A New and Improved System for Excluding the Diagnosis of Deep Venous Thrombosis

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The summary below is from the full report titled "Management of Suspected Deep Venous Thrombosis in Outpatients by Using Clinical Assessment and D-dimer Testing." It is in the 17 July 2001 issue of Annals of Internal Medicine (volume 135, pages 108-111). The authors are C Kearon, JS Ginsberg, J Douketis, M Crowther, P Brill-Edwards, JI Weitz, and J Hirsh.

What is the problem and what is known about it so far?

Normally, blood clots do not form inside deep veins. When they do (a condition known as deep venous thrombosis or deep-vein thrombosis), they can lead to tissue damage or even fatal complications. Blood thinners can prevent most of those problems, but because these drugs can cause serious bleeding, it is important to use them only when necessary. Diagnostic accuracy is therefore very important. When deep venous thrombosis is suspected, a patient's medical background, symptoms, and results on physical examination help in estimating whether the probability of deep venous thrombosis is low, moderate, or high (the "prior probability" of disease being present). By themselves, however, these 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 if breakdown products from blood clots are present. Unfortunately, D-dimer can also be found in some people without blood clots. The test is therefore most useful in ruling out deep venous thrombosis when D-dimer is absent.

Why did the researchers do this particular study?

To see if a low prior probability of deep venous thrombosis combined with a negative result on the D-dimer test could reliably rule out the diagnosis and, consequently, if blood thinners could be withheld safely in these patients.

Who was studied?

The investigators studied 445 consecutive outpatients from three different hospitals in Canada who had a first episode of suspected deep venous thrombosis.

How was the study done?

By using a nine-item prediction rule, a vascular technologist or nurse first categorized each patient as having low, moderate, or high pretest probability of deep venous thrombosis. Patients were next seen by a physician who could change the rating on the basis of clinical judgment. D-dimer testing was then performed and the results were interpreted as either negative or positive. Patients with low pretest probability and a negative D-dimer result underwent no further tests, and no blood thinners were prescribed. The other patients had additional testing and received appropriate treatment. All patients were followed for 3 months for evidence of deep venous thrombosis.

What did the researchers find?

One hundred seventy-seven patients had low pretest probability and a negative D-dimer result. Only one of these patients was found to have a blood clot during the follow-up period, indicating that the combination of these findings was 99.4% accurate in ruling out deep venous thrombosis. What were the limitations of this study? The reasons that physicians changed prior probability ratings were not specified, making it difficult to know how well the evaluation system applies in other settings.

What are the implications of the study?

In many patients suspected of having deep venous thrombosis, evaluation of prior probability combined with D-dimer testing may substantially reduce the need for additional diagnostic testing, which is often complex and expensive.