A false positive blood alcohol test reports a positive result when little or no ethanol was consumed. Understanding the mechanisms behind these inaccurate results is important, as they stem from the body’s internal chemistry, the presence of non-drinking substances, and procedural errors in handling the blood sample. The accuracy of the final reported alcohol concentration is dependent on the integrity of every step, from the moment of collection to the laboratory analysis.
Internal Biological Processes
The human body can produce substances that either mimic ethanol or are converted into it, leading to a false positive reading. The most direct example is a rare medical condition called Auto-Brewery Syndrome (ABS), or gut fermentation syndrome. In individuals with ABS, fungi or yeasts, such as Candida albicans, ferment ingested carbohydrates into ethanol in the gastrointestinal tract. This endogenous ethanol production results in measurable blood alcohol concentrations even without alcohol consumption. The syndrome is often linked to underlying conditions like small intestinal bacterial overgrowth (SIBO).
Severe metabolic states, such as diabetic ketoacidosis (DKA), can also create a false positive scenario. During DKA, the body produces high levels of ketones, including acetone. While modern, specific blood tests can usually distinguish between ethanol and acetone, older or less specific alcohol assays may register the high concentration of acetone as a positive result. Acetone can also be metabolized into isopropanol (isopropyl alcohol), which can register on certain tests, complicating the measurement of genuine ethanol.
Chemical Interference from Non-Ethanol Substances
A false positive can also be caused by the ingestion or absorption of non-drinking alcohols that share chemical similarities with ethanol, known as congeners. The most common toxic alcohols that interfere with testing are methanol, ethylene glycol, and isopropyl alcohol. These compounds are found in household products, such as windshield washer fluid, antifreeze, and rubbing alcohol.
Isopropyl alcohol is metabolized in the body into acetone, which can cross-react with some alcohol detection methods. Methanol and ethylene glycol are highly toxic and are metabolized into dangerous substances. While these toxic alcohols might not always be mistaken for ethanol, their presence can sometimes be detected by less specific enzymatic assays used for screening. The definitive forensic method, gas chromatography, is designed to separate and identify these different alcohol compounds, but if a screening test is used alone, the presence of these chemicals can lead to an erroneous initial result.
Certain everyday products also contain small amounts of ethanol or related compounds that can briefly register on highly sensitive tests. These include some cough and cold medicines, mouthwashes, and hand sanitizers, which can cause a temporary elevation in alcohol levels.
Errors in Sample Collection and Laboratory Analysis
Procedural errors during the collection and analysis phase represent a significant source of false positive results. The collection process itself can introduce contamination if the venipuncture site is not cleaned properly. Using an alcohol-based antiseptic wipe to sterilize the skin before the blood draw, and then failing to allow it to dry completely, risks introducing external ethanol directly into the sample, artificially elevating the blood alcohol concentration (BAC).
The risk is why non-alcohol solutions like povidone-iodine are often mandated for forensic blood draws. Contamination can also occur if the person drawing the blood uses an alcohol-based hand sanitizer and then touches the needle or the collection site. Trace amounts of ethanol from these sources can be enough to skew a result, especially if the person’s true BAC is low.
Post-Collection Fermentation and Storage
The integrity of the blood sample relies heavily on proper preservation and storage. After blood is drawn, the sample naturally decomposes. This decomposition, or post-collection fermentation, occurs when bacteria or yeast convert glucose in the sample into ethanol.
To counteract this, collection tubes must contain both an anticoagulant and a preservative, such as sodium fluoride. Sodium fluoride inhibits the enzymes that facilitate this conversion, stopping the fermentation process. If an insufficient amount of preservative is used, or if the preservative is not adequately mixed, fermentation can occur, leading to a falsely elevated BAC reading over time.
Storage conditions are crucial for preventing post-collection fermentation; storing the sample at room temperature or warmer accelerates microbial growth and ethanol production. Samples that are not refrigerated and are held for an extended period before analysis are at risk of showing an artificially high reading. Finally, the analytical phase is susceptible to human error, such as miscalibrated equipment, though modern laboratory methodologies like gas chromatography-mass spectrometry (GC-MS) are considered the most reliable for specific ethanol measurement.