A 42-year-old woman on oral contraceptives presents with acute dyspnea and pleuritic chest pain. Vital signs: HR 102/min, RR 24/min, BP 128/82 mmHg, SpO₂ 94% on room air. Chest X-ray is normal. D-dimer is elevated at 850 ng/mL. Wells score is 4.5 (intermediate probability). Which is the most appropriate next investigation?

## Investigation Strategy for Intermediate-Probability PE **Key Point:** In intermediate-probability PE (Wells 4–6), CTPA is the recommended next investigation regardless of D-dimer status, because clinical probability alone guides imaging decisions [cite:Harrison 21e Ch 297]. ### Clinical Decision-Making Algorithm ```mermaid flowchart TD A[Suspected PE]:::outcome --> B{Calculate Wells Score}:::decision B -->|Low: 0-2| C{D-dimer}:::decision B -->|Intermediate: 4-6| D[CTPA]:::action B -->|High: >6| E[CTPA]:::action C -->|Negative| F[PE excluded]:::outcome C -->|Positive| G[CTPA]:::action D --> H{CTPA Result}:::decision E --> H G --> H H -->|Positive| I[Anticoagulation]:::action H -->|Negative| J[PE excluded]:::outcome ``` ### Why CTPA for Intermediate Wells Score 1. **Wells score 4.5 = intermediate probability**: Requires imaging regardless of D-dimer. 2. **D-dimer elevated**: Confirms need for imaging (D-dimer is sensitive but not specific). 3. **Normal CXR**: Favors CTPA over V/Q scan (V/Q is more specific with normal baseline CXR, but CTPA remains first-line in intermediate probability). 4. **Fast turnaround**: CTPA can be performed urgently and guides immediate management decisions. **High-Yield:** The Wells score stratification rule: - **Low Wells (0–2) + negative D-dimer** → PE excluded, no imaging needed. - **Low Wells (0–2) + positive D-dimer** → CTPA. - **Intermediate Wells (4–6)** → CTPA (regardless of D-dimer). - **High Wells (>6)** → CTPA (regardless of D-dimer). ### Comparison: CTPA vs V/Q in This Context | Feature | CTPA | V/Q Scan | |---|---|---| | **Normal CXR** | Excellent (high specificity) | Also excellent | | **Intermediate Wells** | First-line | Alternative if CTPA contraindicated | | **Speed** | Fast (5–10 min) | Slower, requires nuclear medicine | | **Availability** | Widely available | Limited availability | | **Radiation dose** | Moderate | Low | **Clinical Pearl:** V/Q scan is preferred in pregnancy or severe renal impairment; otherwise, CTPA is standard in intermediate-probability cases. ### Why Other Options Are Incorrect - **V/Q scan**: While valid in low-risk patients with normal CXR, CTPA is preferred for intermediate Wells score. - **Repeat D-dimer in 1 week**: Inappropriate; intermediate Wells score mandates imaging now, not serial D-dimer. - **Empiric anticoagulation**: Unjustified without confirmatory imaging; exposes patient to bleeding risk without diagnosis.