CDT blood testing

Gail Evans

By Gail Evans, Technical Trainer at AlphaBiolabs
Last reviewed: 29/04/2026

Here, we discuss CDT blood testing and how it works.

Read the full article below.

At a glance

  • A carbohydrate deficient transferrin (CDT) test is a blood alcohol test used to establish the percentage of transferrin in the bloodstream that is carbohydrate deficient.
  • Alcohol consumption can significantly impact the levels of CDT in the body, meaning laboratory analysis can be used to measure these levels and draw a conclusion about alcohol use over a defined period.
  • There are many circumstances where a CDT test might be used, including by the DVLA when assessing fitness to drive following a drink driving conviction, and in safety and welfare cases, child custody and access disputes, and during recovery from alcohol misuse or dependency.
  • A CDT test can be used to detect chronic and excessive alcohol consumption for approximately four weeks prior to the blood sample being collected.

Table of contents

What is a blood alcohol test?

A blood alcohol test is a form of alcohol testing that uses a blood sample to determine whether a person has been drinking alcohol chronically and excessively over a defined period.

According to the World Health Organisation (WHO), if a person consumes more than 60g of alcohol (7.5 units) daily over several months, this is deemed to be chronic and excessive consumption.

There are several types of blood test for alcohol, each of which offers a different insight into a person’s alcohol consumption, depending on their level of use and the effects of this alcohol use on the body.

Find more about our Blood Alcohol Testing

What is a Carbohydrate Deficient Transferrin (CDT) blood test?

A Carbohydrate Deficient Transferrin test, also known as a CDT blood alcohol test, is used to establish the percentage of transferrin in the bloodstream that is carbohydrate deficient.

Transferrin is a protein largely made in the liver that regulates the absorption of iron into the blood, and transports iron to the parts of the body that need it.

Alcohol consumption can significantly impact the levels of carbohydrate deficient transferrin in the body, meaning laboratory analysis can be used to measure these levels and draw a conclusion about alcohol use over a defined period.

People who do not drink, or drink moderately, will have lower CDT levels in their blood. However, people who consume 50-80 grams of alcohol at least five days a week for two weeks prior to a CDT test will have significantly higher levels, meaning that CDT testing is a good indicator of excessive alcohol use.

If a person stops drinking, their CDT levels will usually return to normal levels within four weeks.

How does a CDT alcohol test work?

A CDT blood alcohol test requires a blood sample to be collected, usually from the donor’s arm.

This sample is then analysed at the laboratory, to determine the percentage of carbohydrate deficient transferrin present in the blood.

Depending on the CDT levels present, this can indicate that the donor has been drinking alcohol excessively in the four-week period prior to the blood sample being collected.

People who do not drink, or drink moderately, will have lower CDT levels in their blood.

People who consume 50-80 grams of alcohol at least five days a week for two weeks prior to a CDT test will have significantly higher levels of CDT in their blood.

When is CDT alcohol testing used?

There are many circumstances where a CDT alcohol test might be used, including:

Assessing fitness to drive

For anyone convicted of a drink driving offence and considered a high-risk offender, the Driver and Vehicle Licensing Agency (DVLA) will assess their fitness to drive before returning their licence following a disqualification.

The DVLA medical assessment is performed by a DVLA appointed doctor and will usually include a blood test to assess CDT levels in the blood, a brief physical examination, and a medical interview.

The results of a CDT test for the DVLA are assessed using a traffic light system of CDT cut-off levels. These levels vary and will be assessed depending on the driver’s circumstances i.e. whether they have a history of alcohol dependency or misuse, or no history of alcohol dependency or misuse.

In some cases, the driver might want to instruct a private CDT blood test – such as that offered by AlphaBiolabs – ahead of the DVLA blood test, for added reassurance that they are likely to pass the official test.

Order a CDT blood test for DVLA

You can find out more about the DVLA medical by visiting https://www.gov.uk/driving-disqualifications/disqualification-for-drink-driving.

Safety and welfare cases

Where the safety and welfare of a child or vulnerable person is of concern, due to a parent or guardian’s alcohol misuse or dependency, local authorities or family courts might use CDT alcohol testing – alongside other assessments and alcohol tests including blood, hair, nail, and breath testing – to gain insight into a person’s historic alcohol consumption.

This means that CDT blood testing can play an important role in allowing authorities to make decisions surrounding a child or vulnerable person’s care.

Child custody and access disputes

For private legal matters where custody and/or access to children is in dispute, and a parent/guardian has a history of alcohol misuse or dependence, the court might order a parent or guardian to undergo routine alcohol tests including CDT blood alcohol testing.

This is often used alongside other assessments, and the judge in the case will carefully consider the results of any alcohol tests alongside other relevant evidence when making decisions about child custody and/or access.

Recovery from alcohol misuse/dependency

Some recovery specialists will use CDT blood testing, alongside other alcohol tests, to establish a baseline for their client’s misuse or dependency.

This can then be considered when recommending a suitable treatment plan for the patient. Ongoing CDT blood alcohol tests can also be used to determine whether a patient has been successful in reducing their alcohol consumption during treatment.

How accurate is a CDT blood test?

CDT blood testing is an extremely accurate form of testing that can be used to detect heavy drinking and is second only to Phosphatidylethanol (PEth) testing for the insight it provides into a person’s historic alcohol consumption.

Drinking alcohol to excess over a prolonged period is often the leading cause of elevated CDT levels in the blood.

However, whereas PEth is a direct biomarker of alcohol, meaning it is only present in the body when alcohol has been consumed, CDT is an indirect biomarker.

This means that in addition to excess alcohol consumption, it is possible for other factors to affect a person’s CDT levels, including liver disease, certain medications, and other underlying health conditions.

How far back can a CDT test detect alcohol?

A CDT blood alcohol test can be used to detect chronic and excessive alcohol consumption for approximately four weeks prior to the blood sample being collected.

What affects the results of a CDT blood test?

Although elevated levels of CDT are suggestive of excess alcohol consumption/misuse, there are other factors that can affect the results of a CDT alcohol test.

These include:

  • Certain liver disorders/diseases
  • Hormones
  • Biological factors such as genetic variations
  • Pregnancy – CDT testing cannot be carried out during pregnancy or for two months after birth
  • Iron deficiency

Where can I get a CDT test?

Whether you are a member of the public who needs a peace of mind CDT test before your DVLA fitness-to-drive medical, or a privately-instructed legal test for other official matters (e.g. child custody disputes), AlphaBiolabs can help.

Established in 2004, we have extensive experience providing award-winning, accredited alcohol testing – including CDT blood alcohol tests – for members of the public, the legal sector, and the workplace sector.

Our toxicology laboratory is equipped with state-of-the-art technology and uses the very latest scientific methods, so you can be confident of receiving accurate, reliable results.

It’s easy to order a peace of mind CDT Blood Test online now. Alternatively, call our friendly and discreet Customer Services team on 0333 600 1300 or email info@alphabiolabs.com to discuss your requirements.

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Gail Evans, AlphaBiolabs

Gail Evans

Technical Trainer at AlphaBiolabs

A professionally-trained forensic scientist, Gail joined AlphaBiolabs in 2012 and holds the role of Technical Trainer.

Her day-to-day responsibilities include delivering in-depth training sessions both internally and externally, covering DNA, drug, and alcohol testing.

Throughout her career at AlphaBiolabs, Gail has held a variety of roles, including within the Legal and Workplace sectors of the business.

Before joining the company, Gail was a practicing forensic scientist with 25 years’ experience working for the Forensic Science Service, attending scenes of crime, and analysing physical and biological material with potential evidential value.

20 thoughts on “CDT blood testing”

  1. I have my results form DVLA they say my Which did not look good.. I had a blood test done CDT test . That said I do not meet with the medical standard to drive.. I am on medication for for my arm which might be factor ? For my resort. I have been drinking in the past not so much now .I drink very little now .Any way here from you soon..

    Reply

    1. Hi Allan. It is highly unlikely that medication would impact a CDT result. Whilst there are some genetic reasons or certain diseases that could potentially impact a CDT result, scientific studies show that 95% of raised CDT results are due to alcohol consumption.

      Reply

  2. Hi, I have Fatty liver disease and my Gamma GT is high. I am not drinking now a days. Will the test still shows elevated CDT levels?

    Reply

    1. Hi Jon. If the bile ducts are obstructed or damaged by disease removal of CDT from the blood is reduced leading to a raised CDT level. This can be excluded in the case in question by the normal biochemical liver tests as the disease needs to be severe for there to be an effect on CDT. A medical review with a practitioner to diagnose a condition of the bile ducts could be undertaken if a raised CDT is obtained. Alternatively, a PEth test can be undertaken at the same time as the CDT test to demonstrate the CDT is not elevated due to excessive alcohol use.

      Reply

  3. Hi, I have had 2 CDT tests one was 0.6 and one was 0.7. I had abstained from alcohol for a longer period for the 0.7 result. Thus does this mean there is a natural ‘score’ that I would obtain regardless of alcohol consumption.
    Also would abstinence of 3 months result in a normal reading (the key factor is to abstain for longer than 4 weeks)?

    Thanks

    Reply

    1. Hi Pete

      Consumption of alcohol leads to an increased production of transferrin molecules that are deficient in carbohydrates, hence the term carbohydrate-deficient transferrin (CDT). People who consume little or no alcohol will have a CDT value less than 1.6%; however, due to biological variability in the creation and elimination of CDT in one individual (excluding pre-menopausal women), testing at different time periods will provide slightly different test results, but all will be below the threshold of 1.6%.

      As a minimum of one to two weeks of sustained heavy alcohol drinking is needed to increase CDT above the cut-off value, in addition to the half-life of transferrin in blood of 7 to 14 days, it provides an indication of alcohol consumption over the two to four weeks prior to the collection only. Therefore, the key period covered by this test is the approximate one-month period prior to the sample collection.

      Reply

  4. Good afternoon,
    I am giving blood tomorrow and have my CDT test next week, will this affect the result?
    Kind Regards
    Paula

    Reply

  5. Will one binge drink of 5 pints having not drank for the previous 6 weeks raise CDT level for a test in 11 days?

    Reply

    1. The Society of Hair Testing (SoHT) definition of chronic excessive alcohol drinking corresponds to an average consumption of 60 grams of pure ethanol (alcohol) per day, over several months (this equates to 7.5 units of alcohol, or approximately 3 pints of beer or 2 to 3 large glasses of wine per day, over a prolonged period of time).

      In relation to your question – we are unable to say for certain as multiple factors must be considered and we have limited details provided. That being said it is unlikely that a single isolated drinking session of this level would raise a CDT level when the donor has been abstinent otherwise over the last 6 weeks.

      Reply

  6. my name is Tayo,
    Hi my name is Tayo, i have just receive my dvla medical test letter inviting me to book for a cdt test.
    I do drink even on till the date i receive the letter, i will like to confirm with you if staying completely out of alcohol for exactly minimum of 4 weeks before going for the DVLA CDT examination appointment, meaning before collecting my blood for the CDT Test, will this be your advise to prevent failing my DVLA CDT Medical Examination Alcohol Test? or i can take just light drink as long i dont exceed approved WHO weekly suggested alcohol unit consumption.
    Thanks

    Reply

  7. Hi – I have a CDT test for the DVLA later today. I last consumed more than the recommended daily amount on 19th December 2024. I’ve since then consumed around 12 Units across the 20+ days but exercise regularly (15k steps a day/Run 4-6 hours a week) – Should i rearrange my appointment or should this be ok?

    Reply

    1. Hello Tommy,

      As we state above, the only way to guarantee that you will pass a DVLA CDT test is to completely abstain from drinking alcohol for at least four weeks prior to taking the test. We offer a CDT test to give you peace of mind ahead of your DVLA CDT test.

      Reply

  8. hi
    Is it normal for DVLA to only give me a driving license that is valid for 1 year?
    in 2017, I was disqualified from driving for 36 months, in the first analysis the DVLA found that I was dependent on alcohol. because the result of the CDT 1.9 . then I went back to do the CDT after a year its was 0.6 and then was 0.7 and then was 0.6.
    I sent the renewal request again, but my license has already expired, am I fit to drive under section 88?

    Reply

  9. Hello
    I had a CDT test back in 2021 and i must admit was drinking 5 bottles of beer a day and had a reading of 2.9 (bad) but over the years have cut back massively. I’ve contacted the dvla to have another test as i need my licence back now (been using public transport since 2021and its not as bad as people say) but anyway i have not had any alcohol, not even non alcohol beers since 1st January 2025 as of writing this which is about 9 weeks and by the time i send my D1 form to the dvla and they reply to me to go for a test i would say it would be in and around the 13 to 14 weeks that i would not of touched any alcohol, that being said could you say that my CDT levels would be back to normal around 1.6 and just to say i am fit walk about 3 to 4 miles a day and eat healthly

    Reply

  10. I see that the key period covered by the alcohol blood test is the approximate one-month period prior to the sample collection. If I’m taking the test to get a baseline reading, does it matter if the one-month period prior to collection is 27 days or 30?
    Thanks,
    Phyllis

    Reply

    1. Hi Phyllis. As mentioned above, a CDT blood alcohol test can be used to detect chronic and excessive alcohol consumption for approximately four weeks prior to the blood sample being collected. Therefore, the difference between 27 days and 30 days prior to collection is not significant for establishing a baseline reading.

      Reply

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