Is the prevalence of the factor V Leiden mutation in patients with pulmonary embolism and deep vein thrombosis really different?

INTRODUCTION: Previous investigations have suggested a lower prevalence of the factor V Leiden mutation in patients with pulmonary embolism, as compared to patients with deep leg vein thrombosis.
METHODS: We studied unselected patients with pulmonary embolism, in whom we also assessed the presence of deep vein thrombosis by ultrasonography. We assessed the prevalence of heterozygosity for the factor V Leiden mutation and compared the outcome of patients with a normal ultrasound (primary pulmonary embolism) to those with an abnormal ultrasound (combined form of venous thromboembolism). Furthermore, we performed a literature search to identify all articles regarding the prevalence of heterozygous factor V Leiden mutation in patients with primary deep vein thrombosis, primary pulmonary embolism and a combined form of venous thromboembolism. We calculated a (common) odds ratio for these 3 manifestations of venous thromboembolism, including the current findings.
RESULTS: In 92 patients with proven pulmonary embolism, 25 (27%) had also an abnormal ultrasound. In these patients, the prevalence of the factor V Leiden mutation was 24% (95% CI 9%-45%), whereas the mutation was present in 5 of 67 patients with primary pulmonary embolism (7%; 95% CI 2%-16%). The literature analysis indicated the common odds ratio for the presence of heterozygous factor V Leiden mutation in patients with primary deep vein thrombosis, primary pulmonary embolism and the combined form of venous thromboembolism to be 7.9 (95% CI 5-12), 3.5 (95% CI 2-6) and 6.8 (95% CI 3-14), respectively.
CONCLUSION: In patients with primary pulmonary embolism the prevalence of the factor V Leiden mutation appears to be half of that reported in patients with primary deep vein thrombosis. The mechanism remains unclear.