Feature Story

Using new D-dimer tests to rule out venous thromboembolism

February 2000
Karen Sandrick

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 tests—the latex agglutination procedures that have been used in the past to evaluate disseminated intravascular coagulation—are 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 rapid—yielding 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 vague—shortness 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 notes.

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 fastest—yielding 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 Journal. 1998;317:1037-1040).

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 thromboembolism.

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 test.

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 proximal veins.

Proceed slowly
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.