Diagnosis of pulmonary embolism

Abstract

No single noninvasive test for pulmonary embolism is both sensitive and specific. Some tests are good for "ruling in" pulmonary embolism (e.g., helical CT) and some tests are good for "ruling out" pulmonary embolism (e.g., D-dimer); others are able to do both but are often nondiagnostic (e.g., ventilation-perfusion lung scanning). For optimal efficiency, choice of the initial diagnostic test should be guided by clinical assessment of the probability of pulmonary embolism and by patient characteristics that may influence test accuracy. This selective approach to testing enables pulmonary embolism to be diagnosed or excluded in a minimum number of steps. However, even with the appropriate use of combinations of noninvasive tests, it is often not possible to definitively diagnose or exclude pulmonary embolism at initial presentation. Most of these patients can be managed safely without treatment or pulmonary angiography by repeating ultrasound testing of the proximal veins after one and 2 weeks to detect evolving deep vein thrombosis. Helical CT and MRI are rapidly improving as diagnostic tests for pulmonary embolism and are expected to become central to its evaluation.

Fig. 1: A diagnostic algorithm for pulmonary embolism (estimated frequencies of test results and associated prevalences of pulmonary embolism for a hypothetical cohort of 1000 outpatients) [1]. If a very sensitive D-dimer assay is used, it can be the first test performed: a negative result excludes pulmonary embolism regardless of clinical assessment category and a positive test can be followed by a ventilation–perfusion scan [2]. A ventilation–perfusion scan can be performed as the initial test without using clinical assessment of the probability of pulmonary embolism as part of the diagnostic process [3]. Pulmonary angiography or helical CT may be considered if the clinical assessment of pulmonary embolism probability is low, particularly if a D-dimer test has not been done [4]. Additional testing (e.g., helical CT, bilateral venography) may be considered if overall assessment suggests a high probability of pulmonary embolism (e.g., 50%–80%), symptoms are severe or cardiopulmonary reserve is poor [5]. Venography should be considered if there is an increased risk of a false-positive ultrasound result (e.g., previous venous thromboembolism, equivocal ultrasound findings, preceding findings suggest low probability of pulmonary embolism [e.g., ≤ 10%]) [6]. It is reasonable not to repeat ultrasound testing, or to do only 1 more ultrasound after 1 week, if preceding findings suggest a low probability of pulmonary embolism (e.g., ≤ 10%) [7]. If helical CT is used in place of ventilation–perfusion lung scanning: (i) intraluminal filling defects in segmental or larger pulmonary arteries are generally diagnostic for pulmonary embolism; (ii) all other findings (i.e., a normal CT scan or intraluminal filling defects confined to the subsegmental pulmonary arteries) are nondiagnostic and can be managed as shown for a nondiagnostic lung scan.

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Figure. Helical CT of the pulmonary arteries with intraluminal filling defects in the lobar artery of the left lower lobe (solid arrow) and the main artery of the right lung (open arrow) in a patient with a chest deformity.

Figure. Conventional pulmonary angiogram of the right lung with intraluminal filling defects in the lobar artery and segmental and subsegmental arteries of the lower lobe.