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Imaging procedures for diagnosing blood clots
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CHICAGO—New research indicates that a diagnostic strategy using computed tomographic pulmonary angiography (CTPA) may be a safe alternative to conventional lungs scans (known as ventilation-perfusion scans) for excluding the diagnosis of pulmonary embolism (blood clots in the lung vessels), although CTPA may detect more clots, according to a study in the December 19 issue of JAMA.

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Pulmonary embolism is a common and serious medical condition leading to the hospitalization or death of more than 250,000 people in the U.S. each year, and is estimated to result in 5 percent to 10 percent of all deaths in U.S. hospitals. Pulmonary embolism remains one of the most difficult conditions for clinicians to diagnose accurately, with timely, accurate tests essential to start appropriate therapy, while avoiding the risks of therapy to patients diagnosed as not having a pulmonary embolism, according to background information in the article.

For 30 years, ventilation-perfusion (V/Q) lung scanning had been the non-invasive imaging procedure of choice in patients with suspected pulmonary embolism. In the last decade, CTPA was introduced as an alternative non-invasive test and has been adopted rapidly, despite some concerns about the sensitivity (the proportion of affected individuals who have a positive test result for a condition the test is intended to reveal) of this method.

David R. Anderson, M.D., of Dalhousie University, Halifax, Nova Scotia, Canada, and colleagues conducted a comparison of CTPA with V/Q scanning to determine if CTPA is a safe, reliable alternative to V/Q scanning, and does not miss the detection of clinically important pulmonary blockages. The trial was conducted at four Canadian and one U.S. tertiary care centers between May 2001 and April 2005 and included 1,417 patients considered likely to have acute pulmonary embolism. Patients were randomized to undergo either V/Q scanning (n = 716) or CTPA (n = 701). Patients in whom this testing did not indicate pulmonary embolism did not receive anti-thrombotic therapy and were followed-up for three months.

Of the patients randomized to CTPA, 133 (19.2 percent) were diagnosed with pulmonary embolism or deep vein thrombosis in the initial evaluation period; 101 (14.2 percent) of patients in the V/Q scanning group had a similar diagnosis. Both groups of patients were treated with anti-coagulant therapy. The overall rate of venous thromboembolism (the composite of deep vein thrombosis and pulmonary embolism) found in the initial diagnostic period was significantly greater in patients randomized to the CTPA strategy (difference, 5 percent).

Of those in whom pulmonary embolism was considered excluded, 2 of 561 patients (0.4 percent) randomized to CTPA vs. 6 of 611 patients (1.0 percent) undergoing V/Q scanning developed venous thromboembolism in follow-up (difference, -0.6 percent).

“The results of our study are reassuring given previous reports of relatively low sensitivity of CTPA for the diagnosis of pulmonary embolism,” the authors write.

“ unanticipated finding in our study was that CTPA resulted in a significantly greater number of venous thromboembolism diagnoses than did V/Q scanning,” they add. “Further research is required to confirm whether some pulmonary emboli detected by CTPA may be clinically unimportant, the equivalent of deep vein thrombosis isolated to the calf veins, and not require anti-coagulant therapy.” (JAMA. 2007;298(23):2743-2753.


In an accompanying editorial, Jeffrey Glassroth, M.D., of the Feinberg School of Medicine, Northwestern University, Chicago, writes that the findings by Anderson and colleagues have clinical implications.

“First, clinicians should consider the likelihood of pulmonary embolism in a structured manner based on patients’ presenting histories and physical examinations much the way Anderson and colleagues did, and based on those assessments, proceed, as necessary, to D-dimer testing [a blood test to diagnose thrombosis]. These 2 steps may substantially reduce the probability that pulmonary embolism, at least large clots, are present and obviate the need for additional study. Where significant concern remains, including some patients whose pulmonary embolism probability may not be very high but whose comorbidities put them at great risk were an embolism to occur, additional testing should be pursued. If readily available, lower extremity ultrasound studies to search for deep vein thrombosis to treat those patients found to have such clots is a reasonable next step.”
(JAMA. 2007;298(23):2788-2789.

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