CARBOHYDRATE DEFICIENT TRANSFERRIN (CDT)
Part of the DVLA medical those classed as high risk offenders are required to take involves providing a sample of blood. The blood sample taken will be sent off for laboratory testing in order to determine the CDT levels in the sample provided. The result of the blood test, alongside other information will then be used to make a decision as to whether or not a driving licence will be issued.
What is carbohydrate deficient transferrin?
Transferrin is a glycoprotein found in the blood that transports iron. Amino acids bind the iron to the transferrin, once this amino acid process has completed, the transferrin undergoes further modification by the addition of carbohydrate side-chains.
It is normal for the transferrin in the blood to have 3-5 carbohydrate side-chains, however people who misuse alcohol have a higher proportion of transferrin in their blood which only have 0-2 carbohydrate side-chains. Hence the term carbohydrate deficient transferrin.
This is because alcohol affects the enzymes that regulate transferrin side-chains. Alcohol hinders the enzymes which add the carbohydrate side-chains while at the same time encourages the enzymes that remove the side-chains.
A person who consumes little to no alcohol will have less than 1.6% of their blood transferrin in the carbohydrate deficient form while people who misuse alcohol and drink to excess will typically have a higher proportion of their blood transferrin in the carbohydrate deficient form e.g. 3-10% .
What must CDT levels be in order to pass the DVLA medical?
The DVLA CDT test results will be assessed using a traffic light system of green, amber and red which will correspond to a range of %CDT cut off levels for licensing purposes.
%CDT cut off levels differ for drivers with a history of alcohol dependence or misuse
For drivers with a history of alcohol dependence
GREEN - %CDT = 1.6% or less
A CDT level of 1.6% or less suggests no recent alcohol intake. This means that a CDT level up to and including 1.6% would be acceptable as evidence of there being no recent alcohol intake in someone classed as alcohol dependent.
RED - %CDT = 1.7% - 2.2% or more
A CDT level between 1.7% and 2.2% or more suggests recent excess alcohol intake.
For drivers with a history of alcohol misuse
A CDT level of up to 2.2% is acceptable as evidence of controlled drinking in someone with a previous history of alcohol misuse only. Otherwise the amber levels below would apply.
For drivers with no history of alcohol dependence or misuse
GREEN - %CDT = 2.2% or less
A CDT level of less than 2.2% or less is considered to be in the normal range. A person whose CDT level falls within this range is identified as consuming little or no alcohol and is compatible for licensing.
AMBER - %CDT = 2.3% - 2.9%
A CDT level between 2.3% and 2.9% is considered to be indicative of possible problematic alcohol consumption. A person whose CDT levels fall within this range is identified as someone who may drink to excess or binge drinks regularly and will trigger further enquiries with that persons general practitioner (GP) before a licensing decision is made.
RED - %CDT = 3% or more
A CDT level greater than 3% is considered to be indicative of alcohol consumption in a dependent manner. A person whose CDT levels fall within this range is identified as someone who is dependent on alcohol and a driving licence will be refused.
The %CDT cut off levels were up to date at the time of publishing although the medical advisory panel may review and change the cut off levels at any time.
It is worth noting that the medical standards of fitness to drive for medical professionals published by the DVLA  states that persistent alcohol misuse confirmed by medical enquiry and/or evidence of otherwise unexplained abnormal blood markers will result in refusal of a licence.
The word unexplained abnormal blood markers means that if CDT levels are increased by something which can be explained, other than alcohol misuse, then this may not lead to a refusal in licensing.
Alcohol misuse is the most common reason for increased CDT levels and in most studies carried out on persons who are potentially misusing alcohol, the test has 95% specificity i.e. 19 out of 20 times the increased CDT levels are a result of excessive alcohol consumption.
False positives are rare, however there can be several causes of increased CDT levels other than alcohol misuse, including advanced cholestic liver disease, transferrin variants and carbohydrate deficient glycoprotein syndrome .
How much alcohol will it take to increase CDT levels in the bloodstream?
According to a publication available from the National Institute of Alcohol Abuse and Alcoholism CDT levels appear to elevate after consuming 60-80 grams of alcohol per day (approximately 4 to 6 440ml cans of fosters) for a period of 2 or 3 weeks and CDT levels will normalize with a mean half-life of 2 to 4 weeks abstinence .
Can binge drinking increase CDT levels?
Binge drinking can increase CDT levels depending on the frequency of binges and the amount of alcohol consumed. According to Dr Roy Sherwood someone drinking 200-300 grams of alcohol (approximately 14 to 21 440ml cans of fosters) 2 days a week, who abstain from consuming alcohol on the 5 other days could possibly have a CDT level of between 1.5 - 3%.
If the frequency of binges is less, for example someone who has only one binge in a 14 day period while abstaining from consuming alcohol for the other 13 days, they are unlikely to have raised CDT levels; this is because the normal transferrin produced during the 13 day abstinence period would 'dilute' the carbohydrate deficient transferrin resulting from the day of the binge .
How can I work out how many grams of alcohol is in a drink?
One UK unit is 10ml or 8 grams of of pure alcohol 
The number of units in an alcoholic beverage is determined by the size of the drink and the alcohol content of the drink.
The alcohol content of a drink is expressed by the measure ABV (Alcohol by Volume) and can be found labelled on cans and bottles of alcohol which can be written as 'ABV', 'VOL' or 'VOLUME'.
ABV, VOL and VOLUME are measures of the amount of pure alcohol as a percentage of the total volume of liquid in a drink.
You can work out how many units of alcohol that is in a drink by multiplying the strength (ABV as a percentage) of the drink by the volume of the drink (in ml) and dividing the result by 1000.
Strength (ABV %) x volume (ML) ÷ 1,000 = units
For example, to determine the amount of units in a 440ml can of fosters which has an alcohol content of 4.0% VOL.
4.0 (% VOL) x 440 (ML) ÷ 1,000 = 1.76 units
In order to work out the grams of alcohol in a drink, you simply multiply the amount of units by 8 (one unit is equal to 8 grams of alcohol).
1.76 (units in a 440ml 4.0% VOL can of fosters) x 8 = 14.08 (grams of alcohol)
So a 440ml can of fosters, the strength of which is 4.0% VOL contains 1.76 units (which equates to 14.08 grams of alcohol).
Reference: Carbohydrate Deficient Transferrin (CDT) by Dr Roy Sherwood, Consultant Clinical Scientist Medical professional guide to assessing fitness to drive National Institute of Alcohol Abuse and Alcoholism Publication NHS Choices - Alcohol Units