Drivers who have been caught drink driving will be required to complete a Carbohydrate Deficient Transferrin examination, also known as a CDT. The blood sample you provide will be passed to an independent lab who will then pass the results to the DVLA.

The DVLA will look at the information presented to them and make a decision on whether or not to grant the driver their licence back.

What is a CDT?

The CDT test is really quite scientific and looks at a deficiency of carbohydrate side chains on a transferrin.

Transferrin is a glycoprotein that transports iron within the blood. Amino acid binds iron to transferrin. On completion of this process, carbohydrate side-chains are added to the transferrin.

3 to 5 carbohydrate side-chains is what a normal transferrin is expected to have. For those who abuse alcohol, these side-chains drop to 0 – 2. This is why it is called a carbohydrate deficient transferrin. The transferrin is deficient of carbohydrate.

What cause them to be deficient?

When alcohol enters the blood stream, it starts to hinder the enzymes that regulate the side-chains. Alcohol actually encourages the enzyme that removes these side-chains whilst also supressing the enzyme that adds them.

How do drinkers and non-drinkers compare?

A non-drinker or low level drinker will have less than 1.6% of their transferrin carbohydrate deficient. A drinker however will have a much higher level of their blood in this state ranging from 3 to 10%.

What must my CDT levels be to pass?

The DVLA decide who may get their licence back by using a traffic light system.

Green (CDT Pass)

To pass, an individual must have a CDT of 2.1% or less. If this is the case, assuming their is nothing else to cause the DVLA concern, you are likely to get your licence back.

Amber (Further Investigation)

If an individual has a CDT of 2.2% to 2.9%, they will be placed in the amber bracket. This will flag to the DVLA and they are likely to investigate further before issuing a licence back.

An amber scores indicates a potential drinking problem.

Red (CDT Fail)

Those who have a CDT of 3% or more will be refused their drivers licence. These people are considered to higher risk to drive.

It is likely that when applying for drink drivers insurance, you will be required to provide your CDT results.

What else can effect CDT levels?

19 out of 20 times, alcohol is the reason for increased CDT levels. There are a few other reasons though that may cause an increase in CDT. These include carbohydrate deficient glycoprotein syndrome, advanced cholestic liver disease or transferrin variants.

If CDT levels are higher than they should be but not through alcohol use, then you will be required to prove why they are higher. This may be through either further examination by a doctor or additional medical tests.

An individual should also be aware that should a medical professional via examination prove that they regularly misuse alcohol, this will also qualify for refusing to give them their licence back.

Will binge drinking affect CDT?

In short, binge drinking can affect CDT levels depending on the frequency of binging. According to medical research, someone who consumes 14 to 21 cans of beer on 2 days of the week can increase their CDT levels to 1.5% to 2.1%.

Someone consuming the same amount of alcohol but only once in 2 weeks are unlikely to have an increased CDT level.

What level of alcohol will increase CDT?

On average, those consuming 60 to 80 grams of alcohol a day will increase their CDT levels. This works out to roughly 4 to 6 cans of beer a day.

Download the full guide by Dr Roy Sherwood to Carbohydrate Deficient Transferrin