Pulmonary embolism is potentially fatal, yet its diagnosis remains difficult because clinical features are nonspecific, and all available diagnostic tests have significant limitations. Pulmonary angiography is the accepted “gold standard” test, but it is invasive and difficult to interpret, and can give false-negative results. Many other tests have been developed and used in combination to assess the probability of pulmonary embolism in individual patients, including ventilation/perfusion (V/Q) lung scanning, venous compression ultrasonography of the legs, and contrast-enhanced spiral computed tomography (CT) of the chest. Initially, spiral CT was thought to have high sensitivity and specificity, but it is now known that the sensitivity is too low to reliably rule out pulmonary embolism without further testing. Nevertheless, the combination of spiral CT and leg compression ultrasonography might have sufficient sensitivity and specificity to safely exclude pulmonary embolism and avoid unnecessary anticoagulation in many patients. Musset and colleagues conducted a large study in 14 French hospitals to assess the safety of withholding anticoagulation therapy in patients with low or intermediate probability of pulmonary embolism and negative results on spiral CT and leg ultrasonography.

A total of 1,902 consecutive adult patients with suspected pulmonary embolism were eligible for the study. Patients were excluded if they were pregnant; had renal insufficiency, contraindications to the tests, or current anticoagulation therapy; had massive pulmonary embolism with hemodynamic instability; were receiving antidiabetic treatment with metformin; had a life expectancy of less than three months; or refused to participate in the study.

The 1,041 patients all underwent clinical assessment of probability of pulmonary embolism, spiral CT of the chest, and ultrasonography of the legs. Patients with negative findings on both tests and low or intermediate clinical risk of pulmonary embolism were not treated with anticoagulation. Patients with a high clinical probability of pulmonary embolism underwent V/Q scanning or pulmonary angiography. If these results were normal, no anticoagulant was given. Anticoagulants were given to any patient who had positive spiral CT, ultrasonography, V/Q scan, or pulmonary angiography. All patients were followed for three months.

The 1,041 patients studied had a median age of 66 years (range, 18 to 97 years). Pulmonary embolism was diagnosed by at least one test in 360 patients (34.6 percent). Of the 601 patients who had negative findings on spiral CT and ultrasonography, 525 were classified as having a low or intermediate clinical probability of pulmonary embolism(see accompanying figure). Eighteen of these patients received anticoagulation for an additional indication during the follow-up period. Of the remaining 507 patients, nine experienced venous thromboembolic events during the follow-up period. Four of these patients had symptoms and positive testing for thromboembolic events, and the other five died from causes attributed to pulmonary embolism. Anticoagulation also was withheld from the 70 patients who had a high clinical probability of pulmonary embolism but negative V/Q lung scan or pulmonary angiography. None of these patients experienced any venous thromboembolic event during the three-month follow-up.

The authors conclude that pulmonary embolism occurred in only 1.8 percent of patients with low or intermediate clinical probability and negative testing on both spiral CT and venous ultrasonography who were not treated with anticoagulants. They advise that anticoagulant therapy can safely be withheld in patients with a low or intermediate probability of pulmonary embolism and negative spiral CT and ultrasonography. Anticoagulation can probably also be avoided in high-probability patients with negative testing if more advanced screening such as V/Q lung scan or pulmonary angiography also is negative.