CDT blood testing

Here, we take a closer look at Carbohydrate Deficient Transferrin (CDT) blood testing, what it is, its accuracy and how it works.

Gail Evans, Alphabiolabs

By Gail Evans, Technical Trainer at AlphaBiolabs
Last reviewed: 07/31/2023

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.

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 analyzed 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.

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

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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. Before joining the company, Gail was a practising forensic scientist, attending scenes of crime, and analyzing physical and biological material with potential evidential value. Gail also holds qualifications in chemistry and is a Lead Auditor for the ISO 9001 standard, the international standard for quality management.

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