Suspected pulmonary embolism: choosing the right test
The case
A 52-year-old woman presents to the emergency department with a 6-hour history of acute dyspnoea and right-sided pleuritic chest pain that is worse on deep breathing. Earlier today she coughed up about 10 mL of bright red blood. Ten days ago she returned from a 14-hour flight from London to Sydney with minimal movement during the flight. She has no history of thromboembolism, recent surgery, cancer or leg swelling. Respiratory rate is 26/min, heart rate 112/min, blood pressure 128/78 mmHg and oxygen saturation 92% on room air. The lungs are clear, with tenderness over the right lower chest wall. ECG shows sinus tachycardia without right heart strain. What is the most appropriate next investigation?
Options
- ACT pulmonary angiography
- BQuantitative D-dimer assay
- CVentilation-perfusion scan
- DLower limb venous duplex ultrasound
- EBedside echocardiography
Think it through before you read on. Which single option is best, and why are the other four wrong?
Show the answer and explanation
Answer
A. CT pulmonary angiography
Why this is the right answer
The first step in suspected pulmonary embolism (PE) is to estimate the pre-test probability. Using the Wells score, this woman has heart rate above 100 (1.5), haemoptysis (1) and PE as the most likely diagnosis (3), giving at least 5.5 points, which is 'PE likely' [VERIFY]. Recent long-haul immobility adds to the clinical suspicion.
When the probability is high, D-dimer should not be used. D-dimer is a rule-out test for low-probability patients. In a high-probability patient a negative result cannot safely exclude PE and a positive result adds nothing, so the test only delays imaging.
CT pulmonary angiography (CTPA) is the standard first-line imaging in Australian emergency departments. It confirms or excludes PE directly, shows the clot burden, and can reveal alternative diagnoses such as pneumonia or pneumothorax. Therapeutic Guidelines and the Australian and New Zealand guidance on PE support this approach [VERIFY].
While waiting for the scan, a patient with high probability and no bleeding risk should be started on anticoagulation, because delay increases the risk of a second, larger embolus.
Ventilation-perfusion scanning is the alternative when CTPA is contraindicated: contrast allergy, significant kidney impairment, or pregnancy where breast radiation is a concern.
Why the other options are wrong
- B
D-dimer is only useful to rule out PE when the pre-test probability is low; here it is high.
- C
V/Q scan is second-line, reserved for contrast allergy, renal impairment or pregnancy, and needs a normal chest X-ray.
- D
Leg ultrasound can miss PE when there are no leg symptoms; a negative scan does not exclude it.
- E
Echocardiography helps risk-stratify a confirmed PE but cannot diagnose it directly.
High-yield takeaway
High Wells score means skip the D-dimer and go straight to CTPA, starting anticoagulation while you wait if bleeding risk is low.
Reference: Therapeutic Guidelines: Cardiovascular, Pulmonary embolism (2023)
Common questions
When is D-dimer useful in suspected PE?
Only when the pre-test probability is low. A negative result then safely rules out PE. It should not be ordered when the probability is high.
Should anticoagulation start before the CTPA result?
Yes, if the probability is high and the bleeding risk is low. Waiting for the scan risks a further embolus.
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