Dec. 19 (HealthDay News) -- Pulmonary embolism -- a life-threatening blood clot in the lungs -- can be very difficult to diagnose, but new research suggests a technique using CT scanning might be slightly more effective than lung scans at spotting such clots.

Canadian researchers found that computed tomographic pulmonary angiography (CTPA) detected pulmonary embolisms in 19.2 percent of a group of study volunteers who'd had symptoms of a pulmonary embolism, compared to 14.2 percent of the group who underwent the more standard test, ventilation/perfusion (V/Q) lung scanning.

More important, a pulmonary embolism was missed in only two out of the 561 people who underwent CTPA versus six out of 611 people who'd had V/Q scans. If a pulmonary embolism goes undetected and untreated, almost one-third of people with this type of blood clot will die.

"CTPA can safely be used to exclude the diagnosis of pulmonary embolism," said study author Dr. David R. Anderson, head of the division of hematology at Dalhousie University and Capitol Health in Halifax, Nova Scotia.

"We ended up diagnosing more patients with pulmonary embolism with CTPA rather than V/Q scanning. That was a bit of a surprise," said Anderson, who added that this finding needs to be investigated further. "Sometimes, when you detect very small blood clots that are isolated to single vessels, clinically, we wonder if such a small abnormality is the cause of symptoms."

Results of the study are published in the Dec. 19 issue of the Journal of the American Medical Association.

"In general, the tendency is that when a clot is found, unless there's a specific contraindication, the clot is treated, because we recognize that clots can be very serious, and they can recur," said Dr. Jeffrey Glassroth, vice dean and chief academic officer at the Feinberg School of Medicine at Northwestern University in Chicago. Glassroth is also the author of an accompanying editorial.

However, the anticoagulant medicines used to treat blood clots aren't without risk. In some people, these medications can cause excessive bleeding that can be life-threatening. So, it's important to accurately diagnose pulmonary embolism. "There's a definite downside to treating unnecessarily," Anderson noted.

For the past 30 years, V/Q scanning has been the tool most often used to diagnose pulmonary embolisms. The test requires an injection of a radioactive tracer and the inhalation of radioactive gas. CTPA has been around for about 10 years and also involves using an injected radioactive tracer but doesn't require the inhalation portion included in V/Q scans. The cost of the two procedures is similar, but there's a slightly higher chance of a reaction to the contrast material used in CTPA, and the radiation dose from the imaging technique is higher with CTPA, according to Glassroth.

While CTPA quickly became an accepted alternative, it wasn't clear how the two methods compared to each other. To answer that question, Anderson and his colleagues recruited 1,417 people being treated for symptoms of pulmonary embolism.

Seven hundred and one of the study volunteers had CTPA to rule out pulmonary embolism, while 716 had V/Q scans. Almost 20 percent of those undergoing CTPA were diagnosed with a pulmonary embolism, compared to 14.2 percent of the V/Q volunteers.

A pulmonary embolism was missed in just 0.4 percent of those who had CTPA versus 1 percent of those who had V/Q scanning, according to the study.

"We did confirm that the newer modality (CTPA) was at least as safe as V/Q," said Anderson.

And, although doctors may still be debating which clots need immediate treatment, Glassroth said the good news from this study is that "we have ever-improving capabilities for detecting clots."

SOURCES: David Anderson, M.D., professor, medicine, and head, division of hematology, Dalhousie University and Capitol Health, Halifax, Nova Scotia, Canada; Jeffrey Glassroth, M.D., vice dean, chief academic officer, and professor, medicine, Feinberg School of Medicine, Northwestern University, Chicago; Dec. 19, 2007, Journal of the American Medical Association