Next Steps After Negative Chest X-ray and Bilateral Leg Doppler for Suspected Pulmonary Embolism
The next step in evaluating a patient with suspected pulmonary embolism after negative chest X-ray and bilateral leg venous Doppler ultrasound should be CT Pulmonary Angiography (CTPA), which is currently the standard of care for PE detection. 1
Diagnostic Algorithm
- A negative leg ultrasound does not exclude pulmonary embolism, as studies have shown that proximal clot is found in only 23-52% of patients with confirmed PE 1
- CTPA is the imaging test of first choice and is more sensitive than other studies such as V/Q scans, with the ability to detect filling defects in distal subsegmental arteries as small as 2-3 mm in diameter 1
- Multidetector CTPA has been shown to have high sensitivity and specificity for the diagnosis of PE and can determine the presence or absence of PE with sufficient certainty without the need for additional imaging tests 2
Special Considerations
- For patients who cannot receive IV contrast (due to kidney disease or contrast allergy), a ventilation-perfusion (V/Q) scan is the recommended alternative 3, 4
- In pregnant patients, there is ongoing debate about whether CTPA or V/Q scan is preferable, with some guidelines recommending V/Q scanning due to concerns about maternal radiation exposure 1
- For patients with high clinical probability of PE and a negative CTPA, additional testing may be warranted 3
Clinical Risk Assessment
- Before proceeding with CTPA, clinical probability assessment should be considered 5
- In patients with low clinical probability and negative D-dimer, PE can be safely excluded without further imaging 3, 5
- For patients with intermediate/high probability or positive D-dimer, proceed with CTPA 4, 5
Important Caveats
- A single normal leg ultrasound should not be relied upon for exclusion of subclinical DVT 1
- While a negative chest X-ray and negative bilateral leg Doppler significantly decrease the likelihood of PE, they do not exclude it 1
- In patients with high clinical suspicion for PE despite negative initial tests, further imaging with CTPA is warranted to avoid missing this potentially fatal diagnosis 1
- The presence of DVT on ultrasound would have eliminated the need for further testing, as treatment would be identical whether or not PE is present 1
Ventilation-Perfusion Scan Considerations
- V/Q scanning is particularly useful in patients with chronic kidney disease or contrast allergy 4
- A normal V/Q scan has high negative predictive value for excluding pulmonary embolism 4, 6
- V/Q SPECT (Single Photon Emission Computed Tomography) offers improved diagnostic accuracy compared to traditional planar V/Q scanning 7