A blood alcohol test is a laboratory procedure designed to accurately measure the concentration of ethyl alcohol (ethanol) in a person’s bloodstream. This measurement, expressed as a percentage of blood alcohol concentration (BAC), determines the level of intoxication. A false positive occurs when the test reports a measurable amount of ethanol, or a higher amount than is actually present, even if the individual consumed no or significantly less alcohol than suggested. This discrepancy can result from internal biological processes, external chemical interference, or procedural mistakes during sample collection or analysis.
Internal Physiological Sources
Some false positive results originate from rare metabolic conditions where the body produces ethanol internally. The most direct cause is Auto-Brewery Syndrome (ABS), a condition in which certain fungi or bacteria in the gastrointestinal tract ferment ingested carbohydrates. These microorganisms, similar to those used in brewing, convert sugars and starches into ethanol and other byproducts within the gut. The ethanol is then absorbed into the bloodstream, leading to an elevated BAC without the person consuming alcohol.
Other metabolic disturbances can also interfere with blood alcohol tests. While diabetic ketoacidosis primarily produces acetone, which mainly interferes with breath tests, general metabolic imbalances can be associated with elevated levels of lactate and lactate dehydrogenase (LD/LA). If these substances are present in high concentrations, certain enzymatic blood tests relying on alcohol dehydrogenase (ADH) methods may register a false increase in ethanol. This occurs because the elevated LD/LA levels influence the enzymatic reaction, interfering with the measurement process.
Cross-Reactive Substances
Many standard laboratory blood alcohol tests, especially those used for rapid screening in clinical settings, employ the enzymatic Alcohol Dehydrogenase (ADH) method. This method works by using the ADH enzyme to oxidize ethanol, a reaction that produces a measurable signal. The problem of cross-reactivity arises because this enzyme is not perfectly selective and can also react with other alcohols that share a similar chemical structure to ethanol.
Non-ethanol alcohols, such as isopropyl alcohol (found in rubbing alcohol) and methanol (found in solvents), are known to cause this chemical interference. When these substances are present in the bloodstream, the ADH enzyme attempts to metabolize them, generating the same measurable product as ethanol. While these non-ethanol alcohols are highly toxic, their chemical similarity can lead to a false positive reading for ethanol if the lab analysis is not confirmed by a more specific method like Gas Chromatography (GC).
Errors in Sample Handling and Analysis
The most common category of false positives involves procedural failures that occur after the blood is drawn. A major issue is the inadequate use of preservatives in the collection tube. Blood contains glucose and, often, microorganisms like bacteria or yeast; if the blood sample is not properly preserved, these microbes can begin to ferment the glucose into ethanol while the sample is stored or transported.
To prevent this post-collection fermentation, collection tubes must contain an anticoagulant, such as potassium oxalate, and a preservative, typically sodium fluoride. Sodium fluoride inhibits the enzymes responsible for microbial fermentation, preventing the artificial creation of ethanol, known as neo-ethanol formation. Improper storage conditions, such as storing the sample at room temperature or warmer, accelerate microbial activity and increase neo-ethanol production.
Another potential source of error is contamination of the sample. If the venipuncture site is cleaned with an alcohol-based antiseptic swab, residual ethanol can be introduced into the sample during the blood draw, resulting in a falsely elevated reading. Contamination can also occur if laboratory equipment or glassware is not completely free of residual alcohol from cleaning agents or reagents. These procedural mistakes can artificially raise the measured BAC, incorrectly suggesting a higher concentration than the individual actually had.