What Blood Test Shows Alcohol Use?

Several blood tests can reveal alcohol use, but they differ dramatically in what they detect and how far back they can look. A straightforward blood alcohol concentration (BAC) test only catches ethanol that is still circulating in your system, which means it is useful for hours, not days. For a longer window, clinicians and employers increasingly rely on phosphatidylethanol (PEth), a direct alcohol metabolite stored in red blood cell membranes that can remain detectable for up to four weeks after the last drink. Older indirect markers like GGT, CDT, and MCV are still ordered but are less specific and easier to confuse with non-alcohol-related conditions. The test that gets ordered depends on the question being asked: Did this person drink tonight? Have they been drinking regularly? Are they abstinent?

Blood Alcohol Concentration and How Quickly It Disappears

The most familiar alcohol blood test simply measures the ethanol itself. After a drink, ethanol moves into the bloodstream and is broken down primarily by liver enzymes. In most forensic and clinical situations, the body clears alcohol at a roughly steady rate once the blood level is above about 20 mg per 100 mL. For someone who drank on an empty stomach, that rate runs between 10 and 15 mg per 100 mL per hour; with food in the stomach, it tends to be faster, around 15 to 20 mg per 100 mL per hour.¹ At low concentrations, elimination slows and shifts to a different pattern, with research on drunk drivers showing this shift occurs below about 0.19 g/kg.²

What this means practically is that a BAC test is only informative within a few hours of drinking. Someone who had several drinks the night before will almost certainly test at zero by late morning. That makes BAC useful for traffic stops, emergency rooms, and workplace incidents where the question is “are they impaired right now?” but useless for detecting a pattern of drinking over days or weeks.

EtG and EtS for the In-Between Window

Ethyl glucuronide (EtG) and ethyl sulfate (EtS) are minor metabolites that the body produces as it processes ethanol. They linger in blood longer than ethanol itself, though not by a huge margin. In a controlled drinking study, blood EtG peaked around five hours after drinking and remained detectable for roughly 8 to 12 hours; EtS peaked a bit earlier, at about three hours, and had a similar detection window of 5 to 12 hours.³ Another study found EtG and EtS reaching maximum concentrations four to seven hours after consumption, with EtG distributing more into serum than into whole blood.⁴

EtG and EtS fill a gap between a straight BAC reading and the longer-range biomarkers discussed below. They are especially valuable in forensic work: because they are only produced when the body metabolizes real ethanol, their presence in postmortem blood can help distinguish genuine drinking before death from ethanol generated by bacteria during decomposition. One study confirmed that EtG and EtS levels remained stable and provided accurate insight into ethanol sources even in decomposed remains and in people who had diabetes, a condition that can confuse ethanol readings.⁵

PEth, the Longest-Range Direct Marker

Phosphatidylethanol, universally called PEth, has become the gold standard for detecting sustained alcohol use through a blood test. It is a phospholipid formed only when ethanol is present and the enzyme phospholipase D swaps ethanol into a membrane phospholipid instead of water. Because red blood cells lack the enzyme that normally breaks PEth down, the molecule accumulates in their membranes and clears slowly. Estimates of its elimination half-life range from about three days after a single drinking event up to five to ten days in heavier or more chronic exposure.⁶⁷ In someone who has been drinking regularly and then stops, PEth can remain above the detection limit for up to 28 days.⁸

That weeks-long detection window is what makes PEth so powerful. A single blood draw can indicate whether someone has been drinking over the past two to four weeks, even if they stopped several days before the test. A validation study measuring PEth elimination over consecutive samples found a mean half-life of about 6.2 days, though some individuals showed half-lives exceeding 10 days.⁹ That individual variability matters: two people drinking the same amount can produce quite different PEth levels, which makes it better at detecting whether someone drank at all than at estimating exactly how much.

Sensitivity at Low Drinking Levels

PEth is excellent at catching heavy drinkers but less reliable at catching light ones. When 75 participants consumed about 20 grams of ethanol (roughly a glass and a half of wine) on three consecutive evenings, PEth 16:0/18:1 was only positive in about a quarter of them after the first day, rising to about half after three days of low drinking, using a standard cutoff of 10 ng/mL.¹⁰ The researchers suggested that lowering the cutoff would improve detection of light-to-moderate consumption, though doing so increases the risk of false positives. For now, a negative PEth result does not rule out an occasional glass of wine, but a strongly positive result is very hard to explain away without real alcohol exposure.

How PEth Compares to Older Markers

In head-to-head comparisons, PEth consistently outperforms the traditional markers. In a study of people undergoing occupational and pre-employment medical exams, PEth was positive in about two-thirds of cases at a lower cutoff, while CDT, GGT, and MCV flagged far fewer. Every single person who tested positive on CDT also tested positive on PEth.¹¹ A separate analysis found that PEth identified about 90% of all positive cases (positive on either PEth or CDT), compared to 57% for CDT alone.¹²

In outpatient treatment programs where the goal is to catch relapses, PEth detected relapses alone in about 43% of cases, while PEth and CDT together caught another 38%, and the remaining relapses were picked up by EtG and EtS.¹³ No single marker catches everything, but PEth comes closest.

The Older Indirect Markers Still in Use

Before PEth became widely available, clinicians relied on a panel of indirect markers. These tests do not measure alcohol or any substance the body only makes in alcohol’s presence. Instead, they measure biological changes that heavy drinking tends to cause, which means other conditions can produce the same abnormalities.

GGT (Gamma-Glutamyl Transferase)

GGT is a liver enzyme that rises when the liver is stressed. Chronic heavy drinking is one common cause, but so are obesity, certain medications, bile duct problems, and other liver diseases. One study found that at a cutoff of 50 IU/L, GGT had perfect specificity (no false positives in that sample) but sensitivity that ranged widely, from 56% to 100% depending on the stage of drinking.¹⁴ Another clinical study found even lower sensitivity, around 57%, and specificity of only 63%.¹⁵ GGT is cheap and widely available, so it still gets ordered, but a normal GGT does not mean someone is not drinking, and an elevated one does not prove they are.

CDT (Carbohydrate-Deficient Transferrin)

CDT is more specific to alcohol than GGT. Chronic drinking disrupts the normal addition of sugar chains to transferrin, a protein that carries iron in the blood. The mechanism involves ethanol and its byproduct acetaldehyde suppressing the liver enzymes responsible for attaching those carbohydrate chains while simultaneously boosting an enzyme that strips them off.¹⁶ CDT is considered a marker of chronic intake exceeding roughly 60 grams of ethanol per day, equivalent to about five or six standard drinks daily over an extended period.¹⁷

CDT is not without pitfalls. Genetic variants of transferrin can interfere with the test methods, producing misleading results in people who do not drink at all.¹⁸ And certain liver diseases, particularly primary biliary cirrhosis, can elevate CDT without any alcohol involvement.¹⁹ It is the kind of test that works reasonably well for confirmed heavy drinkers but can produce both false positives and false negatives in the gray zone of moderate consumption.

MCV and Liver Enzymes

Mean corpuscular volume (MCV) measures the average size of red blood cells. Long-term heavy drinking tends to enlarge them, a condition called macrocytosis. Research comparing alcoholics consuming 40 to 500 grams of ethanol per day with moderate drinkers and abstainers found that the heaviest drinkers had the highest MCV values, but even moderate drinkers showed a response: the upper normal limit for MCV shifted from 96 femtoliters in abstainers to 98 in moderate drinkers.²⁰ But macrocytosis has many other causes, including vitamin B12 and folate deficiency, thyroid disorders, and certain medications, so an elevated MCV on its own proves very little about drinking.

The ratio of the liver enzymes AST and ALT is another classic signal. In alcoholic liver disease, AST is typically higher than ALT. In patients without cirrhosis, this ratio was at or below 1.0 in about two-thirds of cases, but in those with cirrhosis, nearly 70% had a ratio of 2.0 or higher.²¹ The catch is that this ratio reflects liver damage more than it reflects drinking itself. Someone in the early stages of alcohol misuse with a healthy liver will likely have a normal ratio. The AST/ALT pattern is also not static; because AST has a shorter half-life (around 18 hours) compared to ALT (around 36 hours), the ratio changes depending on how recently alcohol exposure occurred.²²

Practical Differences Between Testing Contexts

The test that gets ordered depends on who is asking and why. In emergency medicine and traffic stops, a direct BAC is all that matters, because the question is about current impairment. In addiction treatment and court-ordered monitoring, clinicians want to know about the past weeks, so PEth and CDT are the primary tools. In occupational health screening, PEth has increasingly replaced the older panel of GGT, CDT, and MCV because it outperforms all of them at identifying regular heavy drinkers while producing fewer false positives from unrelated liver conditions.¹¹

Prenatal care is another setting where PEth has proven valuable. Self-reports of alcohol use during pregnancy are unreliable for obvious reasons. A systematic review found that PEth was above the detection limit in about 5% of women who reported abstaining during pregnancy, jumping to 26% of light drinkers, 55% of moderate drinkers, and 100% of heavy drinkers. Interestingly, testing the newborn’s blood was even more sensitive than testing the mother’s blood for moderate exposure.²³

Why Sex and Body Composition Affect Results

Women generally reach higher blood alcohol concentrations than men after consuming the same amount of alcohol, even when the dose is adjusted for body weight. Part of this comes down to body water content: women typically have proportionally less water to dilute the ethanol. Women also show less first-pass metabolism of alcohol in the stomach, meaning more ethanol reaches the bloodstream intact. One study found that gastric emptying was about 42% slower in women and their volume of alcohol distribution was about 7% smaller, both of which push BAC readings higher.²⁴ Once alcohol is in the system, though, the per-hour elimination rate is similar between sexes on a body-weight basis.²⁵

These differences matter because they mean two people who split a bottle of wine at dinner could show meaningfully different BAC readings and, over time, different biomarker levels. This is not just an academic point; it shows up in occupational testing and clinical assessments where fixed cutoffs are applied equally to everyone.

How Dried Blood Spots Are Changing Collection

Traditional PEth testing requires a standard venous blood draw, kept properly refrigerated until it reaches the lab. That creates logistical barriers, especially in remote or outpatient settings. Dried blood spots (DBS) offer an alternative: a finger prick produces a small drop of blood on a filter card, which is then mailed to a lab at room temperature. A validated method using volumetric DBS devices showed precision within acceptable limits and gave results that closely matched liquid venous blood, with capillary and venous DBS samples producing virtually identical values.⁹

There is one significant technical problem, though. If the person has alcohol in their blood at the moment of collection, the enzyme phospholipase D can continue converting it into PEth on the filter card as the blood dries, artificially inflating the result. In testing, blood from people who had zero PEth but were spiked with ethanol showed substantial PEth formation after just three hours at room temperature on standard cards.²⁶ The solution is to incorporate a chemical inhibitor of phospholipase D directly into the collection device. Sodium orthovanadate has proven effective at blocking this post-collection artifact entirely.²⁶ Newer commercial devices now include this inhibitor, but anyone undergoing PEth testing by finger prick should ideally not have consumed alcohol very recently, or the sample should be collected on an inhibitor-treated card.

Postmortem Alcohol Testing and Its Unique Challenges

Determining whether a deceased person was drinking before death poses a specific puzzle. After death, bacteria in the body can ferment sugars and produce ethanol on their own, creating a false-positive alcohol reading that has nothing to do with drinking. Forensic toxicologists look for “higher alcohols” like 1-propanol and isobutanol, which are byproducts of microbial fermentation and serve as qualitative indicators that the ethanol in a postmortem sample was produced by microbes rather than consumed.²⁷

EtG and EtS are valuable here because the body only produces them through genuine enzymatic metabolism of ethanol while alive. Finding EtG and EtS alongside ethanol in a postmortem sample is strong evidence of actual drinking. Their levels stayed stable and reliable even in decomposed remains and in diabetic decedents, whose blood chemistry can otherwise produce confounding artifacts.⁵ PEth is similarly useful in this context because it cannot be formed postmortem without the specific enzymatic reaction that requires living red blood cells and circulating ethanol.

Choosing the Right Test and What to Expect

If your doctor orders “alcohol labs” as part of a general health workup, you will most likely see GGT and liver enzymes (AST, ALT) on the panel, because these are included in standard metabolic and hepatic function tests. They may raise a flag for heavy drinking but do not specifically test for alcohol. If your doctor suspects alcohol misuse or wants confirmation, they may add CDT. For the most definitive picture of recent drinking over weeks, PEth is the test to request or expect, particularly in substance abuse treatment, legal monitoring, or occupational medicine. In urine, EtG testing extends the detection window well beyond what blood EtG can offer, which is why urine EtG panels are common in court-ordered testing. Blood EtG and EtS are more commonly encountered in forensic investigations.

One common misconception is that drinking a lot of water, exercising, or taking supplements can “flush” these markers from the blood before a test. None of these strategies meaningfully accelerates the clearance of PEth, CDT, or GGT. PEth sits embedded in red blood cell membranes and only clears as those cells are naturally recycled. CDT reflects weeks of altered liver glycosylation. GGT is an enzyme whose production takes time to normalize after the liver stops being stressed. The only reliable way to produce a clean test is sustained abstinence over a period that matches the marker’s detection window, which for PEth means roughly three to four weeks of no alcohol at all.