A letter from the CBR stating that your driving fitness is being investigated almost always causes alarm. Such an investigation often follows an arrest for driving under the influence. One blood value plays a leading role in it: the CDT value. Understanding what that value does and does not say helps you enter the process with a realistic picture.
What is CDT and why does the CBR look at it?
CDT stands for carbohydrate-deficient transferrin. It is a form of the iron-transport protein transferrin that occurs in raised amounts with prolonged, heavy alcohol use. Unlike a breath test, which only measures the most recent use, CDT gives a picture over weeks. That is why the CBR uses the CDT value in its investigation into driving fitness: it can be an indication of a pattern of excessive drinking, not of a single evening.
We think it is important to frame this soberly: a raised CDT value is a signal, not a verdict. The CBR weighs the result together with other data, such as a conversation with an examining doctor and possibly further investigation. A single number does not on its own determine whether you keep your licence.
What the CBR investigation involves
The process runs in steps. The table below gives an indication of the procedure and a rough timeline. The exact details and time limits can differ per situation; the CBR informs you about your specific process.
| Step | What happens | Indicative timing |
|---|---|---|
| 1. CBR notification | You receive notice that your driving fitness is being investigated | After a report, often by police or justice |
| 2. Fitness examination | Examination by an independent doctor, including blood testing (incl. CDT) | Within a few weeks of the notification |
| 3. Assessment of results | Doctor and CBR weigh CDT, liver values and the conversation together | Weeks after the examination |
| 4. Fitness decision | Fit, unfit, or fit under conditions | Follows the assessment |
| 5. Possible recovery or re-examination | If unfit, a follow-up process may follow | Time limit depends on the decision |
How long before CDT drops again?
CDT responds slowly, and that works both ways. After stopping or sharply reducing alcohol, the value declines gradually; it can take several weeks before a raised CDT value returns to the normal range. That makes sense, because the value reflects a pattern over time, not a snapshot. The Trimbos Institute, the Dutch knowledge centre for substance use, places alcohol firmly in the context of long-term health risks, and the WHO too stresses that there is no fully risk-free level of alcohol use.
CDT, EtG and blood: three windows side by side
CDT does not stand alone. For driving fitness and for personal checks, several alcohol markers are often used side by side, because each covers a different window. That distinction determines which marker answers your question.
- CDT in blood. Reflects a pattern of heavy use over several weeks, and is thus the marker the CBR uses to assess a drinking pattern.
- EtG, often in hair. Can look back much further, depending on hair length even months, giving a longer historical picture.
- Alcohol in blood or breath. Shows recent, current use and says nothing about the weeks before.
Anyone seeing a raised CDT value is therefore looking at weeks, not at last night. That is exactly why the CBR considers this marker suitable for distinguishing a structural pattern from a one-off slip.
An important nuance: CDT is not infallible
A raised CDT value can point to heavy alcohol use, but other causes exist too, such as certain rare liver conditions. That is why an examining doctor never looks at CDT alone, but combines it with other liver values and the clinical picture. A single deviant result is therefore not a final verdict, and interpretation should always be done by a doctor.
Measuring your CDT value yourself
If, apart from any official CBR process, you want to know for yourself how your CDT value stands, you can have it tested yourself, with only you seeing the result. The alcohol (CDT) test targets this marker specifically. Read more about proving alcohol in alcohol detection: what CDT, EtG and a blood test show, and on detection in how long is alcohol detectable and the EtG hair test.
A self-test is explicitly not a substitute for the official CBR investigation and has no formal status within it. Every result at Zuivertest includes a professional assessment from a BIG-registered doctor. A test can provide clarity, but for your licence process the CBR is decisive; for health questions, consult your GP.
Sources
- CBR, investigation into driving fitness and driving under the influence (cbr.nl)
- Trimbos Institute, alcohol and health risks (trimbos.nl)
- WHO, no risk-free level of alcohol use (who.int)
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Zuivertest
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