Studies that identify patients at highest risk of thrombosis are scarce, according to background information in the article. It is unclear what risks are for various types and stages of cancer.

Jeanet W. Blom, M.D., of the Leiden University Medical Center, Leiden, the Netherlands, and colleagues conducted a study to identify individuals with cancer with an increased thrombotic risk, evaluated different tumor sites, the presence of distant metastases, and carrier status of gene mutations.

The study (Multiple Environmental and Genetic Assessment [MEGA]) included 3,220 patients, aged 18 to 70 years, with a first deep venous thrombosis of the leg or pulmonary embolism (blood clot in the lungs), between March 1, 1999, and May 31, 2002, at 6 anticoagulation clinics in the Netherlands. There were 2,131 control participants (partners of the patients). Both groups reported via a questionnaire on acquired risk factors for venous thrombosis. Three months after discontinuation of the anticoagulant therapy, all patients and controls were interviewed, a blood sample was taken, and DNA was isolated to ascertain the gene mutations, factor V Leiden and prothrombin 20210A, both linked to thrombosis.

The researchers found that the overall risk of venous thrombosis was increased 7-fold in patients with a malignancy vs. persons without malignancy. Patients with hematological (related to blood) malignancies had a 28-fold increased risk of venous thrombosis, followed by those with lung cancer and gastrointestinal cancer. The risk of venous thrombosis was highest in the first few months after the diagnosis of malignancy (53 times greater risk). Patients with cancer with distant metastases had a higher risk vs. patients without distant metastases (20 times greater risk). Carriers of the factor V Leiden mutation who also had cancer had a 12-fold increased risk vs. individuals without cancer and factor V Leiden. Similar results were indirectly calculated for the prothrombin 20210A mutation in patients with cancer.

"Assuming a baseline risk of 1 to 4 patients with venous thrombosis per 1,000 per year, a 5 percent prevalence of factor V Leiden and a 2 percent prevalence of the prothrombin 20210A mutation, among 10,000 patients with cancer, we would expect 8 to 34 patients with venous thrombosis due to factor V Leiden or the prothrombin 20210A mutation. Screening for factor V Leiden and the prothrombin 20210A mutation and subsequent prophylactic anticoagulant therapy with an effectivity of 80 percent would prevent annually 7 to 27 venous thrombotic events per 10,000 patients with cancer screened (numbers needed to screen: 700-2,700), which does not make screening a useful strategy. Rather than screening for factor V Leiden or the prothrombin 20210A mutation, it may be more cost-effective to consider prophylactic anticoagulant therapy for patients with cancer who have an increased risk to develop venous thrombosis," the authors conclude.

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