What Can Cause a False Positive PEth Test?

Phosphatidylethanol, commonly called PEth, is one of the most specific blood-based markers for alcohol consumption, and genuine false positives from the analytical chemistry itself are extremely rare. PEth forms only when ethanol reacts with cell membrane phospholipids, so the molecule shouldn’t appear in someone who hasn’t been exposed to alcohol at all. That said, “false positive” in practice covers more than just an instrument error. Several real-world scenarios can push a PEth result above a reporting threshold even when the person being tested did not deliberately drink, from contaminated blood samples and transfusions to problems with how a specimen was handled after collection.

Why PEth Is Hard to Fake in the First Place

PEth belongs to a family of phospholipids that the body does not make on its own. It is produced when the enzyme phospholipase D, which normally acts on other phospholipids in cell membranes, encounters ethanol and swaps its usual water molecule for an ethanol molecule in a reaction called transphosphatidylation.¹ Because PEth only forms when ethanol is physically present, the test has a built-in chemical specificity that most other alcohol biomarkers lack. A validated LC-MS/MS assay for PEth in dried blood spots showed no interference from other blood components, and a separate method validation confirmed no meaningful lipid interference or ion suppression from unrelated molecules.²³ In short, common medications, foods, and medical conditions don’t trick the instrument into seeing PEth that isn’t there. The problems that do arise almost always involve ethanol getting into the sample or the patient’s blood from an unexpected source.

PEth Can Keep Forming After Your Blood Is Drawn

This is probably the most well-documented route to a misleadingly elevated result. If you had any alcohol in your bloodstream at the time of the blood draw, the enzyme phospholipase D doesn’t stop working just because the blood left your body. It continues converting membrane phospholipids into PEth inside the collection tube. The formation rate depends on how much alcohol is present and how long the sample sits. Research has shown that PEth formation in blood samples is linear for the first several hours and scales with blood alcohol concentration, with formation rates climbing from very low levels at a blood alcohol of 0.01% up to roughly fifteen times faster at 0.3%.⁴ After three days at high blood alcohol concentrations, PEth levels in those samples climbed to over 400 ng/mL, which would be interpreted as heavy chronic drinking in most clinical settings.

The practical implication is straightforward. If someone has a drink or two the night before a scheduled PEth test and still has some residual alcohol in their blood at the time of the draw, the sample can continue manufacturing PEth during transport and processing. The longer the tube sits before the lab runs it, the more PEth accumulates. This isn’t the test malfunctioning. It’s a pre-analytical artifact where the chemistry that’s supposed to happen only in a living person keeps happening in a tube. One study examining authentic blood samples found that some specimens exceeded the initial concentration by more than 15% as early as day one of storage when blood alcohol was above 0.1 per mille.⁵

Alcohol Vapor Exposure During Dried Blood Spot Preparation

Dried blood spots (DBS) are an increasingly common collection method for PEth testing, especially in remote or outpatient settings. But a study specifically investigating this format found a surprising vulnerability. When PEth-negative blood samples from a non-drinking volunteer were allowed to dry in the presence of ethanol vapor, every single one turned PEth-positive. After just four hours of drying near ethanol fumes, the results resembled those of a moderate drinker. The same researchers observed that methanol and isopropanol vapors could trigger the formation of their own phosphatidyl derivatives through the same enzymatic pathway.⁶

This matters because alcohol-based hand sanitizers, disinfectant wipes, and sanitizing sprays are everywhere in medical settings. If a blood spot card is left to dry near freshly applied hand sanitizer or in a poorly ventilated space where alcohol-based cleaning products were just used, the drying droplets of blood can absorb enough ethanol vapor to generate measurable PEth. You wouldn’t know it happened, and neither would the lab unless their collection protocol specifically accounts for it.

Blood Transfusions Can Transfer Someone Else’s PEth

PEth is embedded in red blood cell membranes. If you receive a blood transfusion and the donor had been drinking, their PEth-containing red blood cells enter your circulation and show up on your test. A case report described exactly this scenario in a patient being evaluated for liver transplant eligibility. The patient’s PEth came back elevated, raising concerns about ongoing alcohol use. But the elevation turned out to reflect the donor’s drinking history rather than the patient’s.⁷

Researchers have separately noted that cautious interpretation is warranted for anyone who has received red blood cell transfusions, since a donor’s drinking pattern could feasibly influence the recipient’s PEth result.⁸ This is particularly relevant in hospital populations awaiting organ transplant, where abstinence from alcohol is often a condition for remaining on the waitlist. A falsely elevated PEth from transfused blood could, in the worst case, disqualify someone from a life-saving transplant.

Mouthwash and Other Incidental Ethanol Exposure

Many people worry that alcohol-containing mouthwash could trigger a positive PEth result. The concern isn’t unreasonable: some mouthwash formulas contain upwards of 20% ethanol, and you swish them around your mouth twice a day. A controlled study tested exactly this by having participants use an alcohol-based mouthwash on a regular schedule and then measuring their blood PEth levels. The result was reassuring. None of the participants reached the commonly used reporting threshold of 20 ng/mL.⁹

That said, the researchers made a point worth remembering: even sub-threshold exposures matter when the stakes are high, and extraneous ethanol sources should be considered in clinical decision-making. If a cutoff lower than 20 ng/mL is being used, which some labs do use, regular mouthwash exposure could conceivably nudge a result into positive territory. The broader lesson applies to any non-beverage ethanol source: certain liquid medications, herbal tinctures, and cooking extracts contain ethanol. None is likely to produce a strongly positive PEth result on its own, but they can add low-level background noise to the measurement.

The Cutoff Problem

Part of what makes the “false positive” question so loaded is that there’s no universally agreed-upon cutoff for what constitutes a positive PEth test. Different labs and clinical programs use different thresholds, commonly 20 ng/mL or 35 ng/mL for the main PEth species (16:0/18:1), though some use thresholds as low as 10 ng/mL. A study examining PEth levels after low-dose alcohol consumption found enormous interindividual differences in how much PEth people generate from the same amount of drinking. At a 10 ng/mL cutoff, the test picked up only about a quarter of consumption days for PEth 16:0/18:1 but caught roughly two-thirds for PEth 16:0/18:2. At the higher 20 ng/mL cutoff, only five samples ever crossed the line for the primary species.¹⁰

What this means practically is that the same PEth level can be called positive by one program and negative by another. A person who had a single glass of wine several days ago might test at 15 ng/mL. At a 20 ng/mL threshold, that’s negative. At 10 ng/mL, it’s positive. Neither result is technically “false,” but the person’s actual drinking behavior might be consistent with social use rather than the problem drinking the test is often used to detect. If you’re being tested in a context where the consequences of a positive result are severe, like child custody proceedings, professional licensing, or transplant evaluation, it’s worth knowing what cutoff is being applied and what it’s meant to detect.

How Red Blood Cell Conditions Affect Results

Because PEth lives in red blood cell membranes, anything that changes how many red blood cells you have or how quickly they turn over can skew your result. In people with anemia or conditions that cause red blood cells to break down faster than normal, PEth can clear from the blood more quickly than expected, potentially making a drinker look like they haven’t been drinking. Research found that PEth elimination was significantly faster in patients with enhanced red blood cell turnover and signs of alcohol-related hemolytic anemia.¹¹ That’s technically a false-negative scenario, not a false-positive one, but the inverse matters too. Someone with an unusually high hematocrit, meaning more red blood cells per unit of blood, could have a higher whole-blood PEth concentration simply because there are more cell membranes in the sample.

Researchers have proposed correcting PEth concentrations for hematocrit to get at the actual amount of PEth per red blood cell rather than per volume of whole blood. This correction improves the test’s accuracy, especially in patient populations where anemia is common, such as people with liver disease or chronic illness.¹² Most clinical labs don’t routinely perform this correction, though, which means the raw PEth number you receive on a report doesn’t account for whether your red blood cell count is unusually high or low.

Sample Storage and Transport Conditions

Even when no alcohol is present in the sample at collection, the way the specimen is stored and shipped can affect the final number. PEth in whole blood stays stable for up to about 28 days at all temperatures including room temperature, but beyond that, it starts to degrade. At standard freezer temperature, concentrations dropped by roughly 19% in EDTA tubes after 90 days, while samples stored at ultra-cold temperatures remained stable for the entire period.¹³ Degradation usually works against the test’s sensitivity rather than causing false positives, since PEth breaks down over time. But the stability picture gets messy when you consider that different PEth species degrade at different rates, and that room-temperature storage showed wildly inconsistent results across individual samples, with differences of more than 40% between specimens after 30 days.⁵

The bottom line for storage is that delays and warm conditions introduce uncertainty in both directions. A sample shipped across the country in summer heat without cold packs might gain PEth if residual alcohol is present, or lose PEth through degradation, or both. Short transport times and cool storage reduce this problem dramatically.

Endogenous Alcohol Production

Auto-brewery syndrome is a rare condition in which fungi or bacteria in the gut ferment carbohydrates into ethanol inside the body. People with this condition can have measurable blood alcohol levels without drinking. Because PEth formation requires only that ethanol be present in the blood, regardless of its source, someone with auto-brewery syndrome could theoretically produce PEth endogenously. Case reports linking auto-brewery syndrome to positive PEth results do exist in the gastroenterology literature, typically in patients with underlying gut conditions like Crohn’s disease or small intestinal bacterial overgrowth. This isn’t a test error in the analytical sense; the PEth is real, and the ethanol is real. But it’s a false positive in the practical sense that the person never drank alcohol.

Auto-brewery syndrome is genuinely rare, and diagnosing it usually requires a controlled carbohydrate challenge under medical supervision. If someone consistently tests positive for PEth despite credible claims of abstinence, and especially if they have gastrointestinal disease or diabetes, the possibility is worth raising with a physician.

How PEth Stacks Up Against Other Alcohol Markers

Understanding PEth’s false-positive risk is easier if you compare it to the markers it increasingly replaces. Carbohydrate-deficient transferrin (CDT) has been the standard indirect biomarker for heavy drinking for decades, but it’s far less specific. CDT can be elevated by liver disease, genetic variants in transferrin, pregnancy, and iron deficiency, none of which have anything to do with alcohol. PEth consistently outperforms CDT in head-to-head studies. In one large occupational screening study, every single CDT-positive case was also PEth-positive, but PEth caught additional cases that CDT missed entirely.¹⁴ In an outpatient treatment setting, PEth alone detected relapses in over 40% of cases where other markers had missed them.¹⁵

Ethyl glucuronide (EtG) in urine is another commonly used marker, but it has its own notorious false-positive problem from hand sanitizers, fermented foods, and even ripe fruit. EtG also has a much shorter detection window, typically two to three days, compared to PEth’s window of up to several weeks. Hair EtG extends the window but introduces its own complications from hair treatments and environmental exposure. A study comparing all of these markers in driver’s license reinstatement cases found that PEth and hair EtG were each more sensitive than CDT for detecting chronic excessive drinking.¹⁶ The tradeoff is that PEth’s pre-analytical vulnerabilities, particularly post-sampling formation, require more careful specimen handling than a simple urine EtG test.

The Two PEth Species and What They Reveal

Most clinical labs measure PEth 16:0/18:1, the most abundant species. But a second species, PEth 16:0/18:2, is synthesized more quickly after drinking and also disappears faster. Research has found that the ratio between these two species shifts predictably over time after the last drink. The longer someone has been abstinent, the higher the ratio of 16:0/18:1 to 16:0/18:2, because the slower-clearing species persists while the faster one fades.¹⁷

This differential behavior has a practical use for anyone contesting a result. If your PEth 16:0/18:1 is mildly elevated but PEth 16:0/18:2 is undetectable or very low, that pattern is more consistent with a single episode days ago than with ongoing heavy drinking. A drinking study with 16 volunteers showed that some participants still had PEth 16:0/18:1 above 30 ng/mL from drinking events that occurred well before a requested abstinence period, illustrating how a previous drinking episode can linger on this marker long after alcohol has left the body.¹⁸ Whether this represents a “false positive” depends on the question being asked. If the question is “has this person consumed any alcohol in the past two weeks,” a lingering result from moderate social drinking is a true positive. If the question is “does this person have an ongoing alcohol problem,” the same result could be deeply misleading.

Inhibiting Post-Collection Formation

Given that the biggest practical threat to PEth accuracy is continued PEth production inside the collection tube, researchers have looked at ways to shut that process down. Adding phospholipase D inhibitors to blood samples at collection can block the enzyme from creating new PEth after the draw. This approach is especially relevant when samples will be transported over long distances or when the person being tested might have residual blood alcohol at the time of collection.¹⁹ Dried blood spots offer a partial solution because the blood dries quickly and the enzymatic reaction slows dramatically once the liquid environment is gone, though as discussed earlier, this introduces the separate risk of vapor contamination. Neither approach has become universal in clinical practice, which means the quality of your PEth result still depends heavily on the collection site’s protocols.

If you’re facing a PEth test in a high-stakes context, a few things are worth verifying. Ask whether the sample will be refrigerated promptly after collection. Find out how long transport typically takes to the testing lab. If you’ve recently received a blood transfusion or have a condition that affects your red blood cells, make sure the ordering provider is aware, because the raw number on the report won’t reflect those factors on its own.