A 42-year-old woman with suspected pulmonary embolism has moderate renal impairment (eGFR 35 mL/min/1.73m²) and a documented contrast allergy. CTPA cannot be performed. What is the most appropriate next investigation to confirm or exclude PE in this patient?

Explanation

## Investigation Strategy When CTPA is Contraindicated **Key Point:** When CTPA cannot be performed due to renal impairment and contrast allergy, the **V/Q (ventilation-perfusion) scan** is the recommended first-line alternative investigation to confirm or exclude pulmonary embolism. ### Rationale for V/Q Scan **High-Yield:** In patients with suspected PE and contraindications to CTPA (contrast allergy and/or renal impairment): 1. **V/Q scan** uses inhaled radiolabelled gas (ventilation) and IV-injected technetium-labelled macroaggregated albumin (perfusion) — **no iodinated contrast** is required 2. It directly images the pulmonary vasculature for perfusion defects, making it the most appropriate **direct** investigation for PE when CTPA is unavailable 3. Sensitivity ~98%, specificity ~97% when interpreted using PIOPED II criteria in patients with normal chest X-ray 4. Safe in renal impairment — the tracer dose is negligible and does not cause contrast nephropathy 5. Safe in contrast allergy — no iodinated contrast is used ### Comparison of Available Options | Investigation | Pros | Cons | Use in This Case | |---|---|---|---| | **V/Q Scan** | No iodinated contrast, directly images PE, well-validated | Requires normal/near-normal CXR for best accuracy; radiation | **BEST CHOICE** | | **CUS + Follow-up** | Non-invasive, no contrast | Detects DVT in only ~50% of PE cases; misses isolated PE; indirect evidence only | Adjunct, not primary PE investigation | | **Pulmonary Angiography** | Highest sensitivity/specificity | Invasive, requires iodinated contrast (contraindicated here) | NOT suitable | | **MRPA** | No iodinated contrast | Gadolinium-based agents carry NSF risk in eGFR <60 (significant risk <30); limited availability; lower diagnostic accuracy | NOT preferred | **Warning:** Gadolinium-based contrast agents carry a risk of nephrogenic systemic fibrosis (NSF) in patients with eGFR <30 mL/min/1.73m², and caution is warranted between 30–60 mL/min/1.73m². This patient's eGFR of 35 places her in the caution zone, making MRPA a less safe and less validated option compared to V/Q scanning. ### Why CUS Alone is Insufficient **Clinical Pearl:** Compression ultrasound (CUS) of the lower limbs detects DVT in approximately **50% of confirmed PE cases**, meaning roughly half of all PEs are isolated (no detectable proximal DVT). A negative CUS therefore does **not** exclude PE and cannot serve as the primary investigation to confirm or exclude PE. CUS is a useful adjunct but not a substitute for direct pulmonary imaging. ### Algorithm When CTPA Contraindicated ``` Suspected PE + CTPA contraindicated (contrast allergy / renal impairment) ↓ V/Q Scan (first-line alternative) ↓ High probability → Treat for PE Normal → PE excluded Non-diagnostic → CUS ± clinical risk stratification ± repeat imaging ``` **Reference:** British Thoracic Society Guidelines for the investigation and management of acute PE (2023); PIOPED II study; Harrison's Principles of Internal Medicine, 21st edition — Chapter on Pulmonary Thromboembolism.