What is the most sensitive investigation for diagnosing Pulmonary Embolism (PE)?

Most Sensitive Investigation for Pulmonary Embolism

Computed Tomography Pulmonary Angiography (CTPA) is the most sensitive investigation for diagnosing pulmonary embolism (PE). 1, 2

Comparison of Diagnostic Tests for PE

CTPA

• Sensitivity: 83%
• Specificity: 96%
• Considered the method of choice for imaging pulmonary vasculature 2
• Allows adequate visualization of pulmonary arteries down to subsegmental level 2
• Has become the de facto clinical "gold standard" for PE diagnosis 3
• Has replaced catheter pulmonary angiography and V/Q scanning as first-line imaging method 3

Ventilation/Perfusion (V/Q) Scan

• High frequency of non-diagnostic results (inconclusive in up to 50% of cases) 2
• Reserved for patients with contraindications to CTPA 1
• Limited by lower sensitivity compared to CTPA

Pulmonary Angiography

• Historically considered the "gold standard" but rarely performed now 2
• Invasive procedure with significant risks:
  • Procedure-related mortality: 0.5%
  • Major non-fatal complications: 1%
  • Minor complications: 5% 2
• Not readily available in all centers 2

ABG (Arterial Blood Gas)

• Not mentioned as a primary diagnostic tool for PE in current guidelines
• Low sensitivity and specificity for PE diagnosis
• Not recommended as a standalone test for PE diagnosis

ECG

• Not a primary diagnostic tool for PE
• May show signs of right heart strain but lacks sensitivity and specificity

Chest X-ray

• Often normal in PE
• May show nonspecific abnormalities
• Insufficient sensitivity to rule out PE
• Mainly useful to exclude other causes of symptoms

Leg Phlebography

• Invasive procedure
• Has been largely replaced by non-invasive tests like compression ultrasonography
• Not recommended as first-line investigation for suspected PE

Diagnostic Algorithm for PE

1. Initial Assessment: Clinical probability assessment using validated tools (Wells score or Geneva score) 1

2. D-dimer Testing:

   • For patients with low or intermediate clinical probability
   • High negative predictive value in non-high risk patients
   • Should not be used in high clinical probability patients 2, 1

3. Imaging:

   • CTPA is the recommended first-line imaging test when indicated 2, 1
   • V/Q scan reserved for patients with contraindications to CTPA 1

Important Considerations

• CTPA can identify alternative diagnoses in approximately 35% of patients without PE 1
• The negative predictive value of CTPA varies with clinical probability (96% in low probability patients but only 60% in high probability patients) 1
• CTPA may miss isolated subsegmental PE, though the clinical significance of these is uncertain 1
• In pregnant women, V/Q scanning may be preferred to reduce fetal radiation exposure 1

Conclusion

Among the options presented (ABG, ECG, Chest X-ray, and Leg phlebography), none are considered highly sensitive for PE diagnosis. The most sensitive investigation for PE diagnosis according to current guidelines is CTPA, which was not listed among the options. Of the listed options, chest X-ray would be the most commonly used in the initial workup, but primarily to rule out alternative diagnoses rather than to confirm PE.