From the Guidelines
To rule out pulmonary embolism (PE), a CT pulmonary angiogram (CTPA) is the recommended imaging study, as stated in the most recent guidelines 1. This test involves intravenous contrast administration followed by rapid CT scanning of the pulmonary arteries. The patient should have an IV placed, and renal function should be checked before administering contrast. Patients should be instructed to hold their breath during the scan for optimal image quality. CTPA directly visualizes filling defects in the pulmonary arterial tree, allowing for definitive diagnosis of PE with high sensitivity (>90%) and specificity (>95%). Some key points to consider when ordering a CT to rule out PE include:
- Using age-adjusted d-dimer thresholds in patients older than 50 years to determine whether imaging is warranted 1
- Reserving ventilation-perfusion scans for patients who have a contraindication to CTPA or if CTPA is not available 1
- Not obtaining any imaging studies in patients with a d-dimer level below the age-adjusted cutoff 1 The test takes approximately 5-10 minutes to perform, and results are typically available within hours. For patients with contrast allergies or renal insufficiency, a ventilation-perfusion (V/Q) scan may be considered as an alternative. D-dimer testing prior to imaging may be appropriate in patients with low clinical probability of PE to potentially avoid unnecessary radiation exposure, but should not delay imaging in patients with moderate to high clinical suspicion 1.
From the Research
Diagnostic Tests for Pulmonary Embolism
To rule out pulmonary embolism (PE), the following diagnostic tests can be considered:
- Computed Tomography Pulmonary Angiography (CTPA) is the imaging test of first choice 2
- Compression ultrasonography and ventilation-perfusion scintigraphy are reserved for patients with concomitant symptomatic deep vein thrombosis or a stringent contraindication for CTPA, respectively 2
- Lower extremity venous compression ultrasonography (CU) is both sensitive and specific for the diagnosis of femoropopliteal DVT 3
- V/Q scan, CT pulmonary angiography, and lower extremity ultrasound remain the diagnostic imaging modalities of choice 4
Safety of Ruling Out Acute Pulmonary Embolism by Normal CTPA
A normal CTPA result alone can safely exclude PE in all patients in whom CTPA is required to rule out this disease 5
- The pooled incidence of VTE at 3 months was 1.2% based on a normal CTPA result as a sole test
- The negative predictive value of a normal CTPA result was 98.8%
CT Angiography Protocol
For lean patients, a low-kilovoltage pulmonary CT angiography protocol with 40 mL of iodinated contrast material can provide sufficient vascular enhancement 6
- Iodixanol (320 mg I/mL) and iomeprol (400 mg I/mL) are two contrast media that can be used for this purpose
- The overall vascular density of pulmonary arteries down to the segmental level was significantly higher with iodixanol (320 mg I/mL) than with iomeprol (400 mg I/mL) 6