What is the best initial test for pulmonary embolism (PE) in pregnancy?

Best Test for Pulmonary Embolism in Pregnancy

The best initial test depends on clinical presentation: perform chest X-ray first in all cases, then if signs/symptoms of DVT are present, start with bilateral lower extremity compression ultrasound; if no DVT signs and chest X-ray is normal, proceed with lung scintigraphy (V/Q scan); if chest X-ray is abnormal, proceed with CT pulmonary angiography (CTPA). 1, 2

Initial Clinical Assessment and Triage

Start with chest radiography (CXR) as the first radiation-associated procedure to guide subsequent imaging choices and minimize radiation exposure. 1, 2 This strong recommendation from the American Thoracic Society helps triage between V/Q scanning and CTPA based on whether the CXR is normal or abnormal. 1

If DVT Signs/Symptoms Present

Perform bilateral compression ultrasound (CUS) of lower extremities first if the patient has leg swelling, pain, or other DVT symptoms. 1, 2 This approach prioritizes avoiding radiation exposure when possible:

• If CUS is positive: initiate anticoagulation immediately without further pulmonary imaging 1
• If CUS is negative: proceed to chest radiography and then pulmonary vascular imaging 1

The "LEFt" criteria can help identify high-risk patients: symptoms in the Left leg, calf circumference difference >2 cm, and First trimester presentation predict DVT in 58.3% of cases when 2-3 variables are present. 1

If No DVT Signs/Symptoms

Proceed directly to pulmonary vascular imaging rather than lower extremity ultrasound to improve cost-effectiveness and diagnostic efficiency. 1

Pulmonary Vascular Imaging Selection

Normal Chest X-Ray: Choose V/Q Scan

Lung scintigraphy (V/Q scan) is preferred over CTPA when the chest X-ray is normal. 1, 2 This strong recommendation is based on:

• Lower maternal breast radiation: V/Q delivers 0.28-0.37 mGy to breast tissue versus 44 mGy with CTPA 3
• Diagnostic yield: 75-94% of V/Q scans provide diagnostic results (high probability, very low probability, or normal) in pregnant women with normal CXR 1
• Similar accuracy: False-negative rates are comparable between V/Q (0.5%) and CTPA (0.4%) 4
• Fetal safety: Both modalities deliver fetal radiation well below the 50,000 μGy safety threshold 1

The lifetime breast cancer risk increase from a single CTPA in a 25-year-old woman is 1.011, making radiation minimization important. 2

Abnormal Chest X-Ray: Choose CTPA

CTPA is preferred over V/Q scanning when the chest X-ray is abnormal because V/Q scans lose accuracy in the presence of parenchymal lung disease. 2, 4 CTPA also provides the advantage of identifying alternative diagnoses. 2

Important Caveats and Pitfalls

D-Dimer Testing

Do not use D-dimer to exclude PE in pregnancy. 1 The American Thoracic Society gives this a weak recommendation with very-low-quality evidence, but emphasizes avoiding false negatives. D-dimer levels rise physiologically throughout pregnancy, with only 4.2% of third-trimester women with non-high pre-test probability having negative D-dimer. 5

Technical Challenges with CTPA

CTPA yields inconclusive results in 6-36% of pregnant women due to suboptimal vascular opacification and respiratory motion artifacts. 2 In the third trimester specifically, 43% of studies may be inconclusive. 5 Optimize CTPA protocols with:

• Automated bolus triggering
• High iodine flux and concentration
• Reduced anatomical coverage
• Iterative reconstruction techniques 2, 5

Avoid repeated CTPA after non-diagnostic studies due to cumulative radiation exposure. 2

Management of Inconclusive Results

Initiate empiric anticoagulation while awaiting further testing if clinical suspicion is high and an initial imaging study is inconclusive or unavailable. 2, 4 PE accounts for 20% of maternal deaths in the United States, making the risk-benefit ratio favor treatment during diagnostic uncertainty. 4

For indeterminate CTPA results:

• Assess for DVT symptoms and perform bilateral CUS if present 4
• Consider V/Q scanning if chest X-ray was normal 4
• Start therapeutic low-molecular-weight heparin if clinical suspicion remains high 4

Contrast Agent Safety

Iodinated contrast agents for CTPA are not associated with neonatal adverse events and should not be withheld due to fetal concerns. 6 Modern CTPA techniques deliver fetal radiation doses well below complication thresholds (actual doses <1 mGy versus 50-100 mSv threshold). 5

Algorithm Summary

1. All patients: Obtain chest X-ray first 1, 2
2. DVT signs/symptoms present: Bilateral lower extremity CUS → if positive, treat; if negative, proceed to step 3 1
3. Normal CXR: V/Q scan 1, 2
4. Abnormal CXR: CTPA 2, 4
5. Inconclusive or unavailable imaging with high suspicion: Empiric anticoagulation 2, 4