Quest Diagnostics resource guide
CDT blood testing
Carbohydrate-deficient transferrin
An indirect marker associated with sustained heavy alcohol use, interpreted with clinical context.
A pattern marker, not a countdown.
CDT measures a change in a blood protein.
Sustained use
A pattern over time
Transferrin changes
A biological response
Clinical review
Result + medical context
Conceptual explanation, not a patient result or proof of alcohol use. Liver conditions and genetic variants can also affect CDT.
What this test tells you
Quest associates elevated CDT with heavy drinking sustained for at least two weeks. Certain liver conditions and genetic transferrin variants can also elevate it. This is not a fixed look-back window.
What collection involves
Quest test 16985 prefers 1 mL of serum and lists fasting as preferred. Confirm preparation with the ordering provider or collection site.
Keep in mind
CDT is not a direct measure of alcohol. A qualified reviewer must consider medical factors and the assay used.
A common question
Why is CDT separate from the detection-window graph?
Quest describes an association with sustained use, not a guaranteed number of days in which every drinking event is detectable.
Read the original resources.
Source material behind this guide.
Reviewed October 26, 2026. Laboratory guidance can change; the current order and report take priority.