How Good Are the New CT Scanners for PE?

Abstract & Commentary

By Stephanie B. Abbuhl, MD, FACEP, Vice Chair, Department of Emergency Medicine, The Hospital of the University of Pennsylvania; Associate Professor of Emergency Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA. Dr. Abbuhl reports no relationships with companies having ties to the field of study covered by this CME program.

Source: Perrier A, et al. Multidetector-row computed tomography in suspected pulmonary embolism. N Engl J Med 2005;352:1760-1768.

The purpose of this prospective study was to assess whether a strategy of D-dimer testing and multidetector-row computed tomography (CT)—without the use of lower-limb ultrasonography (US)—would safely rule out pulmonary embolism (PE) in emergency department (ED) patients suspected to have PE. All patients were followed for three months. Of 1014 patients screened, 18% were excluded due to predefined protocol criteria, while another 7% were excluded for violations of the study protocol.

Patients were categorized into one of three pretest probability categories, based upon their Geneva scores. In patients with either a low or intermediate clinical probability, an ELISA D-dimer was used to rule out PE with a level below 500ug/L. In all patients with a D-dimer level above the cut-off, both a lower limb proximal venous compressive US and a multidetector-row CT scan were performed. If either CT or US results were positive, the patient was given anticoagulation treatment. If both were negative, the patient was discharged without anticoagulation treatment.

In patients with a high pretest probability of PE, no D-dimer test was done and both CT and US imaging were done. If either was positive, the patient was anticoagulated, but if both were negative, a pulmonary angiogram was performed. The results of CT imaging were considered inconclusive if there were technical problems or if only a subsegmental PE was diagnosed. Inconclusive CT results were followed by a VQ scan or a pulmonary angiogram for final diagnosis.

The prevalence of PE was 26%. In the 82 patients with a high pretest probability of PE, CT imaging was positive in 78 (95%), and 37 of the 78 had proximal deep vein thrombosis (DVT) (47%; 95%CI, 37-58%). Only one patient had DVT and a negative CT scan, and that patient was treated. Three patients had negative CT, US, and angiogram results. Of the 674 low/intermediate pre-test probability patients, 34% had a negative D-dimer test and an uneventful follow-up. In the remaining 442 patients, CT results were positive in 109 and 2 additional patients with negative CT results were identified with DVT on US. CT and US results were negative in 318 patients in whom the three-month risk of thromboembolism was 1.7% (95%CI, 0.7-3.9).

Commentary

The main question this study addresses is: What is the additional value of looking for proximal DVT in patients with a negative CT scan after initial screening with both a pretest probability assessment and a D-dimer? Only 0.9% (95% CI, 0.3-2.7) of patients had proximal DVT despite a negative CT scan. This low rate is consistent with at least one previous study1 but significantly less than the rate in another.2 With such a marginal improvement in overall detection of thromboembolism (VTE) with US, the authors determined that the three-month VTE risk in patients untreated after a negative CT scan alone would have been 1.5% (95%CI, 0.8-3.0), a risk similar to that after pulmonary angiography and similar to other outcome studies. Because the study did not truly manage patients without US, the authors concluded that “a larger outcome study is needed before this approach can be adopted.”

Our dilemma is whether we can adopt this strategy now or be cautious and continue to do US as a complementary study when CT imaging is negative. The weight of evidence clearly is suggesting that this is not necessary, at least in patients with low-to-intermediate probability and especially in outpatients.

It is worth noting that the number of high pretest probability patients in this study was small (i.e., only 10% of all the study patients) and a larger outcome study where patients are managed without US would be reassuring in this group. The authors, in fact, made the suggestion that US be eliminated in those patients without a high probability of PE, but this recommendation is buried in the discussion and undoubtedly will be missed by many readers.

In addition, this series adds to other studies that have validated the use of ELISA D-dimer as an initial test in patients with low-to-intermediate pretest probability. Not a single thromboembolic event occurred in the 220 patients with low-intermediate probability with a normal D-dimer test result who were left untreated.

References

1. van Strijen MJ, et al. Single detector helical computed tomography as the primary diagnostic test in suspected pulmonary embolism: A multicenter clinical management study of 510 patients. Ann Intern Med 2003; 138:307-314.

2. Musset D, et al. Diagnostic strategy for patients with suspected pulmonary embolism: A prospective multicentre outcome study. Lancet 2002;360:1914-1920.