Prevention of Recurrent Venous Thromboembolism (PREVENT)

BACKGROUND:

Venous thromboembolism is associated with more than 300,000 hospitalizations and results in thousands of deaths annually. Conventional therapy consists of intravenous heparin followed by oral anticoagulants usually given for three to six months. The recommended intensity of oral anticoagulants (warfarin) has been derived from clinical trials. Such therapy is usually quite effective. However, some patients develop recurrent disease after the oral anticoagulants are stopped. A recent randomized study evaluated the optimal duration of oral anticoagulant therapy. After acute treatment with heparin, subjects were treated with oral anticoagulants for either six weeks or six months with a target INR * of 2 to 2.85. There was no difference in mortality in the two groups. Recurrence was not seen while the patients were under treatment. When anticoagulants were stopped, recurrent thrombosis was documented in 18 percent of the patients treated for six weeks and in 9.5 percent of those treated for six months. The period of greatest risk of recurrence for the six weeks patients was immediately after therapy was stopped. There was a linear increase in cumulative risk of 5 to 6 percent per year for both treatment groups during the following 18 months.

For patients who have experienced idiopathic venous thrombosis, the risk of recurrence may continue even after several months of conventional therapy. Further prophylactic therapy might be beneficial for the patients who are at risk for late recurrence. But, because of the presumed risk of bleeding and inconvenience of monitoring standard warfarin therapy, most physicians usually limit treatment to three to six months.

In 1997, Simioni showed a cumulative recurrence rate of VTE of 39.7 percent among those with factor V Leiden mutation, with all recurrences occurring within three years, a rate 2.4 times higher than among individuals without the mutation. The factor V Leiden mutation is found in 4 to 6 percent of Caucasians and is the single most important cause of thromboembolism in a variety of conditions. Heterozygous carriers with the mutation have VTE at a younger age than do noncarriers. Among those with first VTE, the prevalence of the mutation is 15 to 40 percent and among those with a family history of VTE, as high as 50 percent. However, in a large study of men participating in the Physicians Health Study, those individuals with the mutation had an increased rate of VTE over time. These age-specific incidence rate differences ranged from 1.23 to 5.97 in those aged 70 or older. These data suggest that confounders other than genetic predisposition are important in the development of VTE.

* The INR or international normalized ratio is the ratio of patient prothrombin to control prothrombin multiplied by the international sensitivity index. The INR was introduced by the World Health Organization to standardize control of anticoagulant therapy internationally.