BMJ  2005;331 (6 August), doi:10.1136/bmj.331.7512.0-e

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Negative CT scans can rule out pulmonary embolism in low risk patients

Question Can clinicians rely on a negative computed tomography (CT) scan to rule out suspected pulmonary embolism?

Synopsis Previous studies question the value of using CT scanning alone to rule out suspected pulmonary embolism. These investigators thoroughly searched multiple databases—including Medline, the Cochrane Registry of Controlled Trials, and Science Citation Index—and relevant journals for English language articles meeting selection criteria. Included studies used contrast enhanced CT as the initial triage test to rule out the diagnosis of acute pulmonary embolism, had an appropriate clinical follow-up of at least three months, and had a prospective design. The gold standard to establish the validity of testing to rule out pulmonary embolism was the rate of subsequent venous thromboembolic events after anticoagulation therapy was withheld. Two reviewers independently abstracted data and a third party arbitrated discrepancies. From the initial search that found 22 studies, 15 studies evaluating a total of 3500 patients met the minimal inclusion criteria, of which seven met the criteria for level 1 diagnostic studies. Three different CT modalities were evaluated (single slice CT, multidetector row (helical) CT, and electron beam CT). Patient follow-up ranged from three months to 12 months. The overall negative likelihood ratio of a venous thromboembolic event after a negative CT scan for pulmonary embolism was 0.07 (95% CI 0.05 to 0.11). There was no significant difference in the risk of a subsequent event based on the type of CT modality used. The risk of subsequent events in studies using additional imaging tests prior to chest CT was not significantly reduced compared with studies that used chest CT imaging only. The reported negative likelihood ratio in this analysis compares favourably with that reported for pulmonary angiography (Henry JW, Relyea B, Stein PD. Chest 1995;107: 1375-8[Abstract/Free Full Text]). A formal analysis found no significant publication bias, but there was some minimal heterogeneity among the results of the various trials.

Bottom line A negative computed tomography scan is as accurate as pulmonary angiography in ruling out suspected pulmonary embolism. Clinicians should strongly consider using clinical decision rules to accurately assess the pretest probability of pulmonary embolism in an individual patient, and then interpret diagnostic tests in light of this probability. For example, a negative scan in a low risk patient rules out pulmonary embolism, while a negative scan in a high risk patient may require further confirmation.

Level of evidence 2a (see www.infopoems.com/levels.html). Systematic reviews of cohort studies displaying worrisome heterogeneity.

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Quiroz R, Kucher N, Zou KH, et al. Clinical validity of a negative computed tomography scan in patients with suspected pulmonary embolism. A systematic review. JAMA 2005;293: 2012-7[Abstract/Free Full Text].

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^* Patient-Oriented Evidence that Matters. See editorial ( BMJ 2002;325: 983[Free Full Text])

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Absence of evidence

Jeffrey Mann
bmj.com, 6 Aug 2005 [Full text]