In a patient with elevated D-dimer (D-dimer test) and negative vascular Doppler, is contrast necessary for a CT pulmonary angiogram (CTPA) to rule out pulmonary embolism (PE)?

CT Pulmonary Angiography with Contrast is Required Despite Negative Doppler

In a patient with elevated D-dimer and negative vascular Doppler, contrast is absolutely necessary for CT pulmonary angiography (CTPA) to rule out pulmonary embolism (PE). A non-contrast CT cannot adequately evaluate for pulmonary emboli 1.

Diagnostic Algorithm for Suspected PE

1. Clinical Probability Assessment + D-dimer:

   • Your patient already has an elevated D-dimer, which indicates the need for further imaging
   • Negative D-dimer would have been sufficient to exclude PE in low/intermediate probability patients 1

2. Vascular Doppler Results:

   • Negative lower extremity Doppler ultrasound does NOT exclude PE
   • The ACR Appropriateness Criteria specifically states that ultrasound duplex Doppler of lower extremities cannot be used alone to rule out PE 1
   • While positive Doppler showing DVT can support PE diagnosis, a negative result requires further investigation

3. CTPA with Contrast - Required Next Step:

   • CTPA is the first-line diagnostic imaging tool after positive D-dimer 1
   • Must be performed WITH IV contrast to visualize filling defects in pulmonary arteries
   • When IV contrast is given during CT acquisition, the study should specifically be performed as a CTPA 1

Why Contrast is Essential

CTPA requires contrast administration because:

• Pulmonary emboli are visualized as filling defects within contrast-enhanced pulmonary arteries
• Non-contrast CT chest has no relevant literature supporting its use for PE diagnosis 1
• The ACR guidelines explicitly state: "There is no relevant literature to support the use of CT chest without IV contrast to assess PE" 1

Important Considerations

• CTPA is highly sensitive (78-100%) and specific (89-100%) for detecting PE 1
• CTPA may occasionally demonstrate other pathology responsible for the patient's symptoms 1
• A negative CTPA has been shown to safely exclude PE in several large-scale outcome studies 1

Common Pitfalls to Avoid

1. Relying solely on negative Doppler results: While DVT and PE are related, a negative lower extremity ultrasound does not exclude PE. Studies show that many patients with PE have no evidence of DVT on ultrasound 1.

2. Ordering non-contrast CT: This will not adequately visualize pulmonary emboli and would result in missed diagnosis of a potentially fatal condition.

3. Assuming elevated D-dimer with negative Doppler rules out PE: The 3-month incidence of venous thromboembolism after a negative CTPA in patients with likely clinical probability is still around 2.0% 2, highlighting the importance of proper imaging.

In conclusion, for your patient with elevated D-dimer and negative vascular Doppler, a contrast-enhanced CTPA is absolutely necessary to properly evaluate for pulmonary embolism and cannot be replaced with non-contrast imaging.