Alcohol breath test: how it works and how reliable it is

A breath test measures alcohol in the air you exhale and converts that into a blood alcohol level. You blow into a mouthpiece for about 20 seconds. What the device cannot do is look back at last night, or at your drinking pattern over recent weeks.

That distinction causes most of the confusion we see. People reach for a breath test to answer a question the device cannot answer.

Below you can read how the measurement works, how accurate it is, and where the limits of this instrument lie.

How does a breath test work?

Alcohol enters your blood through the stomach lining and the intestine. When that blood passes through the lungs, a small part of the alcohol evaporates into the air sacs. A breath test measures that concentration in your exhaled air and converts it using a fixed ratio.

That fixed ratio is also the weak point. It holds on average, not for you personally.

Your breathing, body temperature and the moment of measurement all shift the result. If you blow shortly after your last sip, the device also picks up alcohol left in your mouth and reads too high. That is why police and occupational health services keep a waiting period of about fifteen minutes.

How reliable is a breath test?

That depends on the device, not on the method. Police equipment is calibrated periodically and has a small margin of error. Breath testers from a chemist or the internet are not calibrated, and their reading can be off in either direction.

A reading that is too low is the real risk here.

Anyone who trusts a cheap device and decides they can drive is basing that on an uncalibrated number. If you doubt a police measurement, you have the right to a blood test in the Netherlands. That result takes precedence over the breath reading.

What does a breath test not see?

A breath test only sees alcohol that is in your blood right now. Once the alcohol is broken down, the device reads zero, however much you drank yesterday. So the measurement says nothing about a pattern over weeks.

Picture this: you finish your last glass of wine at 11pm and blow into a device at 7.30am before work. The device will most likely read zero. An EtG urine test that same morning can still come back positive, because it measures a breakdown product rather than alcohol itself.

For those questions a lab looks at different substances. CDT and gamma-GT in blood respond to a pattern lasting weeks. How these markers relate to each other is set out in our overview of proving alcohol use.

I would put it this way: breath measures now, blood and hair measure history.

Breath test, police analysis or blood test: which fits which question?

Each instrument answers one question well. The choice follows from what you want to know: your state right now, or your use over a longer period. The table below sets them side by side.

If you want to know whether your CDT value falls within the reference range, an alcohol CDT test can quantify that.

How long does alcohol stay measurable in your breath?

Your body breaks alcohol down at a fairly fixed rate, roughly 0.1 to 0.15 per mille per hour. One standard drink raises the level by about 0.2 to 0.3. In most adults that drink is no longer measurable in breath after two to three hours.

There is no way to speed that up.

Coffee, a cold shower and fresh air change nothing about the breakdown rate. At best they make you more alert, which is not the same as sober.

What counts as one glass differs per drink. You can read about that in what a standard drink is. How long alcohol stays visible in blood, urine and hair is covered in how long alcohol is detectable.

Where do the limits for alcohol use lie?

The advice from the Dutch Health Council is short: any amount of alcohol can be harmful, so drinking no alcohol is healthier and safer. The RIVM counts men who drink more than 21 glasses a week, and women who drink more than 14, as excessive drinkers.

A breath test says nothing about those limits. It measures a moment, not a week.

So my advice stays simple. Use a breath test for the question of the moment, and not as proof about yesterday or about the past month.

If you want to know what your blood values show across a longer period, discuss that with your GP or have your CDT value measured.

Every blood test result at Zuivertest includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

Sources

• Gezondheidsraad, advice on alcohol: any amount of alcohol can be harmful (accessed July 2026).
• RIVM, Leefstijlmonitor, alcohol use among adults, 2025 figures.
• Thuisarts.nl, "Ik drink vaak alcohol" (accessed July 2026).