Quick answer: if clinical suspicion of pulmonary embolism (PE) is low, apply the PERC rule; if every criterion is absent, no further testing is needed. Otherwise calculate the two-level PE Wells score. A score above 4 means PE is likely: arrange an immediate CTPA, with interim anticoagulation if the scan is delayed. A score of 4 or less means PE is unlikely: send a D-dimer, with the result available within 4 hours, and scan only if it is positive. Haemodynamically unstable PE is treated with unfractionated heparin and consideration of thrombolysis.
PE is a diagnosis that is easy to miss and easy to over-investigate. MRCEM questions usually test which rule to apply, in which order, and when a D-dimer is the wrong test. This guide follows NICE guideline NG158. In practice, always follow your local pathway.
Step 1: is PE a real possibility?
Take a history, examine the patient and request a chest X-ray and ECG to look for other causes. The classic S1Q3T3 pattern is uncommon. Sinus tachycardia is the most frequent ECG finding.
Step 2: the PERC rule
Use the pulmonary embolism rule-out criteria (PERC) only when your clinical impression is that PE is unlikely, with an estimated probability below 15%. PE is ruled out without a D-dimer if all of the following are true:
- Age under 50
- Heart rate under 100 beats per minute
- Oxygen saturation 95% or above on air
- No haemoptysis
- No oestrogen use
- No previous DVT or PE
- No unilateral leg swelling
- No surgery or trauma needing hospital admission in the past 4 weeks
If any criterion is present, move on to the Wells score.
Step 3: the two-level PE Wells score
| Clinical feature | Points |
|---|---|
| Clinical signs and symptoms of DVT (leg swelling and pain on palpation of the deep veins) | 3 |
| An alternative diagnosis is less likely than PE | 3 |
| Heart rate over 100 beats per minute | 1.5 |
| Immobilisation for more than 3 days or surgery in the previous 4 weeks | 1.5 |
| Previous DVT or PE | 1.5 |
| Haemoptysis | 1 |
| Malignancy (on treatment, treated in the last 6 months, or palliative) | 1 |
More than 4 points: PE likely. 4 points or fewer: PE unlikely.
Step 4: test according to the score
| Wells result | Next step |
|---|---|
| PE likely (over 4) | Immediate CTPA. If it cannot be done straight away, give interim anticoagulation and arrange the scan. Do not send a D-dimer |
| PE unlikely (4 or less) | D-dimer with the result within 4 hours. If that is not possible, give interim anticoagulation while waiting |
| D-dimer negative | PE excluded. Stop interim anticoagulation and look for another diagnosis |
| D-dimer positive | Immediate CTPA, or interim anticoagulation while awaiting it |
- Age-adjusted D-dimer: consider it for patients over 50. The usual threshold is age x 10 micrograms/L.
- When CTPA is not suitable: use a V/Q SPECT or planar scan for patients with contrast allergy, severe renal impairment or a high risk from radiation.
- CTPA negative but DVT suspected: arrange a proximal leg vein ultrasound.
Treatment
- Interim anticoagulation: choose a drug that can be continued if PE is confirmed, usually apixaban or rivaroxaban.
- Confirmed PE: offer apixaban or rivaroxaban first line, for at least 3 months.
- Haemodynamically unstable PE: continuous unfractionated heparin infusion and consider systemic thrombolysis, for example alteplase. In cardiac arrest from suspected PE, give thrombolysis and continue CPR for 60 to 90 minutes.
- Low-risk PE: consider outpatient treatment using a validated tool such as PESI, simplified PESI or the Hestia criteria.
Special situations
- Pregnancy: D-dimer and the Wells score are unreliable. Start low molecular weight heparin and investigate with imaging, beginning with leg ultrasound if there are signs of DVT.
- Renal impairment, cancer and antiphospholipid syndrome: anticoagulant choice differs, so check the guideline.
Common exam traps
- Using PERC in the wrong patient: it only applies when clinical suspicion is already low.
- Sending a D-dimer when PE is likely: a negative result does not exclude PE in a high-probability patient.
- Treating a positive D-dimer as a diagnosis: it is sensitive but not specific. It is raised in infection, cancer, pregnancy, after surgery and with age.
- Delaying anticoagulation while waiting for a scan: give interim treatment if imaging is not immediate.
- Thrombolysing a stable patient: thrombolysis is for haemodynamic instability, not for right heart strain alone.
- Being reassured by a normal ECG or chest X-ray: both are often normal in PE.
Summary
- Low suspicion: apply PERC. All eight criteria absent means no further testing.
- Otherwise use the two-level Wells score. Over 4 is likely; 4 or less is unlikely.
- PE likely: immediate CTPA, with no D-dimer.
- PE unlikely: D-dimer within 4 hours, then CTPA only if positive.
- Treat confirmed PE with apixaban or rivaroxaban for at least 3 months.
- Unstable PE: unfractionated heparin and consider thrombolysis.
Frequently asked questions
What Wells score makes PE likely?
More than 4 points on the two-level PE Wells score. Four points or fewer means PE is unlikely.
When can the PERC rule be used?
Only when clinical suspicion is low, with an estimated probability under 15%. All eight criteria must be absent.
Should a D-dimer be sent if the Wells score is over 4?
No. Go straight to CTPA, with interim anticoagulation if the scan is delayed.
What is the age-adjusted D-dimer threshold?
For patients over 50, the threshold is usually the patient’s age multiplied by 10 micrograms/L.
Next steps
Test yourself with our 20 free MRCEM SBA sample questions. For more, explore our Respiratory system resources and the Respiratory System course, or see the full question bank and mock tests in our membership plans.
Clinical content checked against NICE guideline NG158 (venous thromboembolic diseases, updated 2023) on 5 October 2026. This article is for exam preparation. In clinical practice, always follow your local pathway.