Most venous clots begin in the deep veins of the calf. Some stay there and dissolve on their own; some grow upward into the veins of the thigh and pelvis, from where pieces can travel to the lung. The signs are unremarkable individually and unmistakable together, which is why the missed ones are the ones with only one sign. This chapter is about telling a clot from the things that look like it, and about the clots that are not in the leg at all.
1. The leg
- Swelling of one calf, or the whole leg from the thigh down. Measure both calves ten centimetres below the bony point at the top of the shin: a difference of three centimetres or more is one of the formal criteria.
- Pain or tenderness along the line of the deep veins — the inside of the calf, behind the knee, the inside of the thigh — rather than in a muscle belly.
- Warmth and a reddish or dusky colour of the skin.
- Surface veins more prominent than on the other side, because blood is finding another way back.
- Pitting: a fingertip pressed into the swollen area leaves a dent.
It is usually one leg, usually over hours to a couple of days, and it often follows one of the triggers in section 4. About half of DVTs cause no symptoms, and the first sign is the pulmonary embolism. A normal-looking leg does not exclude one.
2. What else looks like a DVT
| Condition | How it differs | The catch |
|---|---|---|
| Torn calf muscle | Sudden pain during exertion, often a "shot in the calf"; bruising appears over days | The swelling can be identical, and a muscle tear is itself a risk factor for a clot in the days after |
| Cellulitis | Skin hot, red, sharply demarcated and spreading; fever; a break in the skin | The two coexist; a cellulitic leg with deep tenderness needs a scan |
| Ruptured Baker's cyst | A known cyst behind the knee; sudden calf pain and swelling | Indistinguishable without ultrasound, and the ultrasound shows both |
| Chronic venous insufficiency | Both legs, long-standing, worse at the end of the day, skin changes | A new one-sided change on top of it is a DVT until proved otherwise |
| Lymphoedema | Non-pitting, chronic, often after cancer surgery or radiotherapy | Cancer surgery is a clot risk factor too |
| Heart or kidney failure | Both legs, with breathlessness or known disease | An asymmetric worsening still needs a scan |
| Superficial thrombophlebitis | A tender cord along a surface vein, usually a varicose vein | See section 5: sometimes it is a DVT, and it always needs an ultrasound of the deep veins |
None of these should be diagnosed instead of a DVT without the score and the tests in the leg pathway. The ultrasound that shows a Baker's cyst or a muscle tear is the same ultrasound that excludes the clot.
3. When it is 999
Otherwise, a suspected DVT is a same-day problem: a GP, an urgent treatment centre or 111 to be directed to the local DVT service, which in most of the UK runs from the emergency department or an ambulatory clinic. It is not something to sleep on: the risk of a piece breaking off is highest in the first days.
4. Who is at risk
The same list as for a pulmonary embolism, because it is the same disease: surgery in the last three months (especially hip, knee, pelvic or abdominal), a hospital admission, a broken leg or a plaster cast, three or more days mostly in bed or a chair, cancer, pregnancy and the six weeks after birth, the combined pill or HRT, a previous clot or a first-degree relative with one, a journey of four hours or more spent sitting still by any means in the previous eight weeks, obesity, and a recent severe infection including COVID-19. Around a third of clots have no identifiable trigger.
5. The arm, the skin, the abdomen and the brain
The arm. Around one DVT in twenty is in an arm. The arm swells from the hand or forearm up, the hand feels heavy, the veins on the chest and shoulder stand out, and there is often a drip line, a pacemaker lead or a chemotherapy port in the vein above. The other pattern is a young, muscular person after heavy overhead effort with no line at all. Both need a scan.
The skin. Superficial thrombophlebitis is a clot in a vein you can see: a firm, tender, red cord along the course of a varicose vein or at an old drip site. It was long treated as a nuisance. It is now known that around one in five is associated with a DVT at diagnosis, and that a clot in the long vein of the thigh close to where it joins the deep system behaves like a DVT. Every thrombophlebitis of the leg above the knee should have a deep-vein ultrasound.
The abdomen. A clot in the veins draining the gut or liver causes abdominal pain, sometimes with bloating, vomiting or jaundice, and is found on a CT scan done for the pain. It is rare, and it needs a search for its cause.
The brain. A clot in the veins that drain the brain (cerebral venous thrombosis) is rare, affects younger adults and women more, and presents as a headache that does not behave: new, persistent, worsening over days, worse lying down or on straining, sometimes with blurred vision, seizures, weakness or drowsiness. It is often called migraine for several days. Any new severe or progressive headache with a neurological symptom, or in someone on the combined pill, pregnant or recently delivered, needs an emergency assessment and a scan of the veins.
Each of these has its own section in the unusual-sites chapter.
6. Where UK and US guidance differ
Not on recognition. The Wells score, the differential and the red flags are shared.