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Negative Results on a Quantitative Latex D-Dimer Test Exclude Deep Leg Clots FREE

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The summary below is from the full report titled “A Diagnostic Strategy Involving a Quantitative Latex D-Dimer Assay Reliably Excludes Deep Venous Thrombosis.” It is in the 20 May 2003 issue of Annals of Internal Medicine (volume 138, pages 787-794). The authors are S.M. Bates, C. Kearon, M. Crowther, L. Linkins, M. O'Donnell, J. Douketis, A.Y.Y. Lee, J.I. Weitz, M. Johnston, and J.S. Ginsberg.

Ann Intern Med. 2003;138(10):I-30. doi:10.7326/0003-4819-138-10-200305200-00002
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What is the problem and what is known about it so far?

Normally, blood clots do not form inside deep veins of the legs. When they do (a condition known as deep venous thrombosis or DVT), they can lead to tissue damage or even fatal complications. Blood thinners, which are used to treat DVT, can prevent most of these problems or help resolve them if they already exist. Because blood thinners can cause serious bleeding, it is important to use them only when necessary. Diagnostic accuracy of DVT is therefore very important. When DVT is suspected, a patient's history, symptoms, and physical examination may help assess whether the probability of DVT is low, moderate, or high. However, clinical history and physical findings are not reliable enough to make a firm diagnosis. Since the body begins breaking down blood clots as soon as they are formed, a simple blood test—the D-dimer test—can determine whether breakdown products from blood clots are present. However, increased amounts of D-dimer can also be found in some people without blood clots. The test is therefore most useful in ruling out DVT when high levels of D-dimer are absent. Also, there are several D-dimer tests. Some take a long time to complete, and some don't perform as well as others.

Why did the researchers do this particular study?

To see whether a low or moderate prior probability of leg clots combined with a negative result on a new, second-generation D-dimer test could reliably rule out DVT.

Who was studied?

556 consecutive outpatients who had a first episode of suspected DVT.

How was the study done?

Using a nine-item prediction rule, doctors categorized each patient as having low, moderate, or high probability of DVT. All patients had blood drawn for the new D-dimer test (MDA d-Dimer [Organon Teknika Corp., now bioMérieux, Inc., Durham, North Carolina]). This test was called a latex immunoassay. It took 30 minutes and was automated (done by a machine) and quantitative (counted the amount of breakdown products). Persons who did not know the patients' probability of DVT interpreted the D-dimer results as either negative or positive. Patients with suspected low or moderate probability of DVT and a negative D-dimer result had no further tests to diagnose DVT. They were not given blood thinners and were followed for 3 months to check for any symptoms or signs of DVT. Patients who developed any symptoms or signs then had special ultrasonography or dye tests to look for clots in the deep veins of the legs. The doctors who did the ultrasonography or dye tests for leg clots did not know the results of the D-dimer test.

What did the researchers find?

Of 556 patients, 485 had low to moderate probability of DVT. Of these, 283 had negative MDAD-Dimer results. Of the 283 patients with negative results, only one was found to have a blood clot during the follow-up period.

What were the limitations of this study?

The results apply only to the MDAD-Dimer test. Other D-dimer tests with rapid turnaround may not be safe for ruling out DVT in patients with moderate probability of clots.

What are the implications of the study?

In patients with low or moderate probability of clots in the deep veins of the leg, a negative MDAD-Dimer result safely rules out DVT.





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