Risks for Persistent Clots in Leg Veins despite Treatment

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The summary below is from the full report titled "Residual Venous Thrombosis as a Predictive Factor of Recurrent Venous Thromboembolism." It is in the 17 December 2002 issue of Annals of Internal Medicine (volume 137, pages 955-960). The authors are P Prandoni, AWA Lensing, MH Prins, E Bernardi, A Marchiori, P Bagatella, M Frulla, L Mosena, D Tormene, A Piccioli, P Simioni, and A Girolami.

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

Sometimes adults get blood clots in their veins, a condition known as deep venous thrombosis (DVT). The blood clots usually form in leg veins. Blood clots in the deep veins of the legs are dangerous because they can break free and travel to the lungs. This problem, called pulmonary embolism (clot in the lung), can be fatal. Doctors typically treat patients with DVT with blood-thinning drugs for 3 to 6 months. The blood-thinning drugs help dissolve clots, reduce the risk that a clot will travel to the lungs, and prevent repeat clots. Some patients may have high risks for repeat clots and may benefit from longer treatment.

Clots and clogged veins can be detected with ultrasonographic scans. The scans use short pulses of sound waves transmitted from a device placed on the surface of the skin to look at whether blood flow in the veins is decreased from clots. Follow-up scans can identify patients with remaining clots despite treatment. But do these patients have high risks for repeat clots?

Why did the researchers do this particular study?

To see whether patients with DVT who have remaining clots in the leg vein after 3 months of treatment have higher risks for repeat clots than patients whose veins are unclogged with treatment.

Who was studied?

313 adults with a single episode of DVT who had been treated with blood-thinning drugs for 3 months.

How was the study done?

The researchers told the patients to stop taking blood thinners. The researchers then scanned the leg with the original DVT using ultrasonography. Patients with evidence of persistent vein clogging had repeat scans every 6 to 12 months for 3 years. All patients were followed for 6 years. They routinely reported any symptoms or signs of new clots. Researchers verified possible new clots in either the legs or lungs with scans or special x-rays taken after injecting dye into the veins (venography). Researchers then looked at the frequency of repeat clots among people who had and had not shown persistent vein clogging after the original DVT.

What did the researchers find?

Rates of scans with no evidence of persistent clots were about 40% at 6 months, 60% at 1 year, 70% at 2 years, and 75% at 3 years. Fifty-eight people had repeat clots in the legs or lungs during the 6-year follow-up period. Most repeat clots (71%) occurred in people who had had evidence of persistent clots on scans.

What were the limitations of the study?

Although patients were told to stop taking blood thinners 3 months after their first DVT, about 20% continued taking blood thinners for another 3 to 6 months. Rates of persistently abnormal results on ultrasonography and repeat clots were probably lower than what would have been seen if all patients had stopped blood thinners at 3 months.

What are the implications of the study?

Patients who have persistent clots despite 3 months of treatment for a first episode of DVT have increased risks for repeat episodes of clots in the legs or lungs. We now need studies to see whether it is beneficial to tailor treatment based on findings of persistent clots.