Study finds ways to improve detection of blood clots in the lung

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.

"Our study spells out the strengths and weaknesses of chest CTs for diagnosing pulmonary embolism, and will help guide physicians on when more tests are needed," added Stein, who also chaired the PIOPED II steering committee.

During a chest CT angiogram, contrast material (dye) to make the blood vessels in the lungs more visible is injected into a vein in the patient's arm. The patient lies on a table as a machine with dozens of detectors rotates around to quickly take X-ray pictures of the blood vessels - in the lungs for the chest CT or in the pelvis or thighs for the leg CT. No additional dye is needed for the leg CT, which can be performed immediately after the chest CT. A computer combines the images to make detailed pictures.

CT scans are noninvasive and well tolerated by most patients. Adverse effects are rare and are primarily related to the iodine-based dye that is injected; possible complications include kidney damage in patients with kidney disease and allergic reaction. The tests involve some exposure to radiation, but the benefits outweigh the risks.

Risk factors for pulmonary embolism include blood clots in the leg or a history of such problems, and certain inherited conditions that increase the risk for blood clotting. Individuals who recently have been treated for cancer, have been bedridden, or have had surgery or suffered a fracture in the past few weeks are also more likely to develop PE. Other risk factors for DVT, which can lead to PE, include sitting for long periods of time, pregnancy and the 6-week period after pregnancy, and being overweight or obese. Women who take hormone therapy or birth control pills are also at increased risk for DVT.

Signs of PE include unexplained shortness of breath, pain with deep breathing, and coughing up blood. Rapid breathing and a fast heart rate can also indicate possible PE. In some cases, there are only signs of DVT, such as swelling of the leg or along the vein in the leg, pain or tenderness in one leg, feeling of increased warmth in the area of the leg that is swollen, and red or discolored skin on the affected leg. Other patients do not experience any symptoms or signs of PE or DVT.