Leg vein ultrasound: why you need a duplex scan and what to expect

2026-10-21

• A duplex scan is the main way to check leg veins: it shows the valves, blood flow and any clots.
• The test is painless, involves no radiation and is safe even in pregnancy.
• No special preparation is needed: comfortable clothes, no cream on your legs, previous reports with you.
• An ultrasound is essential before any varicose vein treatment, and a phlebologist interprets the report.
• With sudden swelling and pain in one leg, do not wait for a scheduled scan: call 103.

You go to the doctor because your legs feel heavy or a twisted vein has appeared on your calf, and the reply is: 'Let's start with an ultrasound.' Questions follow straight away. Will it hurt? Do you need to prepare, as for an abdominal scan? What do the red and blue patches on the screen mean, and how do you make sense of the jargon in the report?

The good news is that a leg vein ultrasound is one of the simplest and safest tests there is. Here is what it shows, who needs it and when, how it is done and what to do with the result.

What is a duplex scan, and how is it different from a 'Doppler'?

An ultrasound probe sends high-frequency sound waves into the tissues and picks up their echoes. The screen shows a black-and-white image in which the doctor can see the vein walls, the inside of the vein and its valves. At the same time, Doppler mode shows which way and how fast the blood is flowing. The combination of these two modes is called a duplex scan.

When colour mapping is added, the test is sometimes called a 'triplex' scan, but it is essentially the same modern duplex. The older Doppler-only test, which gives no picture of the vessel, provides much less information. Today, duplex scanning is the standard for vein disease.

By the way, red and blue on the screen do not mean 'artery' and 'vein'. The colour shows the direction of flow relative to the probe: towards it or away from it.

What does a leg vein ultrasound show?

The enlarged veins you can see under the skin are only the tip of the iceberg. The source of the problem is often deeper, in the large venous trunks that cannot be seen from outside. Ultrasound lets the doctor look there without a single incision and assess:

• whether the deep veins are open, and whether they contain any clots, recent or old;
• how the valves work, and whether there is reflux, meaning blood flowing back down for longer than normal;
• the great and small saphenous veins, the main surface trunks, and the points where they join the deep veins;
• perforator veins, the 'bridges' between the superficial and deep systems;
• the diameter and course of the veins, which matters when choosing treatment.

International guidelines on diagnosing chronic venous disease rely on duplex ultrasound findings. Ultrasound data are also part of the CEAP classification that doctors use to describe the stage of the disease. We explain it in our article on the CEAP classification in plain words.

Who needs a vein scan, and when?

You are usually referred by a phlebologist, surgeon or GP. Most often the test is needed if you notice:

• visible enlarged, twisted veins on your legs;
• ankle swelling by evening, heaviness, a bursting feeling or night cramps; read more about evening symptoms in our article on heavy legs in the evening;
• skin changes around the ankles: darkening, flaking, itching, hardening, or a healed or open ulcer;
• after-effects of a previous thrombosis or vein inflammation.

An ultrasound is essential before any varicose vein treatment, whether sclerotherapy, laser, radiofrequency treatment or surgery: without a 'map' of your veins, the doctor cannot plan the procedure. It is repeated after treatment to check the result.

For a few spider veins with no other symptoms, a scan is not always needed; your doctor will decide after an examination. You can read more in our article on spider veins on the legs. Having a scan 'just in case', without symptoms or signs of disease, is usually unnecessary.

Do you need to prepare for the test?

Unlike an abdominal ultrasound, no special preparation is needed: you can eat and drink as usual. But a few simple steps make the visit easier:

• wear clothes that are easy to remove from your legs, such as shorts under a dress or loose trousers;
• do not put cream or oil on your legs that day;
• bring any previous ultrasound reports, discharge notes from operations and a list of the medicines you take;
• if you wear compression stockings, you will be asked to take them off before the scan.

How a leg vein ultrasound works, step by step

1. You will be asked to undress down to your underwear. Most of the scan is done standing: when you are upright the veins fill with blood and the valves are easier to assess. Part of the examination may be done lying down.
2. A cool gel is applied to your skin so the probe makes good contact and the ultrasound passes through well.
3. The doctor moves the probe along your leg, from the groin crease down the thigh, behind the knee and along the calf to the ankle.
4. To test the valves, you may be asked to bear down as if blowing up a balloon, or the doctor may squeeze your calf several times. At that moment the machine records whether blood flows backwards.
5. To rule out a clot, the doctor presses gently on the vein with the probe: a healthy vein squashes flat completely.
6. Both legs are usually examined, even if only one is troubling you. The whole test takes about half an hour on average, longer if the picture is complicated.

The scan is painless. You may feel slight discomfort only when the probe presses in the groin, or if your leg is already sore. Ultrasound involves no radiation, so it can be repeated as often as needed, including during pregnancy.

How do you read a vein ultrasound report?

The report may contain terms that sound alarming at first. Here are the most common ones:

• 'Veins patent, fully compressible' means no clots were found.
• 'Reflux' or 'valve incompetence' means that blood flows back down for longer than it should when the vein is tested.
• 'GSV' and 'SSV' are the great and small saphenous veins, the main surface trunks of the leg.
• 'Saphenofemoral junction' is the point in the groin where the great saphenous vein joins the deep vein.
• 'Incompetent perforators' are connecting veins whose valves do not work properly.

Do not try to diagnose yourself from the report, and do not be frightened by the numbers. The ultrasound specialist describes what they see, while the conclusions and treatment plan come from a phlebologist, who compares the scan with your examination and symptoms. A great deal depends on the specialist's experience, so it is best to be examined where vein scans are done regularly.

How often should a leg vein ultrasound be repeated?

There is no single schedule. If your doctor found nothing serious and you feel the same, there is no need to repeat the scan every year without a reason. After treatment, a follow-up scan is arranged at intervals set by your doctor. If new symptoms appear, such as swelling, pain or skin changes, it is worth being checked sooner.

The main points

A leg vein ultrasound is a simple, safe way to see what is hidden under the skin and to choose treatment based on an accurate map of your veins rather than guesswork. If your legs have troubled you for a while, do not put off the test: half an hour in the doctor's room may answer questions you have been asking yourself for years.

Sources: 1. De Maeseneer MG, Kakkos SK, Aherne T, et al. 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. 2. National Institute for Health and Care Excellence (NICE). Varicose veins: diagnosis and management. Clinical guideline CG168, 2013. 3. Lurie F, Passman M, Meisner M, et al. The 2020 update of the CEAP classification system and reporting standards. J Vasc Surg Venous Lymphat Disord. 2020;8(3):342–352.

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