Understanding your duplex ultrasound: what the vein scan actually shows

Patients often arrive in my clinic holding a scan report full of phrases like “the great saphenous vein is incompetent, with reflux of 2.4 seconds.” Understandably, it reads as bad news in a foreign language.

The duplex ultrasound is the single most important test in vein care — every sensible treatment decision flows from it — so it is worth understanding what it actually measures. In my practice the duplex scan happens during the consultation and I interpret it there and then, which means the person reading the images is the same person examining your leg and, if needed, treating it.

What “duplex” actually means

Duplex simply means the machine does two things at once. The first is ordinary ultrasound imaging: sound waves build a live picture of the vein — its walls, its diameter, whether it contains any clot. The second is Doppler, which measures movement: it detects blood flow, its speed, and crucially its direction, usually displayed as colour on the screen. (The red and blue you may see on the monitor indicate direction of flow, not arteries versus veins — a common misunderstanding.)

Put together, duplex shows not just what the veins look like but how they are behaving, in real time. There is no radiation and there are no needles or injections — just gel and a hand-held probe moved along the leg.

How we catch a leaking valve in the act

Healthy leg veins contain one-way valves, spaced along their length, that let blood travel up towards the heart and snap shut to stop it falling back. The scan is designed to test those valves under realistic conditions — which is why much of it is done with you standing, since gravity is the whole point.

The test itself is simple: the calf is squeezed, or you are asked to strain gently, to push blood up the vein — then we watch what happens when the push stops. A healthy valve closes in a fraction of a second. If the Doppler shows blood flowing backwards — downwards — for longer than about half a second, that valve has failed. This backward flow is called reflux, and the duration you see quoted on the report is the valve closure time: how long blood kept falling before the flow stopped.

So a phrase like “reflux of 2.4 seconds” means blood was pouring back down that vein for over two seconds after each squeeze. The number is a measurement of a leak, nothing more mysterious than that.

What “the great saphenous vein is incompetent” actually means

The great saphenous vein is the longest superficial vein in the body, running from the inside of the ankle up the inner leg to the groin, where it joins the deep system. “Incompetent” is the traditional medical word for a vein whose valves no longer keep blood moving one way. It is a statement about the valves — not a judgement on the vein’s character.

Why does it matter? Because an incompetent saphenous vein carries a standing column of blood pressing downwards all day, and that pressure inflates the smaller branch veins beneath the skin. Those branches are the visible, bulging varicose veins. In other words, what you can see is usually the downstream consequence; the leak sits higher up, hidden from view. Left alone for years, the same sustained pressure can produce the swelling and skin changes of chronic venous insufficiency.

The scan also checks the deep veins — the main drainage system inside the muscle — for both blockage and reflux, because the state of the deep system shapes what treatment is appropriate.

Why the map decides which treatment fits

By the end of the scan I have a map: which veins leak, where the reflux starts and stops, the diameter of each vein, how straight or winding its course is, and how close it lies to the skin. That map, more than anything else, determines the treatment plan.

A fairly straight saphenous vein of suitable size is typically well suited to endovenous ablation, which seals it from within using heat, or to medical adhesive closure, which seals it with a purpose-designed glue and avoids the need for anaesthetic fluid along the vein. A very winding vein, or scattered surface branches, may point instead towards foam injections or removal of the bulging segments through tiny incisions. And if the deep system is obstructed, the plan changes altogether.

This is also why treating only the visible veins, without closing the leaking source above them, so often ends in recurrence — the tap was left running. A careful scan is what makes treatment targeted rather than cosmetic guesswork.

What to expect on the day

The scan takes roughly twenty to thirty minutes. You will stand for part of it, feel the probe and some calf squeezes, and walk out immediately afterwards — there is nothing to recover from. Because the scan is done during the consultation itself, we look at the findings together on the screen and discuss the options in the same visit, rather than sending you away to wait for a report.

If you have bulging veins with aching or heaviness, one-sided ankle swelling, itching or darkening skin at the lower calf, or a wound near the ankle that is slow to heal, a duplex scan is the logical next step — and sooner is better than later. Vein disease progresses slowly, and a leg mapped early is a leg with simpler options.