A false positive for alcohol occurs when a test registers a positive result for ethanol or its metabolites, even though the subject has not consumed any alcoholic beverages. This phenomenon can occur in breath, blood, or urine testing and is typically due to the presence of substances that chemically mimic beverage alcohol (ethanol). Understanding the different causes of these false readings is important, especially in forensic, medical, or employment-related testing, where an incorrect result can have serious consequences.
Topical and Oral Contaminants
The most common cause of a temporary false positive, particularly on a breathalyzer, is residual alcohol in the mouth, often termed “mouth alcohol.” Breathalyzers are designed to measure alcohol from deep within the lungs, but alcohol lingering in the oral cavity can significantly inflate the reading. This residue dissipates quickly, requiring a 15- to 20-minute observation period before the test is administered.
Many common products contain ethyl alcohol (ethanol). Alcohol-based mouthwashes, for instance, can contain ethanol concentrations as high as 20% to 30%, and if used right before a test, the residue can trigger a high reading. Similarly, certain breath sprays, cough syrups, and liquid cold remedies contain alcohol that can remain on the breath for a short period. Even the use of an asthma inhaler containing specific compounds like albuterol can temporarily cause a false positive, as the methyl compounds may be mistaken for ethanol by the breathalyzer.
Alcohol-based hand sanitizers also pose a risk because of the vapor they emit. If a person uses hand sanitizer immediately before a breath test, the evaporating alcohol can contaminate the air near the device’s intake or the operator’s hands, leading to an artificially high reading. This effect is short-lived, and a retest conducted a few minutes later, once the vapor has cleared, should produce an accurate zero reading.
Metabolic Conditions and Endogenous Production
Some internal biological processes and medical conditions can cause the body to produce compounds that register as alcohol. Auto-Brewery Syndrome (ABS), also known as gut fermentation syndrome, is a rare disorder where an overgrowth of yeast or bacteria in the gastrointestinal tract ferments ingested carbohydrates into ethanol. This ethanol is absorbed into the bloodstream, leading to measurable blood alcohol levels and symptoms of intoxication without any beverage consumption.
Diabetic ketoacidosis (DKA), a serious complication of uncontrolled diabetes or prolonged fasting, is another source of interference. When the body breaks down fat for energy, it produces compounds called ketones, including acetone. Acetone is expelled on the breath, and older or less sophisticated breathalyzers can misidentify high levels of acetone as ethyl alcohol. While modern forensic breathalyzers use infrared technology to differentiate between acetone and ethanol, extremely high concentrations of acetone can still potentially interfere with the reading.
Impaired liver function, often due to liver disease, can indirectly affect the interpretation of alcohol tests, particularly those looking for long-term use. While not causing a false positive for immediate ethanol presence, conditions like non-alcoholic liver disease, obesity, and diabetes can cause false positives for indirect markers of alcohol use, such as the liver enzyme gamma-glutamyl transferase (GGT). Furthermore, impaired liver function could affect the metabolism of trace amounts of alcohol or related compounds, potentially complicating the analysis of blood or urine tests.
Cross-Reacting Compounds and Medications
Certain chemicals, when absorbed into the body, are metabolized into compounds that mimic alcohol or its markers, leading to systemic false positives in blood or urine. Exposure to non-beverage alcohols, such as isopropanol (rubbing alcohol) or methanol (wood alcohol), is a common cause. Isopropanol is converted into acetone, and methanol is converted into formaldehyde and formic acid. The initial presence of these alcohols, or their metabolites, can be mistakenly identified as ethanol by less specific testing methods.
Environmental or occupational exposure to volatile organic compounds (VOCs) can also lead to false positive breath test results. Chemicals found in gasoline, paint fumes, solvents, and adhesives contain VOCs that are inhaled and exhaled on the breath. Although modern breathalyzers are designed to filter out many of these substances, immediate exposure before a test can still cause a positive reading if the device cannot distinguish between certain VOCs and ethanol.
Highly sensitive urine tests, specifically those looking for the alcohol metabolites ethyl glucuronide (EtG) and ethyl sulfate (EtS), are susceptible to cross-reactivity. EtG and EtS tests can detect trace amounts of alcohol absorbed systemically from non-beverage sources like alcohol-based hand sanitizers, mouthwash inadvertently swallowed, or cough syrups. Furthermore, some prescription and over-the-counter drugs may contain trace amounts of alcohol or be metabolized into compounds that chemically resemble EtG, leading to a false positive result reflecting medication use.