Detecting Clots in the Lungs

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The summary below is from the full report titled "Single-Detector Helical Computed Tomography as the Primary Diagnostic Test in Suspected Pulmonary Embolism: A Multicenter Clinical Management Study of 510 Patients." It is in the 18 February 2003 issue of Annals of Internal Medicine (volume 138, pages 307-314). The authors are MJL van Strijen, W de Monyé, J Schiereck, GJ Kieft, MH Prins, MV Huisman, and PMT Pattynama, for the Advances in New Technologies Evaluating the Localisation of Pulmonary Embolism (ANTELOPE) Study Group.

What is the problem and what is known about it so far?

Pulmonary emboli are small pieces of blood clots. They usually break off from larger clots in the veins of the legs and travel through the circulatory system to the lungs. The emboli can block blood flow to the lungs and prevent oxygen from reaching the tissues. They can cause shortness of breath, chest pain, coughing up of blood, and death. Pulmonary emboli can be diagnosed in many ways, including by ultrasonography or dye tests to look for the source of clots in the legs, special blood tests (D-dimer assays), and scan or dye tests of the lung. Helical computed tomography (CT) is one way to scan the lungs for pulmonary emboli. This test uses a computer to display multidimensional images made from x-rays that pass through the body as a CT machine is rotated around the patient. Although helical CT is widely used, some studies suggest that helical CT alone is not a good way to rule out pulmonary emboli.

Why did the researchers do this particular study?

To learn whether a strategy that combines helical CT with ultrasonography of the legs can rule out pulmonary emboli.

Who was studied?

510 patients with suspected pulmonary embolism.

How was the study done?

All patients had helical CT. They were then treated with blood thinners if the scans showed emboli. If an ultrasonography scan showed no emboli and a clear alternative diagnosis, such as pneumonia, patients were not treated with blood thinners for emboli. If a scan showed neither emboli nor alternative diagnoses, serial ultrasonography of the leg veins was done. If a scan showed no leg clots, patients were not treated with blood thinners. All patients were followed for 3 months. They had repeated tests to confirm clots in the legs or lungs if they had any signs or symptoms suggesting clots.

What did the researchers find?

Helical CT showed pulmonary emboli in 24% of the patients and alternative diagnoses in 26%. Only 2 of 248 patients with CT scans that showed neither emboli nor alternative diagnoses had positive ultrasonography results. Evidence of emboli was not seen in 376 patients who did not receive blood thinners. Of these, 4 had suspected clots in their legs or lungs during 3 months of follow-up. Clots were confirmed in 2 patients.

What were the limitations of the study?

The technology of helical CT is changing. Newer CT scanners may detect more and smaller emboli than the types of scans that were used in this study. Accurate diagnosis of leg clots requires serial ultrasonography (one to two repeated tests if previous test results are negative). One fifth of the patients in this study who had ultrasonography did not have repeated tests, and some leg clots could have been missed.

What are the implications of the study?

Helical CT alone rather than helical CT combined with leg ultrasonography may be adequate for ruling out most clinically important pulmonary emboli.