A CDT appointment is a blood test that measures carbohydrate-deficient transferrin, a biomarker used to detect heavy alcohol consumption over the previous two to three weeks. These appointments are most commonly required for driving license reinstatement, court-ordered monitoring, professional licensing, or substance abuse treatment programs. The test itself is a standard blood draw, but what makes it distinctive is what the lab is looking for in your sample.
What CDT Actually Measures
Transferrin is a protein your liver makes to carry iron through your bloodstream. Normally, transferrin molecules have sugar chains (specifically sialic acid) attached to them. When someone drinks heavily over a sustained period, alcohol and its byproducts interfere with how the liver builds these sugar chains. Ethanol disrupts the enzymes responsible for attaching the sugar molecules and may also boost the activity of enzymes that strip them off. The result is a higher-than-normal percentage of transferrin molecules missing their usual sugar coating.
That percentage is what the lab reports. In people who drink little or no alcohol, CDT typically stays below 2%. In habitual heavy drinkers, it rises above 6%. The threshold most labs use to flag a result as elevated is around 1.7% to 2.0%, depending on the testing method.
How Much Drinking Triggers an Elevated Result
CDT levels start rising after roughly two to three weeks of consuming four to six drinks per day. In practical terms, that’s about a bottle of wine, five cans of beer, or half a pint of hard liquor daily. Occasional or moderate drinking generally won’t push CDT into the abnormal range, which is one reason the test is favored over other markers for identifying sustained heavy use rather than a single episode.
Once someone stops drinking, CDT begins dropping within a few days. In most people, levels return to normal after several weeks of abstinence. However, because the decline starts quickly, someone who stops drinking four or more days before their test may already show a result that falls within the normal range, even if they were drinking heavily before that.
Why CDT Is Preferred Over Other Tests
Older alcohol markers like GGT (a liver enzyme) have notoriously inconsistent accuracy. Published sensitivity for GGT ranges anywhere from 37% to 95%, and its specificity is similarly unreliable, falling between 18% and 93%. Elevated GGT can result from many liver conditions, medications, and metabolic issues unrelated to alcohol.
CDT is considerably more specific to alcohol. In one large study comparing the two markers in drivers, CDT was 28 times more likely to be elevated in people over the legal blood alcohol limit, compared to just 6 times for GGT. That precision is why courts, licensing authorities, and treatment programs increasingly rely on CDT as the primary biomarker.
What Can Cause a False Positive
While CDT is more reliable than older tests, a small number of conditions can raise levels without any alcohol involvement. The most common cause of a false positive is end-stage liver disease, including conditions like hemochromatosis, hepatitis C, autoimmune hepatitis, and cirrhosis. Certain genetic transferrin variants can also produce unusual results. In one study, about 12% of patients with severe liver disease had elevated CDT readings unrelated to drinking.
A few other factors occasionally play a role. Some enzyme-inducing antiepileptic medications have been linked to elevated readings. Type 2 diabetes has been flagged in rare cases. Pregnancy can push CDT levels slightly higher, with about 38% of pregnant women in one study reaching borderline values, though all fell back to normal after delivery. If you have any of these conditions, it’s worth mentioning them before or during your appointment so results can be interpreted in context.
What to Expect at the Appointment
A CDT appointment is a straightforward blood draw. A phlebotomist will take a small sample from a vein in your arm, and the entire process takes just a few minutes. There’s no special preparation unique to CDT testing, though some clinics may ask you to fast for eight to twelve hours beforehand if they’re running additional blood panels at the same time. Check with your provider when you schedule the appointment.
If you are told to fast, that means no food or drink other than plain water. Avoid flavored water, coffee (even black), gum, and smoking during the fasting window. Plain water is fine and actually helpful because it keeps your veins fuller, making the blood draw easier. You can usually continue taking prescribed medications unless told otherwise, but ask about any over-the-counter supplements you take regularly. If you accidentally eat or drink something, let the person drawing your blood know so the results can be interpreted correctly.
You can eat and drink normally as soon as the draw is finished. Results typically come back within a few days to a week, depending on the lab.
How Results Are Used
The way your CDT result gets used depends on why the test was ordered. For driving license reinstatement, authorities typically require one or more CDT tests spaced weeks or months apart to demonstrate sustained abstinence. A single normal result shows your level at one point in time, but a series of normal results over several months provides much stronger evidence.
In court-ordered monitoring or treatment programs, CDT results often become part of a broader compliance record. Because the test reflects drinking patterns over roughly two to three weeks, it fills a gap between breath or urine tests (which detect very recent use) and liver panels (which reflect long-term organ damage). Some programs combine CDT with other biomarkers to get a more complete picture.
If your result comes back elevated and you believe it’s inaccurate, the conditions listed above (liver disease, certain medications, pregnancy, genetic variants) are worth discussing with the ordering provider. Repeat testing or additional lab work can help clarify whether alcohol or another factor is responsible.