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If you are before the police court (tribunal de police) judge for driving under the influence of alcohol, you will often hear the same question: was this a one-off, or is there a drinking problem? The answer determines whether the judge imposes an alcohol interlock (éthylotest antidémarrage), whether you are declared unfit to drive and how long the driving ban (déchéance du droit de conduire) will be. A mere statement will not convince any judge. Medical tests will, provided you choose the right ones.
This article complements our overview of drink driving.
When do you need such tests?
In three situations, evidence of your drinking habits can make a difference.
- A threatened alcohol interlock from 1.8 per mille. The alcohol interlock is then compulsory in principle, but the judge can depart from this if you show with concrete evidence that it was a one-off excess. See our article on the alcohol interlock.
- A threatened declaration of unfitness to drive under Article 42. If you show that there is no dependence and no problematic drinking, you can avoid this measure.
- A review after being declared unfit to drive. If you have been declared unfit to drive, you must prove that you are fit again. See our article on unfitness to drive.
Even outside these cases, the results can influence the sentence.
CDT and gamma-GT: the classic blood values
For a long time, courts accepted a series of regular CDT and gamma-GT blood tests as evidence. Some courts still do, but more and more consider them insufficient on their own.
CDT (Carbohydrate Deficient Transferrin) measures a change in a blood protein caused by prolonged, heavy drinking over the last two to three weeks. It is a specific but indirect marker: there is no direct link between the amount you drink and the value. CDT rises mainly with chronic drinking, for example more than 40 grams of alcohol a day for women or 60 grams for men, which is about a bottle of wine or one and a half litres of beer. With binge drinking, when someone drinks nothing during the week and a lot at the weekend, CDT barely rises.
Gamma-GT is a liver enzyme that rises when the liver is overloaded, in particular because of alcohol. This marker is less specific, as medication or liver disease can also raise the value.
After complete abstinence, CDT usually returns to normal within two to three weeks and gamma-GT within about a month. Two good values therefore do not rule out a problem, and judges know this. That is why they increasingly ask for one of the following tests.
The PEth test
A PEth (phosphatidylethanol) analysis measures average alcohol consumption over the last two to four weeks. The big difference with CDT: PEth is a direct marker. The substance only forms in red blood cells when you drink alcohol. If little or no PEth is found, you have drunk little or nothing.
| PEth value | What it means |
|---|---|
| Below 20 ng/ml | Abstinence or very occasional drinking |
| 20 to 200 ng/ml | Social or moderate drinking |
| Above 200 ng/ml | Excessive or chronic drinking |
PEth has a half-life of four to ten days, which makes it visible for longer than a breath test or CDT. It is a more objective measure than CDT, which more often gives false negatives.
Hair analysis
The breakdown products of alcohol reach the hair root through the blood and are stored in the hair shaft. As hair grows about 1 cm per month, an analysis can look back three to six months.
- An analysis needs at least 3 cm of hair, about the thickness of a pencil, cut close to the scalp.
- For a court case, looking back three months is generally enough.
- For a review after being declared unfit to drive, it is better to look back six months, which corresponds to the required period of abstinence.
Courts and the experts they appoint now often ask for a hair analysis combined with a PEth test. Together, they give the most reliable picture.
Which test should you choose?
| Situation | Recommended |
|---|---|
| Avoiding the alcohol interlock from 1.8 per mille | PEth test and hair analysis (three months), possibly supplemented by CDT and gamma-GT |
| Avoiding a declaration of unfitness to drive | PEth test and hair analysis, plus a report from your doctor or counsellor |
| Review after Article 42 | Hair analysis over six months and one or more PEth tests |
Have the tests carried out by an accredited laboratory and keep the full reports. Start in good time: a hair analysis only says something about a period if the hair grew during that period.
What the tests do not do
A good result does not prove that you were sober on the evening of the facts. For that, the breath analysis or the blood test from that evening is what counts. The tests do, however, show what your drinking habits look like in general. That is precisely what the judge wants to know when deciding on an alcohol interlock or unfitness to drive.
Frequently asked questions
What is a PEth test?
A PEth test is a blood test that measures phosphatidylethanol, a substance that only forms in red blood cells when you drink alcohol. The test gives a picture of your average alcohol consumption over the last two to four weeks. A value below 20 ng/ml indicates abstinence or very occasional drinking, and a value above 200 ng/ml indicates excessive drinking.
Does the judge still accept CDT and gamma-GT?
Some courts still accept them, but more and more consider them insufficient on their own. They are indirect markers that mainly detect chronic drinking and often miss binge drinking. That is why courts and experts increasingly ask for a PEth test and a hair analysis.
How far back does a hair analysis go?
About three to six months, depending on the length of the sample. Hair grows about 1 cm per month: 3 cm of hair therefore gives a picture of three months, and 6 cm of six months. At least 3 cm of hair is needed.
How long do I need to stop drinking to get a good PEth value?
PEth has a half-life of four to ten days and reflects consumption over the last two to four weeks. If you have only just stopped, your value will therefore remain high for some time. Allow at least a few weeks of abstinence for a value below 20 ng/ml, depending on how much you drank before.
Who pays for the tests?
Tests you have carried out yourself to support your defence are at your own expense. If the judge appoints an expert, those costs are treated as legal costs and, in principle, borne by the convicted person if there is a conviction.
Does a good test always mean avoiding the alcohol interlock?
No. From 1.8 per mille, the judge can only depart from it by giving express reasons, and freely assesses whether your documents are convincing. For a repeat offence with more than 1.2 per mille twice within three years, the alcohol interlock is always compulsory. Even the best test results then change nothing.
Have your case reviewed
Are you facing an alcohol interlock or a declaration of unfitness to drive? We look at which tests are the most convincing in your situation and when to have them done. Contact us.
Law as at 29 September 2026. We update this article when the rules change.
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