Synopsis
Select and interpret CTPA within a probability-based pulmonary embolism pathway, recognise technical and clinical limits and act safely on positive, negative or indeterminate results.
- Use the two-level PE Wells score before imaging: a score above 4 is PE likely and leads to immediate CTPA in an eligible stable adult.
- If CTPA cannot be performed immediately in a likely PE and bleeding risk permits, give interim therapeutic anticoagulation while definitive testing is arranged.
- After an adequate negative CTPA or V/Q study, consider proximal leg ultrasound if DVT is suspected; otherwise stop interim therapeutic anticoagulation without stopping separately indicated anticoagulation, pursue alternatives and give recurrence safety-net advice. Treat a technically inadequate study as indeterminate.
Key red flags
Haemodynamic instability, obstructive shock, cardiac arrest or rapidly worsening oxygenation with suspected PE requires a high-risk pathway rather than routine outpatient imaging.
A technically inadequate CTPA cannot exclude embolism; poor contrast timing, motion, low cardiac output and streak artefact must be stated and the diagnostic pathway continued.
An adequate negative CTPA or V/Q study enters the NICE negative-imaging branch; a technically inadequate study must be labelled indeterminate and cannot be used to exclude PE.
Contrast allergy, pregnancy and severe renal impairment alter modality choice and preparation; they do not justify abandoning investigation of a consequential PE.
Right-heart strain on CT can identify adverse risk but is not specific to acute PE and should be integrated with physiology, biomarkers and echocardiography.
Right ventricular enlargement, septal flattening and contrast reflux support pressure overload but can be chronic or caused by other pulmonary vascular disease.
Poor arterial enhancement, respiratory motion or streak artefact can make a scan indeterminate; the report must name the limitation and next action.
Investigation priorities
Confirm or exclude pulmonary arterial emboli and identify right-heart effects or an alternative thoracic diagnosis.
Management branches
A haemodynamically stable adult has suspected PE and a two-level Wells score above 4.
- Check physiological stability, bleeding risk, renal function when available, prior contrast reaction, pregnancy possibility and whether CTPA can be performed immediately.
- Offer immediate CTPA; if it is delayed, give interim therapeutic anticoagulation when appropriate rather than leaving a likely PE untreated.
A stable patient needs definitive PE imaging but has severe iodinated-contrast allergy or severe renal impairment.