A pulmonary embolism is easy to recognise when it looks like the textbook and easy to miss when it does not. The missed ones are the ones this chapter is for. The Healthcare Safety Investigation Branch reviewed delayed and missed PE diagnoses in English emergency departments in 2022 and found the same pattern each time: a plausible alternative explanation, accepted too early.
1. The typical picture
Breathlessness that arrived suddenly — over minutes, or waking with it — in someone who was fine the day before. A sharp chest or flank pain that is worse on breathing in, coughing or moving. A heart rate that stays fast at rest. Sometimes a cough with blood in the sputum. Sometimes a faint or near-faint. Roughly half of people have a leg sign: one calf swollen, warm, tender or aching, or a whole leg heavier than the other. When several of these arrive together nobody misses it.
2. The presentations that get missed
| How it presents | What it gets called | What should prompt the second look |
|---|---|---|
| Breathlessness on its own, gradual over days | Unfitness, asthma, chest infection, anxiety | No wheeze, no fever, normal chest X-ray, and it is new. A clear chest and a fast pulse is PE until proved otherwise. |
| A fast pulse and nothing else | Dehydration, anxiety, "just your heart rate" | Persistent tachycardia at rest with no explanation is a red flag on its own, especially after surgery or immobility. |
| Collapse or fainting with no warning | A simple faint, low blood pressure, a seizure | Syncope is the presenting feature in a meaningful minority of PEs, and in those it usually means a large clot. A faint plus breathlessness or a fast pulse afterwards needs a scan. |
| Pleuritic pain with a normal X-ray | Pulled muscle, costochondritis, pleurisy | Musculoskeletal pain is reproduced by pressing on the spot. PE pain usually is not. |
| Sudden anxiety and "air hunger" | Panic attack | A first panic attack in someone with no history, with a fast pulse and low oxygen reading, is the classic missed PE. |
| Low-grade fever and cough | Chest infection | Lung infarction can cause fever. Antibiotics that are not working after 48 hours, with a fast pulse, need a rethink. |
| New heart failure, or a known lung condition suddenly worse | The known disease | COPD and heart failure both raise PE risk, and a PE is a common cause of a "flare" that does not respond to the usual treatment. |
Oxygen readings can be normal. A pulse oximeter reading of 96% does not exclude a PE. Small clots, and even some large ones, leave the oxygen level untouched at rest. A normal reading is reassuring about the moment, not about the diagnosis.
3. Who is at higher risk
The risk factors are the ones that slow blood in the veins, damage the vein wall, or make blood clot more readily. Any of the following in the last three months makes the diagnosis more likely and should be mentioned at triage:
- Surgery, especially hip, knee, pelvic or abdominal, or any general anaesthetic
- A hospital admission, a broken leg, a plaster cast, or three or more days mostly in bed or a chair
- Cancer, active or under treatment
- Pregnancy and the six weeks after birth
- The combined contraceptive pill, HRT, or testosterone treatment
- A previous DVT or PE, or a first-degree relative with one
- A journey of four hours or more spent sitting still — by air, car, coach or train — in the previous eight weeks
- A recent COVID-19 infection, particularly one that needed hospital
None of these is needed for a PE to occur. Around one in three PEs has no identifiable provocation at all.
4. The leg: finding the source
Most PEs start as a DVT in the deep veins of the calf, thigh or pelvis. The signs are one-sided: swelling of the calf or the whole leg, warmth, tenderness along the inside of the calf or thigh, reddish or bluish skin, and veins that stand out more than on the other side. Measuring the calves ten centimetres below the bony point at the knee and finding a difference of three centimetres or more is one of the formal criteria doctors use. A DVT can also be silent, and about half of people with a PE have no leg symptoms at all, so a normal leg does not rule anything out.
DVT on its own, including the leg pathway, the treatment and the leg afterwards, is the blood clots topic.
5. When to call 999, and when to go to A&E
Go to A&E today (or ring 111 to be directed) for new breathlessness without those features, a one-sided swollen calf, or pleuritic chest pain that has not settled in a few hours. A PE is not a condition to sleep on. The scan takes twenty minutes and the treatment is a tablet; the cost of being wrong the other way is not symmetrical.
6. How a clinician rules it out without a scan: the PERC rule
Not everyone with chest pain and breathlessness needs a CT. NICE allows a clinician who thinks PE is unlikely on the overall picture to apply the pulmonary embolism rule-out criteria. If all eight are true, PE is unlikely enough that no further test is needed: age under 50, pulse under 100, oxygen saturation 95% or higher on air, no coughing of blood, no oestrogen use, no previous DVT or PE, no one-sided leg swelling, and no surgery or trauma needing hospital in the last four weeks. Fail any one and the clinician moves to the Wells score and D-dimer described in the hospital checklist. The rule is for low-suspicion patients only; it is not a way of avoiding a scan in someone who looks unwell.
7. Where UK and US guidance differ
Not here. The recognition, the risk factors, the PERC rule and the Wells score are shared across NICE NG158, the European Society of Cardiology and the 2026 American guideline. The differences start once the clot is confirmed and its severity has to be graded.