Pulmonary Emboli Overdiagnosed by CT Angiography

Tinker Ready

July 05, 2013

The routine use of computed tomography (CT) pulmonary angiography for the detection of pulmonary emboli has led to overdiagnosis of the condition, according to a new study published online July 2 in the BMJ.

In the 8 years after the test was introduced, there was an 80% increase in the incidence of pulmonary embolism but no subsequent drop in deaths. From 1998 to 2006, the detection rate rose from 62.1 to 112.3 per 100,000 US adults, according to the research team led by from Renda Soylemez Wiener, MD, MPH, from the Pulmonary Center, Boston University School of Medicine, and the Center for Health Quality, Outcomes and Economic Research, Edith Nourse Rogers Memorial Veterans Hospital, Bedford, Massachusetts.

At the same time, US deaths from pulmonary embolism dropped only from 12.3 to 11.9 per 100 000. Age-adjusted fatality from pulmonary embolism for hospitalized patients declined by a third, "from 12.1% to 7.8% (P < .001), suggesting that the extra pulmonary emboli being detected are less lethal (given that treatment has not become more effective)," the authors write. "More non-fatal pulmonary emboli dilute case fatality but do not change mortality."

The researchers note that the idea of overdiagnosis of pulmonary emboli might seem "counterintuitive...but the harms are just as real as those of underdiagnosis."

Physicians are trained to suspect pulmonary embolism when a patient complains of shortness of breath, pleuritic chest pain, tachycardia, or signs of right heart strain. Given the lack of sensitivity and specificity of these clinical signs and symptoms, scoring systems have been developed to help clinicians decide which patients to scan. In practice, however, many clinicians skip these scoring systems and go directly to imaging instead, the researchers explain.

Until the late 1990s, ventilation-perfusion (VQ) scanning was the first-line test for pulmonary embolism, with clinicians maintaining a "high threshold" for ordering an invasive pulmonary angiogram, the authors continue. Since the introduction of CT pulmonary angiography, surveys show that most physicians have switched to the noninvasive CT test as first line to detect the "silent killer." Concerns about malpractice, pressure to use new technology, increased availability of CT angiographic scanners, and the ability for these machines to detect other pulmonary causes of the patients' symptoms may be also drive the trend toward increased use of CT pulmonary angiography.

At the same time, the evidence suggests that many small clots are reabsorbed without treatment. The researchers note 1 study that found unsuspected pulmonary emboli in 50% to 60% of autopsies. Similarly, they report another trial that showed a "surprisingly high" number of incidental pulmonary emboli found by contrast CT scans performed for other indications.

The main problem with overdiagnosis of pulmonary embolism is the subsequent treatment with anticoagulation, a leading cause of medication-related death.

"In some studies, complications of anticoagulation are more common than the problem treatment is meant to prevent: recurrent venous thromboembolism," write the authors. "Notably, in the largest case series of patients given anticoagulants for isolated subsegmental pulmonary embolism (n=93), the risk of major bleeding was 5.3% but the risk of recurrent venous thromboembolism was only 0.7%," they cite.

As advocated by the Clinicians Choosing Wisely campaign of the American Board of Internal Medicine, physicians should reserve CT pulmonary angiography for patients who are at intermediate to high risk for pulmonary embolism on the basis of diagnostic algorithms that combine clinical probability and D-dimer tests, the researchers note. Another option would be to consider VQ scan or Doppler ultrasonography of the legs.

"VQ scans may make more sense for younger patients (less radiation), patients with normal lungs (a definitive result is more likely), and patients with renal dysfunction (no nephrotoxic contrast)," the authors write.

"Detection of deep vein thrombosis by ultrasonography of the legs when pulmonary embolism is suspected makes subsequent lung imaging unnecessary because patients need anticoagulation anyway," they conclude.

Although pulmonary embolism is an important cause of death, "the diagnostic zeal and technological advances meant to improve outcomes of patients with pulmonary embolism are double edged swords," the researchers write. To improve patient outcomes, they also call for research to assess which small emboli need treatment. An ongoing study is currently evaluating the safety of withholding treatment for stable patients with isolated subsegmental pulmonary embolism.

This review is the first of a series in BMJ to discuss risks and harms of overdiagnosis, given the expanding definitions of disease and increasing use of new technologies.

The study was supported by the National Cancer Institute's Cancer Research Network Across Health Care Systems fund. The lead author is supported by a grant from the National Cancer Institute and the US Department of Veterans Affairs. The other researchers have disclosed no relevant financial relationships.

BMJ. Published online July 2, 2013. Full text

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