CDT is not measured that way(50/100). It could be your GP was giving you Liver Function Test results - which aren't measured anymore for DVLA purposes. A lot of GP's don't seem to be clued in to the
CDT test.
If you look at my other thread, you can contact DVLA and get results of your blood test.
I agree with white43, the result you have quoted is in relation to LFTs, which include tests for several substances including ALT, ALP, AST, bilirubin and gamma-GT, all related to liver health and function.
As the 'normal range' is generally quoted as 8-65 U/L, the result you have quoted there is probably for gamma-GT. GGT is often looked at as an indicator of excessive/high alcohol consumption, especially in conjunction with abnormal levels of one or more of the other substances tested.
DVLA previously tested for gGT, ALT, AST and mean cell volume (MCV). However, abnormal levels of ALT etc. can be indicative of other underlying issues, not necessarily alcohol related.
Carbohydrate deficient transferrin (cdt) is the substance that the DVLA test for when analysing your blood sample which is a generally reliable indicator of excessive alcohol consumption as it is very rarely affected by other medical conditions (although some individuals seem to have unwaveringly low CDT readings, see white43's consistently low privately conducted CDT tests).
Generally CDT is not run in the normal array of liver function tests as is indicated by virtually everyone who has visited their doctor for advice and the GP has no idea that it is CDT that is tested for and not gGT, effectively sending blood samples for the wrong tests.
CDT levels are represented as a percentage. In my case 1.0%, 0.9% in several other poster's accounts. For the purposes of the DVLA medical, anything under 2.2% is considered to be in the 'green zone'. 2.3%-2.9% is 'amber', prompting further enquiries and above 3.0% indicates consumption of a dependant nature and refusal of the license issuance.