College of American Pathologists
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  D-dimer dance card fills up
  with new tests, uses

  cap today

April 2005
Feature Story

While researchers continue to explore the diagnostic and prognostic potential of D-dimer testing, more diagnostics firms are staking out their claims, entering the market with new assays and refining their existing tests.

Biosite Inc. is the latest company to throw its hat in the ring, announcing in February that its Triage D-dimer test had earned FDA clearance as an aid in evaluating patients suspected of having thromboembolic events, including pulmonary embolism and deep vein thrombosis. Several months before Biosite’s announcement, Roche Diagnostics launched its new point-of-care assay, the Cardiac D-dimer test, which was cleared for use with the company’s Cardiac Reader instrument for the quantitative determination of D-dimer in anticoagulated venous whole blood.

The two new tests provide more options for laboratories that want to add a D-dimer assay to their test menu, but having more options may add to the confusion that still exists about how to use D-dimer testing appropriately to exclude deep vein thrombosis or pulmonary embolism. "Currently, we don’t use D-dimer to rule out DVT or PE. If a physician is considering pulmonary embolism, they do a spiral CT, and if they’re considering DVT in the leg, they do an ultrasound," says Sharon Fong, CLS (ASCP), laboratory manager at Tulare (Calif.) District Healthcare System, a 115-bed acute care hospital.

Fong’s laboratory was one of the first to bring Biosite’s new Triage D-dimer on board. But it is using the test now only for diagnosing disseminated intravascular coagulation, or DIC, because it has had the new assay for only four months and doesn’t yet have enough experience with it to use it as a rule-out test for DVT and PE. Before Triage, the laboratory used a latex agglutination kit, which provided a qualitative result at a cutoff of 80 ng/mL. "Unfortunately," Fong says, "when we used that kit, it seemed like every hospitalized patient was positive for D-dimer because there are so many clinical conditions that will give you a low but significant D-dimer level. Therefore, the latex assay was really not useful for DIC." The Biosite assay, however, measures D-dimer levels up to 5,000 ng/mL or greater, which is helping Fong’s laboratory confirm DIC in more patients than did the latex agglutination assay.

Tulare District Healthcare System may consider using the Biosite Triage D-dimer to exclude PE and DVT in the future, but is taking a wait-and-see posture for now. "We will probably test it to see how it corresponds with the results from our medical imaging studies, and will watch the marketplace and see how everyone else is doing with the Triage compared to the [bioMerieux] Vidas D-dimer Exclusion test, which has the longest track record for PE and DVT rule-out," she says.

Meanwhile, other institutions are formulating protocols to fine-tune the process of using D-dimer to rule out PE and DVT. Michael Brown, MD, MSc, associate professor of emergency medicine and epidemiology at Michigan State University, Grand Rapids, and his colleagues recently instituted an algorithm for D-dimer testing in a hospital setting, using computer pop-up screens with criteria to help clinicians determine how to use D-dimer to provide the most accurate negative predictive value. Physicians participating in the study were first asked to determine the patient’s pretest probability of having pulmonary embolism, and to send subjects with moderate and high pretest probability of having PE for imaging studies. If the patient had a low pretest probability, clinicians were then asked to use seven criteria to determine whether a D-dimer test should be ordered. The criteria state that D-dimer testing is not recommended in patients who have unexplained hypoxia, unilateral leg swelling, or hemoptysis since these patients should be considered to have at least moderate risk for PE. It also urges clinicians not to order D-dimer tests for patients who have had surgery within the last four weeks, are pregnant, or are 70 years or older. In these populations, conditions other than PE or DVT could elevate D-dimer levels, making the test much less specific. D-dimer was also discouraged if the duration of symptoms was four days or more since the sensitivity decreases over time.

The researchers then examined the protocol’s effect on patient outcome (Acad Emerg Med. 2005;12:20-25). "We have a busy emergency department, with over 100,000 visits a year, and we wanted to see how the protocol would affect patient care in a real-world setting. Before we implemented our clinical pathway, there was a lot of variation in how our clinicians used our D-dimer assay, and there were varying degrees of understanding of the proper interpretation of the test," Dr. Brown says. During a four-month period after the protocol was implemented, 71 cases of venous thromboembolism were diagnosed in a population of 1,207 people evaluated for suspected PE. Only one missed case of PE was identified on three-month followup, giving a negative predictive value of 99.9 percent. In addition, 39 percent of the patient population suspected of PE was discharged without being subject to radiologic imaging.

Dr. Brown says these simple criteria could help make D-dimer testing less complicated and more efficient. "In two previous meta-analyses we performed, it was obvious that the specificity of D-dimer was poor, although the sensitivity was good. After reviewing multiple studies, we recognized that the poor specificity most likely reflected comorbid illness and age. In subjects over 70 years of age, the specificity decreases significantly," Dr. Brown says.

The protocol Dr. Brown put in place at Spectrum Health continues to be used, and the clinicians are pleased with it. "Most people don’t understand the limitations of the D-dimer test, and they should use a prediction rule of some type, but most of them don’t. They know about the Wells criteria and the Geneva criteria, but no one uses them because they are hard to remember or too complicated," Dr. Brown says. "Having a simple guideline or clinical pathway is key to successful implementation in the busy clinical setting of the ED."

Though studies like the one Dr. Brown and his colleagues conducted are making D-dimer test protocols easier to use, using D-dimer tests appropriately is still a challenge for many institutions. "What a number of centers have found is that as they’ve introduced new D-dimer assays, almost every year, the volume of D-dimer assays performed has increased, so that in the end introducing these assays may not have been all that cost-effective because more imaging studies are being done because of the assays’ low specificity," says Shannon Bates, MD, an assistant professor in the Department of Medicine at McMaster University, Hamilton, Ontario. Dr. Bates is involved now in several studies at McMaster that are examining the use of D-dimer testing in various populations.

Many institutions, she says, are trying to find a D-dimer test strategy that will work for them. But in addition to developing algorithms, some are tweaking diagnostic thresholds to boost their assays’ specificity. "Some centers are looking at rationalizing D-dimer use by using different cutpoints in different groups of patients. For example, in a low pretest probability group of patients, where the prevalence is low, you can have a slightly lower sensitivity and still maintain a high negative predictive value." In such an instance, Dr. Bates says, instead of having a cutpoint of 0.50 mg/L to rule out venous thromboembolism in someone with a low pretest probability, the institution might have a cutpoint of 1.0 mg/L as a rule-out. "Even though the sensitivity might be slightly lower, the specificity is going to be higher," she says. This strategy is being evaluated now in a multicenter randomized trial.

In an article to be published in the May 2005 issue of Clinical Chemistry, investigators from the Carolinas Medical Center, Charlotte, NC, make a plea for a large management study that would establish new thresholds for D-dimer to rule-out venous thromboembolism in pregnant women. Using the MDA immunoturbidimetric assay from bioMerieux, the researchers measured D-dimer concentrations pre-conception and during each trimester of pregnancy in 50 apparently healthy women. Preconception, 79 percent of the women had D-dimer levels that were lower than the traditional 0.50 mg/L cutoff. In all of the women, D-dimer concentrations increased significantly each trimester, and by the third trimester, none of the 23 women remaining in the study had D-dimer concentrations lower than the 0.50 mg/L cutoff, rendering the test useless at that threshold for ruling out PE and DVT in this population.

But struggling to find the right cutoffs is not a new challenge for labs with D-dimer tests on their menu. Though most assays seem to hover around the 0.50 mg/L cutoff, it’s still not set in stone. "It’s been thought that cutoffs are best determined by ROC curves, and this analysis probably requires a study population of about 200, and labs can’t do that. I always recommend looking to the literature to see if ROC analysis using a sufficient population has been done for the test kit they are using, and that laboratories always try to verify this recommended cutoff in-house with a much smaller population," says Dorothy M. Adcock, MD, medical director at Esoterix Coagulation, Denver. Dr. Adcock herself has contributed to the peer-reviewed literature for D-dimer, most recently having coauthored a study that examined the inter-rater agreement and external validity of Wells criteria in determining pretest probability in patients suspected of having PE (Ann Emerg Med. 2004;44:503-510).

"Different assays, and perhaps the same assay for different indications, may have different cutpoints, which is definitely confusing," Dr. Bates says. "While there are a lot of assays that use a cutoff that’s the equivalent of 0.50 mg/L, there are other assays that have completely different cutoffs." She says D-dimer assay manufacturers should come up with cutpoints that are specific to their assays so that the clinical centers can at least use those as a guideline. "The clinical centers would then need to have ongoing evaluation to make sure the recommended cutpoint is working for them," she says.

As more assays enter the market and more D-dimer assay studies are published, the lack of a standard unit of measure adds to confusion in clinical laboratories and in the research community. "D-dimer can be reported as FEUs, which stands for fibrinogen equivalent units, or as D-dimer units," says Dr. Adcock, who notes that the literature doesn’t always report what units of measure were used. In addition, since 2 FEUs are equal to one D-dimer unit, the numbers aren’t interchangeable. "I think some laboratories may actually be confused as to how they are reporting out results, and may not know whether they are reporting in FEUs or D-dimer units, and probably don’t state on the report which unit of measure they’ve used," Dr. Adcock says.

In fact, the CAP Coagulation Resource Committee has found "significant confusion" among laboratories between FEU and D-dimer units, as well as evidence that many laboratories are using the wrong cutoff for their particular assay, says committee member Elizabeth Van Cott, MD, of Massachusetts General Hospital. To help resolve this problem, the committee recommends that laboratories indicate in the patient report which unit of measure is being used (FEU or D-dimer units), and verify that they are using the correct cutoff as recommended by the D-dimer reagent manufacturer. In addition, the committee recommends that manufacturers clearly indicate in their packaging which unit of measure their assay is generating and the recommended cutoff.

Clinicians should know which types of assays are being used because all D-dimer tests are not created equal. Though there have been few comparative studies of D-dimer assays as a group, a meta-analysis published last year compared the performance of ELISAs, quantitative rapid ELISAs, semiquantitative rapid ELISAs, qualitative rapid ELISAs, quantitative latex assays, semiquantitative latex assays, and whole-blood tests for D-dimer (Ann Intern Med. 2004;140:589-602). "The authors came out fairly strongly in favor of the ELISA assays," says Dr. Bates, who adds that she’s not sure she agrees entirely with their conclusions. "I think the main issue is that you really have to divide your D-dimer assays into different categories and use them according to their different test performance characteristics," she says.

For example, Dr. Bates notes, the SimpliRed D-dimer test from Agen Biomedical, Brisbane, Australia, is unique in that it has a moderate sensitivity but a higher specificity in comparison with other assays. "It’s never going to be used as a stand-alone test. It’s always going to be used in combination with a low pretest probability or a negative compression ultrasound in the case of deep vein thrombosis and with other ancillary testing in the case of pulmonary embolism," she says. "But it does have its advantages. It’s fast, inexpensive, and easy-to-use."

Other D-dimer assays have been evaluated as stand-alone tests, however, and more studies are underway at McMaster University to evaluate at least one of them—bioMerieux’s MDA D-dimer—as a stand-alone test to exclude recurrent deep vein thrombosis. "You might be able to use D-dimer as a stand-alone test in this patient population, which would be important because all of our other diagnostics, like ultrasound or venogram, have pretty important limitations in people who have had clots before," Dr. Bates explains.

But without large studies, she says most health care providers would feel uncomfortable using today’s quantitative D-dimer assays as stand-alone tests. "The only assay that’s probably been used that way widely is the Vidas D-dimer, which has been used a great deal by the Swiss group of Bounameaux and Perrier for the exclusion of pulmonary embolism. I don’t think any of the other assays have been evaluated in that way, however, even though they may have suitable characteristics for it," Dr. Bates says.

Recently, new and revised indications for the use of some D-dimer assays have sparked debate over whether the assays are an aid in diagnosing PE and DVT or whether they are indicated for use in excluding PE and DVT, since their worth is mainly in their negative predictive value. Last fall, the Food and Drug Administration reviewed new performance data for the Advanced D-dimer assay from Dade Behring and cleared a new intended use for it as an aid in the diagnosis of venous thromboembolism, DVT, and PE. And bioMerieux says the Vidas D-dimer Exclusion assay is the only test cleared by the FDA with the indication for excluding both PE and DVT.

Dr. Brown, for one, says the chief function of D-dimer assays is to rule out VTE, DVT, and PE. "D-dimer is definitely used to exclude disease. In fact, ruling out disease is all it should be used for," he says.

In the future, there may be fewer cases of venous thromboembolism, PE, and DVT to rule out, mainly because D-dimer has made inroads into the diagnostic algorithms for these conditions. "Over the past 15 to 20 years," Dr. Adcock says, "the number of individuals who have pulmonary embolism, among those who are sent for a diagnostic test for pulmonary embolism, has decreased dramatically. In a study we performed here in Denver, the prevalence of PE in those patients who were subjected to imaging studies to evaluate for PE was around 10 percent, and that’s really quite low."

Thirty years ago, Dr. Bates adds, the prevalence of PE might have been as high as 30 percent. "Clearly, more and more people are being caught in the net because we have easier tests like D-dimer," she says.

Looking ahead, D-dimer shows promise as an aid in diagnosing a variety of diseases. "What’s interesting about D-dimer is its potential for other indications," Dr. Adcock says. "If you think about it, a D-dimer will be elevated any time you have a clot, whether it’s an arterial clot or a venous clot. Obviously, stroke and heart attack are arterial clots, and therefore those patients can have elevated D-dimers."

Roche, Dade Behring, and Biosite have recognized this, and it’s no coincidence that they’ve positioned their D-dimer assays alongside their cardiac tests on their point-of-care instruments. "People have been using D-dimer assays to predict patients at higher risk of thromboembolism if they have atrial fibrillation," Dr. Bates says. "Once again, though, we have a number of clinical factors that help us predict whether someone is at risk of stroke with atrial fibrillation." These include being over 75 and having a previous stroke, ischemic heart disease, and diabetes.

Perhaps the most obvious use for D-dimer in the future is in diagnosing stroke. "There have been a number of studies showing that D-dimer concentrations might be increased after ischemic stroke or transient ischemic attacks," Dr. Bates says. "Some people have suggested that the D-dimer concentrations might be different in the different types of strokes," she adds, warning that those studies have important limitations. "At this point, I would not use D-dimer to make management decisions in patients with suspected stroke or with stroke because I believe we need to see more studies in that area," she says.

D-dimer has also been shown to be prognostic in terms of evaluating the risk for a recurrence of venous thrombosis. Says Dr. Adcock, "It’s been shown that when patients have a venous thrombosis and are put on standard therapy, if you evaluate a D-dimer level either during therapy or one month after therapy has been discontinued, you have an idea whether patients are at an increased risk for recurrence."

McMaster University is enrolling virtually all of its patients with suspected PE or DVT in one of five clinical trials it is conducting. Researchers there are evaluating D-dimer in suspected recurrent deep vein thrombosis, in suspected first deep vein thrombosis, in suspected pulmonary embolism, in suspected DVT in pregnancy, and in suspected PE in pregnancy. (MDA D-dimer is being used in the first three and SimpliRed D-dimer in the latter two.) It will be another year or two before the trials are completed.

For now, many agree that the biggest challenge facing laboratories in the area of D-dimer testing is ensuring that assays are used in the most rational, efficient way to get the best outcome for patients. "We want our clinicians to have confidence in D-dimer testing," Dr. Brown says, "and if they begin to notice they are getting lots of false-positive results, they may lose confidence in the test and stop ordering it." False-negatives are the main concern, of course, because sending a patient home with a pulmonary embolism "could be disastrous for the patient," Dr. Van Cott says. The key, then, is for pathologists and clinicians to collaborate on strategies that will result in good patient care. "The pathologists probably have the best understanding of the test’s limitations," Dr. Brown says, "while the clinicians probably have the best understanding of how to estimate pretest probability."

Thus the two have to work together because, as he says, "you need both pieces of the puzzle to deliver good patient care."

Sue Parham is a writer in Edgewater, Md.