CDT test: what your value says about alcohol use

Transferrin is the protein that carries iron through your blood. It normally holds two carbohydrate chains. A breakdown product of alcohol disrupts how those chains attach. The result is carbohydrate-deficient transferrin, or CDT.

The laboratory reports the result as a percentage of total transferrin. That percentage climbs the more someone drinks. It reflects alcohol use across weeks, not yesterday.

The value only rises after at least a week of heavy drinking. In the literature that point sits around fifty to eighty grams of alcohol a day. That is roughly five to eight glasses a day. One standard glass counts as ten grams of alcohol in the Netherlands.

The Dutch guideline on alcohol misuse in driving fitness assessments gives two numbers. The upper limit of the normal reference interval is 1.7%. The cut-off used for assessment is 2.0%.

Above 2.0%, chronic heavy drinking is likely. Between 1.7% and 2.0% lies a grey zone. Measurement uncertainty is 0.3 percentage points. A raised CDT value just over the line therefore warrants a repeat.

These numbers belong to the standardised CDT-IFCC method. Some laboratories use CDT HPLC instead. So never place results from two methods side by side.

The CBR can require an assessment of your fitness to drive. This often follows a licence being withdrawn. That assessment includes laboratory testing for alcohol misuse.

The guideline names CDT as the most suitable parameter for it. Gamma-GT comes second. MCV, ASAT and ALAT were dropped in 2020. They proved insufficiently discriminating.

Please note: a test you order yourself is not a CBR assessment. The CBR appoints who carries the assessment out. Our result does not count toward it and does not replace it.

What you can use it for is tracking your own value. Some people do this during a period of abstinence over the longer term. You then know where you stand.

This measurement suits a question about a pattern, not about last night.

Consider it if you want to know how recent weeks read in measurable terms. Or if you are following a period of abstinence and want to see the decline.

If you stop drinking, the value usually returns to normal within two to three weeks. Anyone who drank heavily for years often needs longer. So CDT does fall during abstinence, just not at the same pace for everyone.

Want to know whether last night is still detectable? An EtG test suits that better.

The test is specific, but not infallible. The guideline reports a specificity of 91 to 96 percent. Sensitivity sits lower, between 36 and 51 percent.

That means two things. A raised result usually does have to do with alcohol. But a normal result does not rule heavy drinking out.

Other explanations are rare. One is an inherited fault in how sugars attach to proteins, known as CDG. Pregnancy, iron deficiency and liver disease can also disturb the measurement. In women and in obesity the test is less sensitive.

This measurement needs blood from a vein. The draw takes place at an affiliated collection point near you. You do not need to fast, and no referral is required.

The draw itself takes a few minutes. You receive the CDT percentage back, with the reference range alongside it.