Ultrasound not needed if multislice CT used in PE diagnosis

April 17, 2008

Geneva, Switzerland - Ultrasound is not needed to rule out pulmonary embolism (PE) when multislice computed tomography (MSCT) is used, according to a new study [1]. Ultrasound could still be of interest in patients with a contraindication to MSCT, although it would allow avoiding MSCT in only one of every 11 patients, the results show.

The study, published in the April 19, 2008 issue of the Lancet, was conducted by a group led by Dr Marc Righini (Geneva University Hospital, Switzerland). He commented to heart wire : "Our study has shown that we can simplify the diagnosis of PE and that using MSCT alone is as safe and effective in detecting PE as MSCT and ultrasound of the lower leg. This has been suggested before in previous studies, but this is the first direct head-to-head trial comparing the two approaches and is definitive confirmation that ultrasound is not needed in addition to MSCT, which should save both time and money in the diagnosis of PE."

If a patient has a contraindication to MSCT, such as allergy to the contrast media or renal failure, Righini noted that ultrasound is recommended as the first test, but in the current study this detected deep venous thrombosis (DVT) in only about 10% of patients, and the remaining patients will need to undergo MSCT as well.

In the paper, Righini and colleagues explain that the current diagnosis of PE involves the combination of clinical assessment, a D-dimer blood test, ultrasonography of the leg, and CT of the chest to directly visualize the clot in pulmonary arteries. While first-generation single-slice spiral CT had a low sensitivity (about 70%) for pulmonary embolism, restricting its use as a standalone test, MSCT allows better visualization.

They conducted the current trial to assess whether ultrasonography was also required to diagnose PE in patients undergoing MSCT. In the trial, 1819 outpatients with clinically suspected PE all underwent clinical probability assessment (using the revised Geneva score) and were randomized to D-dimer measurement and MSCT alone or to D-dimer measurement, ultrasonography of the leg, and MSCT.

Identical results in both groups

The primary outcome was the three-month thromboembolic risk in patients who were left untreated on the basis of the exclusion of PE. This was exactly the same (0.3%) in both groups. The prevalence of pulmonary embolism was also the same (20.6%) in both groups. In the group undergoing ultrasonography, this showed a DVT in 9% of patients in whom MSCT was not then undertaken.

"Our results show that ultrasound is no longer required as a safety net for the identification of clots that might have been missed by MSCT," the researchers say.

Noting that the strategy including ultrasonography of the leg is about 20% more expensive than MSCT alone, Righini et al conclude that "our data do not support the routine use of ultrasound. However, ultrasonography might still be an attractive alternative in patients with renal failure or those who have an allergy to contrast dye, especially in the presence of symptoms and signs of deep venous thrombosis."

In an accompanying editorial [2], Drs Paul Kyrle and Sabine Eichinger (Medical University of Vienna, Austria) note that only a quarter of patients with suspected PE actually have the disease, and the diagnostic strategy is to attempt to rule out the disorder by combining clinical assessment, laboratory studies, and imaging techniques. They point out that the previous Christopher Study showed that combining pretest probability, D-dimer, and CT without ultrasonography effectively ruled out PE [3], but that Righini and colleagues are the first to assess this strategy in a randomized trial. They add that this approach will facilitate the diagnostic workup of patients with suspected PE and seems to be cost-effective.

In some patients, particularly in those with severe renal insufficiency or allergy to intravenous contrast agents, CT cannot be used. These patients, as well as those in whom a CT-based strategy is inconclusive, should have ventilation-perfusion lung scanning, Kyrle and Eichinger recommend.

They also point out that the current study also confirms that a negative D-dimer result in patients with low or moderate pretest clinical probability safely rules out pulmonary embolism. "Overall, a third of patients could be managed by clinical assessment and D-dimer testing only, without recourse to imaging techniques," they state. But they add that in patients with a high pretest probability or positive D-dimer test, further imaging studies are needed.

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