This chapter is written so that a patient or a relative can tell whether the right things were done, and so that a clinician can use it as a one-page reminder. The order matters: the score decides whether the D-dimer is useful, the D-dimer decides whether the scan is needed, and the clock decides whether treatment starts before the answer is in.
1. Before anything else: examination, history, chest X-ray
NICE's first instruction for anyone presenting with chest pain, breathlessness or coughing blood is a history, a physical examination and a chest X-ray, to look for the other causes — pneumonia, a collapsed lung, heart failure, a rib fracture. A normal X-ray in someone who is breathless and tachycardic raises the probability of PE rather than lowering it. An ECG is done at the same time: it is usually just fast, but it excludes a heart attack and occasionally shows the strain pattern of a large clot.
2. The Wells score
A structured estimate of probability, which NICE requires before any D-dimer is ordered. The two-level PE Wells score adds up the following:
| Feature | Points |
|---|---|
| Clinical signs and symptoms of DVT (leg swelling, pain on pressing the deep veins) | 3 |
| An alternative diagnosis is less likely than PE | 3 |
| Heart rate over 100 | 1.5 |
| Immobilisation for more than 3 days, or surgery in the previous 4 weeks | 1.5 |
| Previous DVT or PE | 1.5 |
| Coughing up blood | 1 |
| Cancer, treated within 6 months or palliative | 1 |
More than 4 points: PE likely. Go straight to the scan; no D-dimer, because a normal result would not be trusted at that probability. 4 points or fewer: PE unlikely. D-dimer first.
3. D-dimer: what it can and cannot do
D-dimer is a breakdown product of clot. A normal result in someone with an "unlikely" Wells score effectively rules out PE, and no scan is needed. A raised result means only that a clot is possible; it is also raised by infection, surgery, pregnancy, cancer, age and inflammation, so a positive test in a low-probability patient leads to a scan, not to a diagnosis.
- The result should be back within four hours. If it cannot be, NICE says start interim anticoagulation while waiting, in any setting including a GP surgery.
- Age-adjusted threshold. For people over 50, NICE says consider using age × 10 as the cut-off in ng/mL (a 72-year-old's threshold is 720 rather than 500). This reduces unnecessary scans in older people without missing more clots.
- Blood should be taken before any anticoagulant is given, because the drug lowers the D-dimer and can produce a false negative.
- A quantitative point-of-care test is acceptable if the laboratory cannot deliver in time.
4. The scan
CT pulmonary angiogram (CTPA) is the standard test: a CT of the chest timed to when the injected contrast fills the lung arteries, showing the clot directly. NICE says immediately if possible, and if it cannot be done immediately, interim anticoagulation while waiting. It also shows the alternative diagnoses and, in most reports, whether the right side of the heart is enlarged, which feeds the severity decision.
Ventilation-perfusion (V/Q) scanning is the alternative when CTPA is unsuitable: contrast allergy, severe kidney impairment (creatinine clearance below 30), or a high risk from radiation, which in practice means pregnancy and some younger women. SPECT V/Q is preferred over the older planar scan where it is available. It is less definitive when the lungs are already diseased.
Leg ultrasound is not a test for PE, but if the lung scan is negative and a DVT is suspected, NICE says consider a proximal leg vein ultrasound; a DVT found that way is treated the same as a PE would be.
In pregnancy NG158 does not apply. The Royal College of Obstetricians and Gynaecologists pathway uses a chest X-ray, then leg ultrasound if there are leg symptoms, then CTPA or V/Q; D-dimer is not used, because it is raised throughout pregnancy. The radiation to the baby from either scan is well below the level that causes harm, and the risk of an untreated PE is far higher.
5. The bloods, and what each one is for
| Test | Why it is done | When |
|---|---|---|
| Full blood count | Baseline before anticoagulation; anaemia or a low platelet count changes the plan | Before or with the first dose |
| Kidney function (creatinine, eGFR, and creatinine clearance for dosing) | Decides which anticoagulant and what dose; several are restricted below a clearance of 30 or 15 | Same |
| Liver function | Baseline; severe liver disease restricts the choice of drug | Same |
| Clotting screen (PT and APTT) | Baseline, and it can reveal an existing clotting problem | Same, before the first dose |
| Troponin | A marker of strain on the right heart; part of the severity grading, not the diagnosis | Once PE is confirmed or strongly suspected |
| BNP or NT-proBNP | The other strain marker; used with troponin to grade severity | Same |
| Lactate | A raised level means tissues are under-perfused: the "normotensive shock" the 2026 US guideline now names | In anyone unwell or borderline |
| Blood gas | Oxygen and carbon dioxide levels when saturations are low or breathing is laboured | If unwell |
NICE is explicit that treatment must not wait for these results. Baseline bloods are taken, the first dose is given, and the results are reviewed and acted on within 24 hours.
6. Echocardiogram
An ultrasound of the heart is not needed to diagnose a PE, but it is the best bedside test of how the right side of the heart is coping. It should be done in anyone who is unwell, borderline, or has raised troponin or BNP, because a dilated, struggling right ventricle moves a patient from "watch" to "consider reperfusion" in both UK and US guidance. In a collapsed patient too unstable for CT, a bedside echo showing a strained right heart is enough to justify clot-busting treatment.
7. Interim anticoagulation: the four-hour rule in one place
- Wells likely, scan not immediately available: start anticoagulation now.
- Wells unlikely, D-dimer not back within four hours: start anticoagulation now.
- Choose an anticoagulant that can simply continue if the diagnosis is confirmed — usually apixaban or rivaroxaban.
- Take the baseline bloods first, give the dose, review the bloods within 24 hours.
- If the scan is negative, stop the interim anticoagulant (unless the person was already on long-term anticoagulation for another reason).
8. What a negative result should come with
NICE asks that people told they are unlikely to have a PE are also told what symptoms to watch for and where to go if they develop them. If you are sent home with a negative D-dimer and nothing else, that is a gap.
9. Where UK and US guidance differ
The diagnostic pathway is the same in substance: pre-test probability, D-dimer with age adjustment, CTPA, V/Q as the alternative. The US 2026 guideline gives the same weight to echocardiography and adds lactate and other perfusion markers to the routine severity workup, so that a patient with normal blood pressure but early signs of shock is identified rather than filed as "stable". UK practice does the same in good units; it is simply not written into NG158.