A CDT test is a blood test that measures carbohydrate-deficient transferrin, a biomarker used to detect chronic heavy alcohol consumption. It’s the only alcohol biomarker approved by the U.S. FDA for this purpose, and it’s widely used in both clinical and legal settings. If you’ve been asked to take one, it’s most likely related to a DUI case, a workplace fitness evaluation, a liver transplant screening, or monitoring during an alcohol treatment program.
How the CDT Test Works
Transferrin is a protein your liver makes to carry iron through your bloodstream. Normally, transferrin molecules have sugar chains (called glycans) attached to them, each capped with structures called sialic acid residues. When someone drinks heavily over a sustained period, alcohol’s toxic effects on liver cells disrupt the process that attaches those sugar chains. The result is transferrin molecules with fewer sialic acid residues than normal.
These under-decorated versions of transferrin are what the test measures. Specifically, the lab looks at transferrin isoforms carrying zero, one, or two sialic acid residues, collectively called carbohydrate-deficient transferrin. Results are typically reported as a percentage of total transferrin (%CDT). A %CDT at or above 1.7% is generally considered elevated, though the exact cutoff can vary slightly between labs and testing methods.
What Level of Drinking Triggers an Elevated Result
CDT levels don’t rise from occasional or moderate drinking. The test is designed to flag sustained, heavy consumption, typically defined as roughly 60 grams of alcohol per day (about four to five standard drinks) over at least two weeks. A single weekend of heavy drinking is unlikely to push your levels above the threshold.
Once someone stops drinking, CDT levels drop with a half-life of roughly two to four weeks, meaning it takes about that long for levels to fall halfway back to normal. Most people return to baseline within two to five weeks of abstinence. This relatively slow decline is part of what makes the test useful for monitoring: it provides a window into drinking patterns over the previous several weeks, not just the last day or two.
Why CDT Is Preferred Over Other Markers
Several blood tests can suggest heavy drinking, including GGT (a liver enzyme), liver transaminases like AST and ALT, and mean corpuscular volume (MCV, a measure of red blood cell size). The problem with all of these is that they’re not very specific to alcohol. GGT rises with many types of liver disease, medication use, and obesity. MCV has a sensitivity of only 40 to 50% for alcohol abuse. These markers tell you something is stressing the liver, but they can’t reliably tell you it’s alcohol.
CDT is more tightly linked to actual alcohol consumption rather than secondary liver damage. It can even help distinguish between alcoholic liver disease and nonalcoholic fatty liver disease, something GGT and liver enzymes alone cannot do. That specificity is the reason CDT became the FDA-approved standard.
For detecting very recent drinking (within the past few days), other biomarkers are more useful. Ethyl glucuronide (EtG) can be detected in urine for three to five days after heavy intake, and in blood for about 36 hours. Phosphatidylethanol (PEth) covers an intermediate window. CDT fills a different role: it reveals a pattern of sustained heavy drinking over weeks, not a single episode.
Where CDT Testing Is Required
The most common use of CDT testing is in forensic and legal contexts. After a DUI conviction, many jurisdictions require CDT testing before reinstating a driver’s license. The test helps authorities determine whether someone has maintained abstinence or continued drinking heavily. It serves a similar function in workplace evaluations for safety-sensitive jobs like commercial driving, aviation, or operating heavy machinery.
Clinically, CDT is used to monitor patients in alcohol rehabilitation programs. Serial testing over months can confirm whether someone is staying sober during treatment. It’s also used in liver transplant evaluations, where demonstrating sustained abstinence is often a prerequisite for being placed on the transplant list.
What Can Cause a False Positive
While CDT is more specific to alcohol than most alternatives, it isn’t perfect. Several medical conditions can elevate CDT levels without any alcohol involvement. These include primary biliary cirrhosis, hemochromatosis (iron overload), hepatitis C, autoimmune hepatitis, cystic fibrosis, and type 2 diabetes. Certain antiepileptic medications can also interfere with results. Patients on chronic hemodialysis have shown false positives depending on the testing method used.
Genetic variation in the transferrin protein itself is another source of error. At least 20 transferrin variants have been identified in people of European descent, with the B and D variants being most common. Some older testing methods can misidentify these variants as CDT, producing a falsely elevated reading. Modern methods using capillary electrophoresis or HPLC are better at catching these variants, but not all labs use the same technology.
If you have an unexpectedly high result and don’t drink heavily, it’s worth asking your provider whether any of these conditions could explain it and whether confirmatory testing with a different method is available.
How the Test Is Done
CDT testing requires a standard blood draw, typically from a vein in your arm. A fasting sample is recommended, though requirements may vary by lab. No other special preparation is needed. Results are usually available within a few business days, depending on the laboratory.
The International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) maintains a working group dedicated to standardizing CDT measurement worldwide. Their reference method uses HPLC (a type of liquid chromatography), and they support a network of reference laboratories to keep commercial testing methods calibrated against this standard. This matters because different testing platforms can produce slightly different numbers, and standardization helps ensure your result means the same thing regardless of which lab processes it.
What Your Results Mean in Practice
A normal %CDT (below the cutoff, usually around 1.7%) suggests no pattern of heavy sustained drinking in recent weeks. An elevated result is consistent with regular heavy consumption, though it should be interpreted alongside your medical history to rule out the non-alcohol causes listed above.
If you’re being tested as part of a legal or monitoring program, expect repeat testing at intervals. A single normal result shows recent abstinence, but a series of normal results over months provides much stronger evidence of sustained sobriety. Conversely, a rising CDT on serial testing is a reliable signal that heavy drinking has resumed, even if other liver tests remain normal.