In the scenario of pulmonary embolism, which is the most sensitive investigation?

Most Sensitive Investigation for Pulmonary Embolism

Among the options listed, leg phlebography (venous imaging) demonstrates the highest sensitivity for detecting the underlying source of PE, as the majority of pulmonary emboli originate from deep vein thrombosis, and proximal DVT detection itself warrants anticoagulation even without direct PE confirmation. However, this answer requires important contextualization about what "sensitivity" means in PE diagnosis.

Understanding Sensitivity in PE Diagnosis

The question asks about sensitivity, which measures how well a test detects disease when present. Among your listed options:

Why Leg Phlebography Has High Sensitivity

• The majority of patients with PE have proximal deep vein thrombosis, even without clinical signs of DVT 1
• Detection of proximal DVT is itself an indication for anticoagulation treatment, even without direct proof of embolism 1
• Compression ultrasound with color Doppler imaging shows high accuracy in the femoropopliteal segment and is now the investigation of choice for suspected lower limb DVT 1
• Leg vein imaging is specifically indicated following an indeterminate V/Q scan in the assessment of PE 1

Why the Other Options Have Lower Sensitivity

ABG (Arterial Blood Gas):

• Non-specific findings that cannot reliably diagnose or exclude PE 1
• May show hypoxemia and hypocapnia, but these are neither sensitive nor specific for PE

ECG (Electrocardiogram):

• Shows non-specific changes in PE 1
• Classic findings (S1Q3T3, right heart strain) occur in minority of cases and indicate severe PE
• Cannot be used as a sensitive screening tool

Chest X-ray:

• Frequently normal in PE or shows non-specific findings 1
• Used primarily to exclude alternative diagnoses, not to diagnose PE
• Very low sensitivity for detecting PE itself

Critical Context: Modern Diagnostic Standards

It's essential to understand that in contemporary practice, CTPA (CT pulmonary angiography) is the actual gold standard imaging modality for PE diagnosis, though it wasn't among your options 1:

• CTPA has excellent accuracy with sensitivity >95% for segmental or larger emboli 1, 2
• CTPA is now the recommended initial lung imaging modality for non-massive PE 1
• Patients with good quality negative CTPA do not require further investigation or treatment for PE 1

Practical Diagnostic Algorithm

The correct modern approach prioritizes clinical probability assessment first 3, 2:

1. Always begin with validated clinical probability assessment (Wells score or Geneva score) 3
2. For low-to-moderate probability: obtain D-dimer first 3, 2
   • Use age-adjusted cutoff (age × 10 ng/mL) for patients >50 years 3, 2
   • Negative D-dimer safely excludes PE without imaging 3, 2
3. If D-dimer positive or high clinical probability: proceed to CTPA 2
4. Leg imaging (ultrasound) is indicated when CTPA is non-diagnostic or as first-line in patients with clinical DVT 1

Common Pitfalls to Avoid

• Never proceed directly to imaging without clinical probability assessment 3
• D-dimer has high negative predictive value but poor positive predictive value—elevation alone does not diagnose PE 3
• D-dimer specificity drops to only 10% in patients >80 years, making age-adjusted cutoffs essential 3
• Ordering CTPA with elevated D-dimer but low clinical suspicion leads to unnecessary radiation exposure and false positives 3, 2

For your specific multiple-choice question, option (d) leg phlebography is the most sensitive among the listed options, as it detects the DVT source present in the majority of PE cases 1.