % 140 on a negative SimpliRED result. The authors pointed out that the D-Dimer test lacks sufficient sensitivity to rule out PE in moderate or high pretest probability subgroups. Perrier et al evaluated the use of the rapid Vidas ELISA D-Dimer assay. 38 In their study of 444 emergency department patients with suspected PE, 36% were ruled out based on a negative D-Dimer result independently of pretest probability. Owings et al evaluated the SimpliRED assay in consecutive in-patients on a medical and surgical/trauma service. In the surgical subgroup of 114 patients, a negative D-Dimer ruled out PE with 100% sensitivity and NPV. One third of the patients in the cohort would have been spared any further work-up. 60 Bates et al evaluated the rapid MDA D-Dimer assay in 278 patients with suspected PE. Only 3/100 patients with PE had a negative D-Dimer assay yielding a NPV of 97% across all risk groups. Significantly, none of the 278 patients with low or moderate pretest probabilities and a negative D-Dimer result had a PE. 39 Farrell et al highlighted the false negative rate of a D-Dimer assay (SimpliRED ) in the exclusion of VTE. In a population of 125 patients with suspected PE, the NPV was only 83% (75%-91%). However, in the low pretest probability group, the NPV was more than 98%, which was in line with other reports. 61 Data is emerging that a negative D- Dimer result using the highly sensitive rapid ELISA VIDAS D-DIMER assay rules out PE independently of pretest probability. In low-risk groups, the less sensitive point-ofcare SimpliRED assay excludes PE. The combination of a negative SimpliRED result and a non-diagnostic ventilation-perfusion scan reduces the post-test probability of PE to 2.8%. Further studies are required to evaluate the other sensitive D-Dimer assays in suspected PE. Summary Evidence has accumulated confirming the accuracy and cost-effectiveness of incorporating a D-Dimer assay into the diagnostic algorithm of DVT and PE. The rapid ELISA Vidas D-Dimer assay has high sensitivity and NPV for DVT. A negative result rules out proximal vein DVT in all patients. The SimpliRED assay is the only point-of-care D-Dimer test to undergo prospective evaluation in large studies of DVT. This assay is less sensitive but its NPV is high enough to rule out DVT in low-risk patients. Other sensitive assays such as the MDA DD, Tinaquant, Liatest, IL-DD, and Instant IA need further prospective evaluation in large management studies. In high-risk patients, only the Vidas D Dimer has an NPV sufficiently high to rule out DVT. However, most management studies of high-risk patients have recommended a combined, sequential approach utilizing a D-Dimer assay, US, and venography to rule out DVT. Individual results with one assay cannot be extrapolated to another at the present time. Standardization of the D-Dimer assays has not yet been achieved. Further prospective evaluation in large management trials are needed to further evaluate the use of D-Dimer assays in the diagnosis of venous thromboembolism. 1. Anderson FA Jr, Wheeler HB, Goldberg RJ, et al. A population-based perspective of the hospital incidence and case-fatality rates of deep vein thrombosis and pulmonary embolism. Arch Intern Med.1991;151: Carter CJ. The Natural History and Epidemiology of Venous Thrombosis. Prog Cardiovasc Dis. 1994;36(6): Hirsh J, Hull RD, Raskob GE. Clinical features and diagnosis of venous thrombosis. J Am Coll Cardiol. 1986;8(6 Suppl B):114B-127B. 4. Hyers TM. Venous Thromboembolism. Am J Respir Crit Care Med. 1999;159(1): American Thoracic Society. The Diagnostic Approach to Venous Thromboembolism. Am J Respir Crit Care Med. 999;160: Baker WF Jr. Current Concepts of Thrombosis: Prevalent Trends for Diagnosis and Management Med Clin NA. 1998;82(3): Kakkar VV, Flanc C, Howe CT, et al. Natural History of Postoperative Deep-Vein Thrombosis. Lancet. 1969;2: Moser KN, Lemoine JR. Is embolic risk conditioned by location of deep venous thrombosis? Ann Intern Med. 1981;94: Lagerstedt CI, Fagher BO, Olsson CG, et al. Need for long-term anticoagulant treatment in symptomatic calf-vein thrombosis. Lancet. 1985;2:

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A Negative SimpliRED D-Dimer Assay Does Not Exclude the Diagnosis of Deep Vein Thrombosis or Pulmonary Embolus in Emergency Department Patients. Ann Emerg Med. 2000;35(2):

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