Vein Conditions & Symptoms

DVT: The Blood Clot Everyone Fears

What it is, who gets it, what really matters

By Dr Johan Blignaut, Specialist Vein Surgeon · Vein Centres South Africa · Updated September 2026

Few diagnoses frighten patients quite like a blood clot in the leg. Most people know someone who has had one, or have heard of a traveller who collapsed after a long flight. Deep vein thrombosis (DVT) deserves to be taken seriously — but a great deal of the fear around it comes from not knowing which risks are real, which symptoms matter, and what modern treatment actually involves.

The deep dive

What a DVT actually is

Your leg has two sets of veins. The superficial veins run just under the skin — these are the veins that become varicose. The deep veins run within the muscles alongside the arteries, and carry most of the blood from the leg back to the heart.

A deep vein thrombosis is a blood clot that forms in one of those deep veins. Most begin in the calf. Some stay there; others extend upwards into the veins behind the knee and in the thigh.

Doctors describe DVTs by where they sit, because it changes the risk:

• Distal DVT — confined to the calf veins. Lower risk of travelling to the lungs, and some can be safely watched with repeat scans rather than treated.³
• Proximal DVT — in the vein behind the knee or higher. These are the clots most likely to cause a pulmonary embolism, and they are always treated.^1,2

DVT and pulmonary embolism are really two faces of the same disease, which is why you will often see them grouped together as venous thromboembolism (VTE). Overall, roughly 1 to 2 in every 1 000 adults develop VTE each year, with the risk rising steeply with age.⁵

How is that different from superficial phlebitis?

A clot in a surface vein — superficial thrombophlebitis — causes a tender, red, hard cord under the skin, often along a varicose vein. It is usually far less dangerous than a DVT. But it is not always harmless: when it lies close to where a surface vein joins the deep system in the groin or behind the knee, it can extend into the deep vein. European guidelines recommend that a superficial clot within about 3 cm of that junction is treated with full-dose blood thinners, as for a DVT.¹ That is why a superficial clot should be scanned rather than simply assumed to be minor.

Why clots form in the first place

Blood is designed to clot when a vessel is injured and to stay fluid everywhere else. A DVT happens when that balance tips. Three things push it over, and most clots involve more than one of them:

• Slow blood flow. When the calf muscles are not working — after surgery, during illness, in a plaster cast, or on a long journey — blood pools in the deep veins.
• Vein wall injury. Surgery, fractures and other trauma damage the lining of the vein.
• “Stickier” blood. Cancer, pregnancy, oestrogen-containing hormones, inflammation and some inherited conditions make blood clot more readily.

This is not merely an academic point. It explains why the size and duration of the trigger matter so much — and why a long flight in an otherwise healthy person is a far smaller risk than a hip replacement.

Who actually gets DVT?

Risk factors are not all equal. The distinction that matters most — for your risk and for your treatment later — is between a clot that had a clear trigger and one that did not.

Strong, temporary triggers

• major surgery, particularly hip, knee and abdominal or pelvic operations
• a hospital admission with an acute illness
• a leg injury or fracture, especially with a plaster cast or boot
• pregnancy, and above all the first weeks after giving birth

Persistent risk factors

• active cancer and some cancer treatments
• a previous DVT or pulmonary embolism
• certain inherited or acquired clotting disorders
• inflammatory conditions such as inflammatory bowel disease

Smaller contributors

• increasing age and obesity
• oestrogen-containing contraception or oral hormone replacement therapy
• long-distance travel
• varicose veins, which are regarded as a minor risk factor¹⁵

A clot linked to a strong temporary trigger is called provoked. A clot that appears without any identifiable trigger — or with only a very minor one — is called unprovoked. Keep that distinction in mind; it shapes almost every decision about treatment that follows.

The symptoms — and why they can mislead

The classic features of a DVT are:

• swelling of one leg — the calf, or the whole leg
• pain or tenderness, often described as a cramp or a pulled muscle in the calf
• warmth over the painful area
• redness or a darker discolouration of the skin
• surface veins that stand out more than usual

Two things make DVT tricky. First, a substantial number of clots cause few or no symptoms. Second, most painful, swollen calves are not clots — they turn out to be muscle strains, a ruptured Baker's cyst behind the knee, cellulitis, or the swelling of other conditions I have written about separately. Swelling of both legs is less typical of DVT, although it does not rule it out.

This is why DVT cannot be diagnosed or excluded by examination alone.

The warning signs of a pulmonary embolism

A pulmonary embolism may be the first sign of a clot, even without leg symptoms. Treat any of the following as an emergency:

• sudden or unexplained shortness of breath
• chest pain that is sharper when you breathe in
• coughing up blood
• a fast heartbeat
• light-headedness, fainting or collapse

How a DVT is diagnosed

Guidelines recommend a structured approach rather than guesswork.^1,4

1. How likely is a clot?

Your doctor first estimates the probability using a validated scoring system (the Wells score is the best known), based on your symptoms, examination and risk factors.

2. A D-dimer blood test

D-dimer is a breakdown product of clots. A normal result in someone with a low probability makes a DVT very unlikely. A raised result, however, is not proof of a clot — D-dimer also rises after surgery, during pregnancy, with infection and with age — so on its own it cannot make the diagnosis.

3. Venous duplex ultrasound

Ultrasound is the first-line test for suspected DVT.¹ It is painless, uses no radiation and shows directly whether a deep vein is blocked. A healthy vein flattens when the probe presses on it; a vein filled with clot does not.

How DVT is treated today

The treatment of DVT has changed considerably over the past decade. For most patients it is now simpler, safer and often done without a hospital admission.

Blood thinners are the treatment

The medical term is anticoagulants. They do not dissolve the clot directly. Instead, they stop it growing and prevent new clots forming, while your body gradually breaks the existing clot down. The main options are:

• Modern tablet blood thinners — known as DOACs (direct oral anticoagulants). Current guidelines prefer these for most patients, because they are at least as effective as older treatment and do not require regular blood tests.^2–4
• Heparin injections, given under the skin. These are preferred during pregnancy and breastfeeding, and in some patients with cancer, kidney problems or other specific circumstances.
• The older tablet blood thinner that needs regular INR blood tests to adjust the dose. It is still the right choice for some people — for example with certain mechanical heart valves or specific clotting disorders.

Which blood thinner is right for you depends on your kidney function, other medicines, bleeding risk, pregnancy plans and personal circumstances.

Most people can be treated at home

Both American and European guidelines favour treating uncomplicated DVT at home rather than in hospital.^1,2 Admission is reserved for patients with a large clot causing severe symptoms, a significant pulmonary embolism, a high bleeding risk, or other medical problems that need monitoring.

Walking is encouraged

Bed rest used to be standard advice. It is not any more. Once blood thinners have been started, walking does not increase the risk of the clot travelling to the lungs, and there is no benefit in staying in bed.²⁰

How long does treatment last?

Almost everyone with a proximal DVT is treated for at least three months.^2,3 This is the “active treatment” phase. After three to six months, your doctor reassesses and makes a decision that has two sides: the risk of another clot if treatment stops, against the risk of bleeding if it continues.

• If the clot was provoked by a clear, temporary trigger — surgery, a fracture, a plaster cast — the chance of it recurring is low once the trigger has passed, and treatment usually stops.^2,3
• If the clot was unprovoked, or a strong risk factor such as active cancer persists, the risk of another clot is substantially higher. After stopping treatment for a first unprovoked clot, about 1 in 10 people have a recurrence within a year and about 1 in 4 within five years.¹⁸ For these patients, guidelines generally suggest continuing treatment long term, with regular review.^2,3

A useful recent development concerns the dose. When long-term treatment is chosen, the tablet can often be continued at a reduced dose after the first six months. In the RENOVE trial, published in 2025, patients at high risk of recurrence who took a reduced dose had a similarly low rate of new clots to those on the full dose — about 2% over the study period in both groups, although the trial could not formally prove the two were equivalent — and significantly fewer bleeding problems.¹⁰ That makes long-term treatment a more comfortable decision for many people.

What about calf clots?

A clot confined to the calf veins, without severe symptoms or risk factors for extension, does not always need blood thinners. The CHEST guideline suggests that in selected patients, a repeat ultrasound over the following two weeks is a reasonable alternative — treatment is started only if the clot extends.³ Many patients with calf clots are still treated, particularly if symptoms are significant or the risk of extension is higher. This is a decision to make with your doctor, not a reason to skip the scan.

Clot removal procedures and filters

Most DVTs are treated with blood thinners alone. Procedures to break up or remove the clot through a catheter are reserved for selected patients — typically younger people with a very extensive clot in the pelvic and thigh veins and severe symptoms. In the large ATTRACT trial, adding a clot-busting catheter procedure to blood thinners did not reduce post-thrombotic syndrome overall, although it did ease early symptoms and may reduce its more severe forms in carefully chosen patients — at the cost of more bleeding.⁸ The 2025 European Society of Vascular Medicine guidelines recommend that these procedures are performed by experienced teams, and advise against routinely inserting a filter into the main abdominal vein.⁹ A filter is considered only when blood thinners cannot be used, or when clots continue despite adequate treatment.

Long-term leg damage: post-thrombotic syndrome

A pulmonary embolism is the acute danger of DVT. The long-term concern is the leg itself.

As a clot is broken down, it can scar the vein and damage its valves, leaving the vein partly blocked or leaking backwards. Between 20% and 50% of patients develop some degree of post-thrombotic syndrome after a DVT, usually within the first two years.⁶ Symptoms include:

• aching, heaviness and swelling, worse at the end of the day
• cramps and itching
• brown pigmentation and hardening of the skin around the ankle
• in severe cases, a venous leg ulcer

The risk is higher after an extensive clot in the thigh or pelvic veins, a recurrent clot in the same leg, and when the first months of treatment are poorly controlled.

Do compression stockings prevent it?

This is one of the areas where advice has changed. The large SOX trial found that wearing compression stockings routinely after a DVT did not prevent post-thrombotic syndrome,⁷ and the CHEST guideline no longer recommends them for that purpose.³ Stockings do, however, remain very useful for relieving swelling and heaviness — so if they make your leg feel better, they are worth wearing. More on this on our stockings and conservative treatment page.

When the damage is significant

Patients with persistent symptoms after a DVT benefit from a formal venous duplex assessment of both the deep and superficial veins. Sometimes the problem is a narrowing that remains in the pelvic veins, which in selected patients can be treated by opening it with a balloon and stent. Sometimes superficial reflux is adding to the burden and can be treated. And sometimes — importantly — a surface vein is acting as a bypass around a blocked deep vein and should be left alone. This is exactly the kind of situation where treatment should follow a proper map of the veins, not the appearance of the leg.

Flights and long-distance travel

“Economy class syndrome” is probably the most feared cause of DVT, and the most overstated. Long-distance travel is a genuine but weak risk factor. The risk rises with longer journeys and with other risk factors present, and falls back over the weeks after the journey.

The American Society of Hematology guidance on journeys longer than about four hours is clear:¹¹

• If you have no particular risk factors, compression stockings and preventive blood thinners are not recommended.
• If your risk is substantially increased — for example after recent surgery, with a previous clot, active cancer, in the weeks after giving birth, or with two or more risk factors together (such as pregnancy, obesity or oestrogen-containing hormones) — compression stockings or a preventive blood-thinning injection are suggested.

For everyone, sensible habits help: an aisle seat, getting up to walk when you can, calf exercises while seated (flexing the ankles up and down), staying well hydrated, and avoiding sleeping for long periods in a cramped position after alcohol or sleeping tablets.

The pill, pregnancy and hormone therapy

Oestrogen increases the tendency of blood to clot, and this is where some of the most misunderstood risk numbers live. Seeing them side by side helps.

The combined pill, patch and ring

Around 2 in every 10 000 women not using hormonal contraception develop a clot each year. On a combined pill, that rises to about 5–7 per 10 000 with the lower-risk types, and about 9–12 per 10 000 with some of the others; for the contraceptive patch and ring the figure is about 6–12.¹² The risk is highest in the first year of use, and when restarting after a break of four weeks or more. Progestogen-only pills, the hormonal coil and non-hormonal methods do not appear to carry the same increase, although the progestogen injection may raise it.¹⁹ If you have other risk factors, your prescriber can guide you towards a lower-risk option.

Pregnancy and after birth

Pregnancy raises clot risk several-fold, and the first weeks after delivery raise it much further.¹³ The risk is higher again after a caesarean section. Women with a previous clot or other risk factors are often advised to have preventive heparin injections during pregnancy and for about six weeks afterwards. A DVT during pregnancy is treated with heparin injections, which do not cross to the baby; the newer tablet blood thinners (DOACs) are avoided in pregnancy and while breastfeeding.¹

Hormone replacement therapy (HRT)

Oral HRT is associated with a modest increase in clot risk. Oestrogen absorbed through the skin — patches, gels and sprays — does not appear to carry the same increase, according to a large UK study of over 80 000 women with VTE.¹⁴ If you have had a clot or have other risk factors and need HRT, this is worth discussing with your doctor.

After a DVT, oestrogen-containing contraception is generally avoided. Your doctor can advise on the safest alternatives and on hormone treatment while you are taking blood thinners.

Clots after varicose vein treatment

This is a question I am asked almost every week, and it is a reasonable one. Any procedure on the veins carries some risk of a clot.

With modern endovenous treatment — such as radiofrequency ablation — that risk is low. In pooled data from studies of heat-based ablation of the main thigh vein (the great saphenous vein), quoted in the European venous guidelines, a clot extending from the treated vein into the deep vein at the junction occurred in about 1.4% of patients, a DVT in about 0.3%, and a pulmonary embolism in about 0.1%.^15,21

You may see the term EHIT (endothermal heat-induced thrombosis) — or its newer name, ablation-related thrombus extension. It describes a clot at the point where the sealed vein meets the deep vein. It is specific to vein ablation, is generally considered less dangerous than a typical DVT, and small extensions usually resolve with observation or a short course of treatment.^15,16

How the risk is managed

• Assessing each patient's clot risk before treatment. Guidelines recommend routine risk stratification rather than giving every patient blood thinners.^15,16 In our practice, every patient is assessed before their procedure — with particular attention to a previous clot, a known clotting disorder and the size of the vein being treated — and preventive blood thinners are given to those whose risk warrants it.
• Walking straight after the procedure. Endovenous treatment is done under local anaesthetic, and you walk out of the room.
• Scanning when symptoms appear. Guidelines differ on routine check scans. European guidelines suggest a scan one to four weeks after treatment may be considered, while American guidelines recommend against routine early scanning after heat-based ablation in average-risk patients who feel well.^15,16 We scan promptly if you develop new calf pain, significant swelling or any breathing symptoms.

If you are planning treatment and have had a clot before, tell us at your consultation. It does not usually rule treatment out, but it does change the plan.

Should I be tested for a clotting disorder?

Many patients expect a blood test for inherited clotting conditions after a DVT. For most people it is not needed. Current American Society of Hematology guidance suggests testing only in selected situations — for example, when a clot was triggered by hormones or a minor temporary factor and the result would change the decision about long-term treatment, or when there is a known inherited clotting disorder in the family and hormones or pregnancy are being considered. It suggests against testing after a clot that followed surgery.¹⁷ In most cases, whether the clot was provoked or unprovoked already tells us what we need to know.

Reducing your risk

• Keep moving. Your calf muscles are your circulation's second heart. Break up long periods of sitting.
• Before surgery, ask about clot prevention. Hospitals assess this routinely; make sure you understand your plan, especially if you go home early.
• If your leg is in a cast or boot, ask whether you need preventive injections.
• Mention a personal or family history of clots to anyone prescribing hormones, and to your surgeon before any operation.
• Maintain a healthy weight and don't smoke — both matter for your veins and far beyond.

So, what really matters?

Understanding your own risk — rather than fearing the worst — is what allows sensible decisions: when to seek help urgently, when a long flight needs a plan, and how to approach vein treatment safely.

For a shorter overview of symptoms, causes and treatment, see our deep vein thrombosis (DVT) condition page.

Frequently asked questions

References

1. Kakkos SK, Gohel M, Baekgaard N, et al. European Society for Vascular Surgery (ESVS) 2021 Clinical Practice Guidelines on the Management of Venous Thrombosis. Eur J Vasc Endovasc Surg. 2021;61(1):9–82. doi:10.1016/j.ejvs.2020.09.023
2. Ortel TL, Neumann I, Ageno W, et al. American Society of Hematology 2020 guidelines for management of venous thromboembolism: treatment of deep vein thrombosis and pulmonary embolism. Blood Adv. 2020;4(19):4693–4738. doi:10.1182/bloodadvances.2020001830
3. Stevens SM, Woller SC, Baumann Kreuziger L, et al. Antithrombotic Therapy for VTE Disease: Second Update of the CHEST Guideline and Expert Panel Report. Chest. 2021;160(6):e545–e608. doi:10.1016/j.chest.2021.07.055
4. National Institute for Health and Care Excellence. Venous thromboembolic diseases: diagnosis, management and thrombophilia testing (NG158). London: NICE; 2020 (last updated 2 August 2023).
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7. Kahn SR, Shapiro S, Wells PS, et al. Compression stockings to prevent post-thrombotic syndrome: a randomised placebo-controlled trial. Lancet. 2014;383(9920):880–888. doi:10.1016/S0140-6736(13)61902-9
8. Vedantham S, Goldhaber SZ, Julian JA, et al. Pharmacomechanical catheter-directed thrombolysis for deep-vein thrombosis. N Engl J Med. 2017;377(23):2240–2252. doi:10.1056/NEJMoa1615066
9. Schlager O, Campello E, Madaric J, et al. 2025 ESVM Guidelines on interventional treatment of venous thromboembolism. Vasa. 2025;54(6):365–381. doi:10.1024/0301-1526/a001211
10. Couturaud F, Schmidt J, Sanchez O, et al. Extended treatment of venous thromboembolism with reduced-dose versus full-dose direct oral anticoagulants in patients at high risk of recurrence (RENOVE): a non-inferiority, multicentre, randomised, open-label, blinded endpoint trial. Lancet. 2025;405(10480):725–735. doi:10.1016/S0140-6736(24)02842-3
11. Schünemann HJ, Cushman M, Burnett AE, et al. American Society of Hematology 2018 guidelines for management of venous thromboembolism: prophylaxis for hospitalized and nonhospitalized medical patients. Blood Adv. 2018;2(22):3198–3225. doi:10.1182/bloodadvances.2018022954
12. European Medicines Agency. Combined hormonal contraceptives: Article 31 referral — benefits continue to outweigh risks. London: EMA; 2014.
13. Practice Committee of the American Society for Reproductive Medicine. Combined hormonal contraception and the risk of venous thromboembolism: a guideline. Fertil Steril. 2017;107(1):43–51. doi:10.1016/j.fertnstert.2016.09.027
14. Vinogradova Y, Coupland C, Hippisley-Cox J. Use of hormone replacement therapy and risk of venous thromboembolism: nested case-control studies using the QResearch and CPRD databases. BMJ. 2019;364:k4810. doi:10.1136/bmj.k4810
15. De Maeseneer MG, Kakkos SK, Aherne T, et al. European Society for Vascular Surgery (ESVS) 2022 Clinical Practice Guidelines on the Management of Chronic Venous Disease of the Lower Limbs. Eur J Vasc Endovasc Surg. 2022;63(2):184–267. doi:10.1016/j.ejvs.2021.12.024
16. Gloviczki P, Lawrence PF, Wasan SM, et al. The 2023 Society for Vascular Surgery, American Venous Forum, and American Vein and Lymphatic Society clinical practice guidelines for the management of varicose veins of the lower extremities. Part II. J Vasc Surg Venous Lymphat Disord. 2024;12(1):101670. doi:10.1016/j.jvsv.2023.08.011
17. Middeldorp S, Nieuwlaat R, Baumann Kreuziger L, et al. American Society of Hematology 2023 guidelines for management of venous thromboembolism: thrombophilia testing. Blood Adv. 2023;7(22):7101–7138. doi:10.1182/bloodadvances.2023010177
18. Khan F, Rahman A, Carrier M, et al. Long term risk of symptomatic recurrent venous thromboembolism after discontinuation of anticoagulant treatment for first unprovoked venous thromboembolism event: systematic review and meta-analysis. BMJ. 2019;366:l4363. doi:10.1136/bmj.l4363
19. Mantha S, Karp R, Raghavan V, et al. Assessing the risk of venous thromboembolic events in women taking progestin-only contraception: a meta-analysis. BMJ. 2012;345:e4944. doi:10.1136/bmj.e4944
20. Aissaoui N, Martins E, Mouly S, et al. A meta-analysis of bed rest versus early ambulation in the management of pulmonary embolism, deep vein thrombosis, or both. Int J Cardiol. 2009;137(1):37–41. doi:10.1016/j.ijcard.2008.06.020
21. Healy DA, Kimura S, Power D, et al. A systematic review and meta-analysis of thrombotic events following endovenous thermal ablation of the great saphenous vein. Eur J Vasc Endovasc Surg. 2018;56(3):410–424. doi:10.1016/j.ejvs.2018.05.008

Guidance reflects the major international guidelines and trials available in September 2026. The European Society for Vascular Surgery's venous thrombosis guideline is being updated for 2027, and this article will be revised when it is published.