A new study of a commonly used imaging test of the chest to detect potentially deadly blood clots in the lung shows that extending the scan to the legs - where the clots typically originate - or adding a standard clinical assessment significantly improves physicians' abilities to accurately diagnose pulmonary embolism.
A sudden and potentially deadly blockage in a lung artery, pulmonary embolism affects an estimated 600,000 Americans each year, making it the fourth most commonly occurring cardiovascular problem in the United States. The multicenter study was funded by the National Heart, Lung, and Blood Institute (NHLBI) of the National Institutes of Health (NIH).
The Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) II is the largest study ever conducted to assess the role of contrast-enhanced multidetector computed tomographic (CT) angiography for diagnosing pulmonary embolism. In the June 1, 2006, issue of the New England Journal of Medicine, PIOPED researchers from eight clinical centers report that chest CT angiography alone detects suspected pulmonary embolism in only 83 percent of patients; in contrast, combined results of the chest CT angiogram and the leg CT scan detect clots in 90 percent of patients. The researchers recommend that physicians consider additional test results before ruling out pulmonary embolism in patients whose scan does not detect clots but whose clinical assessment suggests a high likelihood of pulmonary embolism.
"Imaging technologies are one of the most rapidly evolving areas of medicine, and they greatly expand our ability to diagnose and treat disease," said Elias A. Zerhouni, MD, NIH Director and a board-certified radiologist. "In recent years, chest CT scans have become the most widely used technique for diagnosing pulmonary embolism. But, until now, we have not had enough scientific evidence to really understand how accurate they are for detecting this often-fatal condition."
Pulmonary embolism (PE) leads to death in nearly one-third of untreated cases, but therapies lower the death rate to between 3 percent and 8 percent. In nine out of 10 cases, PE begins as a clot in the deep veins of the leg, a condition known as deep vein thrombosis (DVT). The clot breaks free from the vein and travels to the lung, where it can block an artery. Commonly used treatments for both PE and DVT include anti-clotting medications and injections of clot-busting agents.
"There are many effective ways to prevent and treat blood-clotting diseases," noted NHLBI Director Elizabeth G. Nabel, MD. "Pulmonary embolism is underdiagnosed and therefore often untreated. If detected early, however, blood clots can often be prevented from causing permanent damage or death."
In PIOPED II, researchers compared the accuracy of three ways to diagnose blood clots in 824 patients suspected of having pulmonary embolism: chest CT angiogram alone, chest CT angiogram with venous-phase imaging (leg CT), and chest CT angiogram with an objective clinical assessment known as the Wells Score. The Wells Score is a validated tool to determine the likelihood that a patient has PE based on characteristics such as signs and symptoms, heart rate, and risk factors. A high score indicates that a patient has a high probability of having PE. To determine their accuracy, the tests were compared with the participants' composite results from other validated diagnostic tests for PE.
Overall, the sensitivity (the ability to detect clots) of the combined chest CT and leg CT was 90 percent, compared to 83 percent sensitivity of the chest CT angiogram alone. The specificity (the ability to rule out the presence of clots) of the chest CT alone compared to the chest CT and leg CT combined was similar (about 95 percent). A high clinical probability combined with positive chest CT correctly indicated PE in 96 percent of participants. However, in patients with a high clinical probability, a negative chest CT result did not confidently rule out a diagnosis of PE, the researchers report.
"This study suggests that chest CT angiogram for detecting dangerous blood clots in the lung is good, but sometimes it is not enough," noted Paul D. Stein, MD, director of research education at St. Joseph Mercy Oakland Hospital in Pontiac, Michigan, professor of medicine at Wayne State University, and lead author of the paper. "We can more accurately detect or rule out pulmonary embolism by taking pictures of the leg veins in addition to pictures of the lung arteries."
The results of the chest CT combined with the patient's clinical probability assessment were comparable to the results from the combined chest CT and leg CT scans.