About 80% of patients who present with pulmonary embolism have evidence of deep venous thrombosis in their legs; if deep venous thrombosis is not detected in such patients, it is likely that the whole thrombus has already detached and embolized. Conversely, pulmonary embolism occurs in up to 50% of patients with proximal deep venous thrombosis.
Although a positive D-dimer test (which measures plasma levels of a specific derivative of cross-linked fibrin) indicates that venous thrombosis and pulmonary embolism are possible diagnoses, this test is nonspecific, since it may be positive in hospitalized patients with infection, cancer, trauma, and other inflammatory states and thus cannot inform decisions about treatment. When an ELISA-based D-dimer test is negative, deep venous thrombosis and pulmonary embolism are ruled out in patients with a low or moderate pretest probability, precluding the need for specific imaging studies.
The low-molecular-weight heparin and pentasaccharide preparations have advantages over unfractionated heparin, including greater bioavailability, more predictable dosing, subcutaneous delivery (usually without the need for monitoring), and a lower risk of heparin-induced thrombocytopenia.
Teaching topics from the New England Journal of Medicine - Vol. 358, No. 10, March 6, 2008