Why 0.5 Seconds? The Science Behind the Reflux Threshold Used to Diagnose Venous Insufficiency — and What It Does Not Tell You

When you are assessed for varicose veins, the doctor performs a duplex ultrasound and at some point says: "reflux duration is more than 0.5 seconds." In Korea, as in many countries, the insurer's review of a claim for treatment looks for the same figure. Patients reasonably ask where it comes from, and why half a second in particular.

This article traces the threshold to the studies that established it, sets out how accurately it performs, and then explains something those same studies make clear but which is often left out: 0.5 seconds tells you whether reflux exists. It does not tell you how bad it is.

Valves and reflux duration

Leg veins contain one-way valves that keep blood moving toward the heart. When a valve is competent it closes almost instantly against backward flow.

In a duplex examination, reflux is provoked — by squeezing and releasing the calf, by a standardised pneumatic cuff, or by a Valsalva manoeuvre — and the ultrasound measures how long blood flows backward before the valve closes and flow stops. That is the reflux duration, or valve closure time. A damaged valve closes late or not at all, and the backward flow lasts longer.

Where the threshold came from

1989: a reproducible way to measure it

Van Bemmelen and colleagues at the University of Washington, publishing in the Journal of Vascular Surgery in 1989, studied 192 deep vein segments in 32 healthy people and compared three ways of provoking reflux. They found that rapid deflation of a distal pneumatic cuff was the only method that gave quantitative, reproducible reflux durations at every level of the leg, and they published normal values for the deep veins: in the popliteal vein, median reflux duration was 0.19 seconds, with 95% of values under 0.66 seconds.

That paper did not itself define 0.5 seconds as the cut-off for the saphenous veins. What it did was establish the method by which such a cut-off could be measured — the necessary first step.

1992: 0.5 seconds against the reference standard

Masuda and Kistner, in the American Journal of Surgery in 1992, compared duplex reflux duration against descending venography, the invasive reference standard of the time, in 143 vein segments. A reflux duration greater than 0.5 seconds identified venographic reflux with sensitivity 90%, and a duration of 0.5 seconds or less identified competence with specificity 84% — overall accuracy 88%.

They added a caution that has stood the test of time: durations between 0.5 and 2.0 seconds fell into a grey zone, where venography sometimes showed a competent valve. The threshold performs well; it is not infallible near the boundary.

2003: the cut-offs, vein by vein

The definitive study is Labropoulos and colleagues, Journal of Vascular Surgery, 2003. They scanned 80 limbs in 40 healthy volunteers and 60 limbs in 45 patients, standing, at 16 sites per limb — 1,553 segments in the healthy group alone — using standardised cuff compression.

In healthy superficial veins, reflux lasted a mean of 210 ms and was under 500 ms in 96.7%. In the deep calf veins, under 500 ms in 97.6%. In the femoropopliteal veins, mean reflux was longer (390 ms) and under 990 ms in 99%. In perforating veins, outward flow was under 350 ms in 97%.

From this the modern cut-offs follow directly:

Vein	Abnormal reflux
Superficial veins (great and small saphenous)	> 0.5 s
Deep calf veins	> 0.5 s
Femoral and popliteal veins	> 1.0 s
Perforating veins	> 0.35 s

And a methodological point from the same paper: reflux testing should be performed with the patient standing. A supine examination underestimates reflux.

Why the deep veins get a longer allowance

The femoropopliteal veins are larger, carry more flow, and take longer to close even when healthy — Labropoulos measured a mean of 390 ms in normal subjects. Applying the superficial 0.5-second threshold to them would label a substantial fraction of healthy deep veins as refluxing. The 1.0-second threshold is not a relaxation of the standard; it is the value the normal data support.

Adoption into the international standards

The UIP consensus document on duplex investigation of the lower limb veins (Coleridge-Smith, Labropoulos, Partsch and colleagues, 2006) set out the agreed methodology for the examination and its interpretation. The Society for Vascular Surgery / American Venous Forum guideline (Gloviczki et al., 2011) recommends duplex scanning of the deep and superficial veins for every patient with varicose veins (GRADE 1A) and adopts the Labropoulos cut-offs — more than 500 ms in the superficial veins, more than 1 second in the femoral and popliteal veins — as the definition of pathological reflux. The CEAP classification, revised in 2004 and updated in 2020, is the descriptive framework into which the reflux finding is placed, under its "P" (pathophysiology) component.

In Korea, the domestic vascular and phlebology societies follow the same criteria, and the Health Insurance Review and Assessment Service uses reflux of 0.5 seconds or more on duplex as one of the core indicators of medical necessity for treatment. The insurer's number is the medical community's number.

What 0.5 seconds does not tell you

Here is the part that is rarely explained, and it comes from the same body of research.

A natural assumption is that longer reflux means worse disease — that 2 seconds is worse than 0.6, and 4 seconds worse still. The data do not support that. Neglén, Raju and colleagues analysed 244 refluxing limbs in 182 patients, comparing ultrasound reflux parameters against ambulatory venous pressure, air plethysmography and CEAP clinical class (Journal of Vascular Surgery, 2004). Their conclusion was unambiguous: the duration of reflux cannot be used to quantify severity, and is a purely qualitative measurement. Reflux time did not differ between limbs with skin changes or ulcers (C4–C6) and those without (C1–C3). What did track severity were the velocity and rate of reflux — how fast and how much blood flows backward, not for how long.

So the 0.5-second threshold answers one question well: is this valve incompetent? It does not answer how much does this matter? That comes from the whole examination — which veins reflux, over what length, at what velocity, whether the deep system is involved — together with the clinical picture. A patient with 0.7 seconds of great saphenous reflux and a venous ulcer is more unwell than a patient with 3 seconds and no symptoms.

This matters when reading an ultrasound report, and it matters when a clinic emphasises a large reflux time as though it were a severity score. It is not one.

In closing

The 0.5-second threshold is not an arbitrary hurdle set by insurers. It rests on a reproducible measurement method (1989), validation against the invasive reference standard (1992), and normal-value data from thousands of vein segments (2003), and it has been adopted by every major international guideline since. Half a second is where the normal valve closure time ends and pathological reflux begins.

Two things to take from the underlying science. Reflux should be tested standing. And reflux duration is a yes/no finding — the severity of your venous disease is judged from the whole examination and from your symptoms, not from that one number. If you have been diagnosed with varicose veins, ask not only whether reflux was found but where, how extensive it is, and what your CEAP class is. Those are the findings that determine what, if anything, should be done.

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References

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