Suspected pulmonary embolism: probability and diagnosis

01Purpose and principlesWhat the assessment is for and the core concepts behind it.

PE symptoms and routine tests are non-specific. The safe diagnostic method is sequential: stability, clinical probability, D-dimer only in the appropriate probability group, then definitive imaging.

D-dimer is a rule-out test in low-probability disease, not a confirmatory test. Infection, cancer, pregnancy, inflammation, age and recent surgery commonly elevate it.

Risk scores support rather than replace judgement. PERC applies only after the clinician has judged the pre-test probability low; the Wells score applies once PE is genuinely suspected.

Pregnancy and the puerperium require a pregnancy-specific diagnostic pathway and multidisciplinary imaging choice; the standard NICE NG158 pathway should not simply be copied across.

Key points

Think PE with otherwise unexplained acute dyspnoea, pleuritic chest pain, haemoptysis, syncope, tachycardia, hypoxaemia or signs of DVT; none is diagnostic in isolation.
First assess physiological stability and plausible alternatives. Obtain chest radiography to look for another cause, but a normal film neither proves nor excludes PE.
When overall clinical suspicion is low and another diagnosis is feasible, NICE says the 8-item PERC rule may be considered to decide whether further PE investigation is needed; do not use PERC in moderate or high suspicion.
If PE remains suspected, calculate the 2-level PE Wells score: more than 4 is PE likely; 4 or less is PE unlikely.
PE likely: arrange CTPA immediately, aiming for the result within 4 hours; give interim therapeutic anticoagulation if imaging cannot be completed within 4 hours and bleeding risk permits.
PE unlikely: obtain D-dimer within 4 hours; give interim therapeutic anticoagulation if the result will be delayed beyond 4 hours. A positive result leads to imaging.
Consider an age-adjusted D-dimer threshold for people over 50. Use the locally validated assay threshold rather than inventing a universal unit conversion.
Use V/Q SPECT, or planar V/Q if SPECT is unavailable, when contrast allergy, severe renal impairment or radiation considerations make CTPA unsuitable and the chest radiograph permits useful interpretation.
Obtain FBC, renal and liver function, PT and APTT before anticoagulation when possible, but NICE says not to wait for these results before starting indicated interim anticoagulation.
A negative CTPA ends the PE pathway in most patients, but assess for DVT if clinically suspected and actively pursue an alternative diagnosis rather than stopping at 'PE excluded'.

02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.

High-risk physiologyRed flag

Cardiac arrest, obstructive shock, persistent systolic hypotension, rising lactate, altered consciousness or rapidly escalating oxygen need suggests high-risk PE or another immediately reversible cause.

Typical but non-specific presentation

Sudden dyspnoea, pleuritic pain, tachypnoea and tachycardia are common; haemoptysis and syncope are less frequent but increase concern in the right context.

DVT evidence

Unilateral leg swelling, deep venous tenderness or a recently confirmed proximal DVT materially increases probability and may provide an alternative diagnostic route when chest imaging is unsafe.

Provoking context

Recent surgery or immobilisation, active cancer, oestrogen exposure, pregnancy/puerperium, previous VTE and thrombophilia change pre-test probability but do not diagnose PE.

Important mimicsRed flag

ACS, aortic syndrome, pneumothorax, pneumonia, asthma/COPD, acute heart failure, pericarditis, sepsis and panic symptoms can look similar; assess them in parallel when clinically plausible.

03Method and interpretationA systematic approach to the test and its findings.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
ABCDE observations and 12-lead ECGFirst step
Why
Identify instability and competing emergencies before probability scoring.
Interpretation and limitations
Tachycardia and right-heart-strain patterns support severity assessment but are neither sensitive nor specific; a normal ECG does not exclude PE.
02
Chest X-ray
Why
Seek pneumothorax, pneumonia, oedema or another explanation and judge whether V/Q imaging is likely to be interpretable.
Interpretation and limitations
A normal or non-specific film is common in PE; do not use it as a rule-out test.
03
2-level PE Wells score
Why
Separate PE likely from PE unlikely once PE is clinically suspected.
Interpretation and limitations
More than 4 points is PE likely and proceeds directly to imaging; 4 or less is PE unlikely and proceeds to D-dimer.
04
Quantitative D-dimer
Why
Exclude PE without imaging in the PE-unlikely group.
Interpretation and limitations
Below the assay's validated threshold rules out PE in the appropriate low-probability pathway; a positive result is non-specific and requires imaging. Consider age adjustment over 50.
05
CT pulmonary angiography
Why
Demonstrate or exclude pulmonary arterial filling defects and assess alternative thoracic pathology.
Interpretation and limitations
A positive scan confirms PE. Review clot distribution and right-heart features, but haemodynamic status—not anatomical clot size alone—defines high risk.
06
V/Q SPECT or planar V/Q
Why
Provide a non-iodinated alternative where CTPA is unsuitable.
Interpretation and limitations
A normal perfusion study effectively excludes clinically important PE; non-diagnostic results require further imaging or specialist resolution.
07
FBC, U&E/eGFR, LFT, PT and APTT
Why
Establish bleeding risk, organ function and a baseline for anticoagulant selection.
Interpretation and limitations
Correct major abnormalities and tailor treatment, but do not delay indicated interim anticoagulation while awaiting results.
08
Compression ultrasound of the leg
Why
Identify proximal DVT when leg symptoms are present or chest imaging is difficult.
Interpretation and limitations
A proximal DVT in a compatible presentation usually establishes VTE requiring treatment; a negative scan does not independently exclude PE.

04Clinical next stepsHow the result changes management or prompts escalation.

01Low initial suspicionUse PERC only in the correct populationFirst stepThe clinician judges overall PE probability low and another diagnosis is feasible.

1Confirm the patient is stable and that PE is not moderate/high probability by gestalt.
2Consider all 8 PERC items. If every item is negative, further PE testing may be unnecessary under NICE NG158.
3If any PERC item is positive—or PERC is not appropriate—move to the 2-level Wells pathway rather than ordering an unstructured D-dimer.

02PE unlikelyWells 4 or lessPE remains suspected and the 2-level Wells score is 4 or less.

1Request D-dimer with a result within 4 hours; consider the assay's validated age-adjusted threshold in people over 50.
2If the D-dimer result cannot be obtained within 4 hours, take baseline bloods and give interim therapeutic anticoagulation unless contraindicated.
3Negative D-dimer: stop interim anticoagulation, explain that PE is not identified, assess alternatives and give return advice. Positive D-dimer: arrange CTPA, or suitable V/Q imaging.

03PE likelyWells more than 4The 2-level Wells score is more than 4.

1Arrange immediate CTPA, aiming to complete and report it within 4 hours; do not insert a D-dimer step.
2If CTPA is delayed beyond 4 hours, obtain baseline bloods and start interim therapeutic anticoagulation if bleeding risk permits.
3If iodinated contrast is unsuitable, select V/Q SPECT or planar V/Q based on availability and chest radiograph; seek imaging/specialist advice where the result may be non-diagnostic.
4AlternativePositive imaging: enter the confirmed-PE treatment and severity pathway. Negative imaging: stop interim anticoagulation unless another indication exists, consider leg ultrasound if DVT is suspected, and pursue an alternative diagnosis.

04Unstable patientDiagnosis must run beside resuscitationShock, persistent hypotension, peri-arrest deterioration or arrest.

1Start ABCDE/ALS care, obtain senior critical-care and PE-team input, and correct hypoxaemia without delaying treatment of reversible causes.
2Use focused bedside echocardiography to look for acute RV failure and competing causes such as tamponade; obtain CTPA only if transfer is physiologically safe.
3If high-risk PE is the working diagnosis, use the emergency reperfusion pathway rather than waiting for routine probability tests or D-dimer.

05Procedure and medicine safetyRelevant preparation, treatment and contraindications.

Prevents thrombus propagation while definitive imaging is delayed in a patient whose pathway requires interim anticoagulation.

Interim therapeutic enoxaparin (example parenteral option)

1 mg/kg subcutaneously every 12 hours is a common treatment regimen for symptomatic PE; use actual product strength and adjust/choose an alternative for severe renal impairment.

Check bleeding, platelet count, renal function, recent neuraxial procedure and heparin-induced thrombocytopenia history. Exact LMWH selection and renal adjustment must follow the current product information.

Treats hypoxaemia while diagnosis and definitive therapy proceed; oxygen is not a treatment for the embolus itself.

Controlled oxygen

Titrate to SpO2 94–98% for most acutely ill adults, or 88–92% if at risk of hypercapnic respiratory failure pending blood gases.

Do not give routinely to a patient already within target. Severe hypoxaemia or shock warrants immediate senior airway/critical-care assessment.

06Risks, monitoring and follow-upComplications, safety checks and further assessment.

Repeat respiratory rate, SpO2, blood pressure, heart rate, mental state and perfusion while awaiting tests; a change in stability overrides the original diagnostic branch.
Watch for bleeding after interim anticoagulation and document the exact drug, dose and time so imaging and treatment teams can plan safely.
Track imaging and D-dimer deadlines explicitly; unowned results are a common source of missed PE and duplicated anticoagulation.
Reassess renal function and platelet count when anticoagulation continues, particularly with LMWH or evolving acute illness.
After negative testing, document the alternative diagnosis considered and give clear advice to return for worsening dyspnoea, syncope, haemoptysis or new leg swelling.

07Special situationsVariants, exceptions and circumstances that change the usual approach.

PERC comes before Wells only in genuinely low risk

PERC is not a substitute for Wells in a patient already judged to have meaningful PE probability; using it after deciding PE is likely creates false reassurance.

A positive D-dimer does not make PE likely

It only removes the rule-out shortcut. Probability and imaging still determine the diagnosis.

Anatomical burden is not haemodynamic category

A saddle embolus may be stable, while a smaller clot burden can produce shock in limited cardiopulmonary reserve. Treat physiology and RV failure, not the headline scan phrase.

Baseline tests guide safety, not permission

NICE specifically separates taking baseline bloods from waiting for their results when interim anticoagulation is indicated.

V/Q is an intentional choice

It is particularly useful when iodinated contrast is problematic, but abnormal lungs increase non-diagnostic studies; discuss the modality rather than ordering it reflexively.

08Common pitfallsFrequent interpretation and management errors.

01
Ordering D-dimer in a PE-likely patient and delaying definitive imaging.
02
Using PERC after deciding the patient has moderate or high clinical probability.
03
Treating tachycardia, S1Q3T3 or hypoxaemia as a diagnostic test for PE.
04
Waiting for baseline blood results before giving indicated interim anticoagulation.
05
Sending a shocked patient to CT without an airway, haemodynamic and reperfusion plan.
06
Stopping after a negative CTPA without considering DVT or a clinically important alternative diagnosis.