Estimating the Probability of Deep Venous Thrombosis in Outpatients

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The summary below is from the full report titled "The Wells Rule Does Not Adequately Rule Out Deep Venous Thrombosis in Primary Care Patients." It is in the 19 July 2005 issue of Annals of Internal Medicine (volume 143, pages 100-107). The authors are R. Oudega, A.W. Hoes, and K.G.M. Moons.

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

Deep venous thrombosis (DVT) occurs when blood clots form in the large veins of the legs. Pieces of the clots can break off and travel through the bloodstream to the lungs. The clots can cause serious symptoms and even death if they are not diagnosed and treated quickly. Because people with DVT are treated with blood-thinning medicines that can cause serious bleeding, accurate diagnosis is very important.

Doctors use several strategies to diagnose DVT, including a scan that uses sound waves to look at clotting in the veins (ultrasonography) and blood tests (d-dimer tests) that help measure whether a clot has formed and is breaking down. Doctors may repeat ultrasonography within 1 week if the initial scan is negative. To decide when to do these tests, doctors often assess a patient's medical background, symptoms, and physical examination. They sometimes sum the presence or absence of 9 items (the Wells rule) to help sort patients into low, moderate, or high probability of having DVT. For example, in the original study that developed the Wells' rule, only 3% of patients who were classified as low risk by the rule had DVT. Few additional studies document how well the Wells rule places people into a low-risk group.

Why did the researchers do this particular study?

To see how well the Wells rule classifies outpatients with suspected DVT into a low-risk group.

Who was studied?

1295 adults with suspected DVT seen in 110 primary care practices in the Netherlands. Their average age was 60 years, and 36% were men.

How was the study done?

One hundred ten primary care physicians examined 1295 outpatients with symptoms suggestive of DVT. After completing standard study forms that included the Wells rule, they referred patients to hospitals for D-dimer tests and leg ultrasonography. Patients with normal ultrasounds had repeated ultrasonography 7 days later. The person who did the ultrasonography did not know the results of patients' history, physical examination, or D-dimer test. The researchers then assessed how often DVT was diagnosed by ultrasonography in patients who had been classified as low risk by their scores on the Wells rule. They also assessed how often DVT was diagnosed in patients with low-risk Wells scores and normal D-dimer results.

What did the researchers find?

Approximately 12% of patients who were classified as low risk by physicians' Wells rule assessments had DVT. About 3% of patients with low-risk scores on the Wells rule and normal D-dimer results had DVT.

What were the limitations of the study?

The study used a different method to diagnose DVT than did the original Wells study; it also included patients with previous DVT, while the original Wells study excluded them. Patients with previous DVT have high risks for repeated DVT.

What are the implications of the study?

Low scores on the Wells rule do not safely guarantee low probability of DVT in all outpatients with suspected leg clots.