How CDT Testing Detects Harmful Alcohol Consumption

When gauging alcohol use in their patients, healthcare providers rely mainly on three methods:

• Traditional brief screening
• The blood alcohol concentration (BAC) test
• The carbohydrate-deficient transferrin (CDT) test

The traditional brief alcohol screening isn't as reliable as the other two methods because its accuracy depends solely on the patient being accurate and honest about how much they drink. While the BAC test measures the level of alcohol currently in the bloodstream, CDT testing uses biomarkers to detect a recent history of harmful levels of alcohol consumption.

The CDT test is a better indicator of binge drinking or daily heavy drinking (four or more drinks per day) than the other two tests. It can even help determine if a person with alcohol use disorder has had a relapse.

What Is a CDT Blood Test?

As the name suggests, CDT blood testing measures the level of CDT in the bloodstream. Carbohydrate-deficient transferrin (CDT) is a substance that carries iron to the bone marrow, liver, and spleen. When someone drinks heavily, the level of CDT in their body increases to a point at which it can be measured in a blood sample. Therefore, this increase is a biomarker of excessive alcohol use.

How the CDT Test Works

People who do not drink, or drink only moderately, have lower CDT levels in their blood than those who drink heavily. Some CDT tests use a cutoff of less than 1.7%. People who drink four or more drinks a day, at least five days a week, for two weeks prior to the test show significantly greater levels of CDT.

The CDT test is highly accurate in detecting heavy drinking. Similar to how an A1C test detects blood glucose levels over the previous 90 days, the CDT test detects heavy alcohol consumption over a long period. When a person stops drinking, CDT levels go back to normal after two to four weeks. Likewise, if they resume drinking, the levels once again increase after a few days.

When and Why CDT Is Tested

The CDT test gives a healthcare provider a far more reliable diagnostic tool than the traditional brief alcohol screening test, which relies on a patient's honesty about their drinking habits. Patients who don't consume alcohol excessively tend to self-report accurately, but those who do are more likely to minimize their drinking levels. The greater the misuse, the more likely the person will deny heavy alcohol consumption.

Medical Conditions

Alcohol consumption is contraindicated for many medical conditions. People with diabetes, high blood pressure, hepatitis C, or liver disease should not drink heavily.

One study found that of 799 patients studied, 9% of subjects with diabetes and 15% of those with high blood pressure were consuming alcohol at harmful levels. When extrapolated to the population at large, these results could mean that millions of people with diabetes and hypertension are risking their health by drinking alcohol.

Drug Interactions

Patients who take certain prescription and over-the-counter medications can experience harmful effects if they drink alcohol. Similarly, people who are being treated with opioid painkillers, sedatives, or sleep aids risk central nervous system depression should they drink alcohol heavily. In a medical emergency, CDT testing could uncover alcohol use that's reacting with medications and causing problems.

Recovery

CDT testing also can help healthcare providers monitor abstinence and relapse because the test is sensitive enough to detect increases and decreases in alcohol use.

Some psychotherapists and psychiatrists use the CDT test to determine a baseline level when they first begin treating a patient for alcohol use disorder. In the weeks and months that follow, they can use the CDT test to determine if the person is remaining sober or has had a relapse.

Accuracy of CDT Tests

Researchers have conducted many studies on the effectiveness of using CDT testing to identify heavy alcohol consumption. Those studies find that the test is accurate but not foolproof. Confirmation methods include the use of a questionnaire, a gamma-glutamyl transpeptidase (GGT) test, or an ethyl glucuronide (EtG) test (which detects alcohol consumption in the previous 24 to 72 hours).

False Negatives

In a small percentage of people, heavy alcohol consumption does not raise levels of CDT in the blood. A healthcare provider who suspects heavy drinking despite a negative CDT test may then turn to digital DNA methylation techniques, which rely on blood or saliva samples to detect heavy alcohol consumption.

False Positives

Biological factors such as genetic variants, female hormones, and end-stage liver disease can falsely increase CDT levels. The earliest CDT tests returned false positives due to these factors, but newer tests can identify genetic variants that can cause false positives and negatives, as well as patterns caused by liver disease related to heavy drinking.