CDT Test (for alcohol misuse)
CDT Test (carbohydrate deficient transferrin) is a blood test used to check if a person has been drinking large amounts of alcohol over the previous 7-14 days. A CDT test is used by the DVLA when a person applies for their driving license to be reinstated. The DVLA accepts a CDT cut-off level of ≤ 2.2 %. A CDT test is also used by companies, such as Transport for London (TfL), who want to make sure an employee is not drinking excessively.
In our experience, the DVLA CDT tests can cause a lot of anxiety. Many of our clients order their own CDT test for for their own piece of mind before they attend for their official DVLA assessments. We also test employees of companies, such as Transport for London (TfL), if they bring in a form of photo ID such as passport or driving license.
A CDT test has an advantage over testing for alcohol in urine or blood which is only detectable for 24-48 hours. For the purposes of detecting alcohol misuse, it is not recommended to use only a CDT test. It would be better to also do other tests such as MCV and liver function tests as in our Alcohol Profile.
There is no need for an appointment to do a CDT test. After ordering it, you will receive a confirmation message with a map and directions attached. You can then go to Patient Reception at The Doctors Laboratory to have the blood sample taken. No fasting is required. The laboratory is open 7am – 7pm weekdays and 7am-1pm on Saturdays.
You are required to bring a valid photo ID with you at the of the test such as a passport or driving license.
Results available: 5 working days *. They include a doctor's explanation of the meaning of the results.
A CDT test result is usually available within 5 working days and will be e-mailed to you.
* Indicative timings only. Actual result may be earlier or slightly later.
Often combined with a liver function blood test to check for any damage to the liver from excessive alcohol consumption.
Causes of a high CDT level:
1. Alcohol Misuse (A 19 out of 20 chance): i.e. by far the most likely cause of a high CDT.
2. Liver Disease: Certain liver problems which cause a blockage or damage to bile ducts; e.g. primary biliary cirrhosis, primary sclerosing cholangitis, gallstones or liver/pancreatic cancer. These causes can be excluded if liver function tests (LFTs) are normal.
3. Transferrin Variants: Transferrin is a protein used to transport iron and can undergo changes by the consumption of alcohol. The normal type is C. People who have type D may have a raised CDT. Type D is rare in caucasions but may be present in 10% of Africans. People with this variant cannot use CDT to check for alcohol misuse because CDT will always be high. We would recommend that these clients use a urine EtG test instead.
4. Carbohydrate deficient glycoprotein syndromes: these are very rare syndromes resulting in mental retardation in children.
How much alcohol will cause a high CDT level?
You need to drink the equivalent of 100-150g of alcohol per day to raise CDT levels. This equates to 5 pints of beer, a bottle of wine, or one-third of a bottle of spirits daily.
Intermittent or "binge" drinking can also raise CDT levels. This would have to be in the range of 200-300g of alcohol on two occasions per week (around double the above amount) and no alcohol on the other days. Just one day of excessive drinking with the other days of no alcohol would probably not be enough to raise CDT levels over a 7-14 day period.
High risk offenders with alcohol problems
Defined in terms of the alcohol-related driving convictions below, the courts notify the DVLA of high risk offenders.
An independent medical examination will be arranged when an application for licence reinstatement is received by the DVLA. The assessment includes:
- serum CDT assay
- any further testing indicated.
If a licence is awarded, the ’til 70 licence is restored for Group 1 car and motorcycle driving. Consideration may be given to a Group 2 licence.
If a high risk offender has a previous history of alcohol dependence or persistent misuse but has satisfactory examination and blood tests, a short period licence is issued for ordinary and vocational entitlement but is dependent on their ability to meet the standards as specified.
A high risk offender found to have a current history of alcohol misuse or dependence and/or unexplained abnormal blood test results will have the application refused.
The high risk offender scheme applies to drivers convicted of the following:
- one disqualification for driving or being in charge of a vehicle when the level of alcohol in the body equalled or exceeded either one of these measures:
- 87.5 mcg per 100 ml of breath
- 200.0 mg per 100 ml of blood
- 267.5 mg per 100 ml of urine
- two disqualifications within the space of 10 years for drinking-driving or being in charge of a vehicle while under the influence of alcohol
- one disqualification for refusing or failing to supply a specimen for alcohol analysis
- one disqualification for refusing to give permission for a laboratory test of a specimen of blood for alcohol analysis.