Blood-Alcohol Tests Positive for Acetaldehyde
Acetaldehyde is the fingerprint of fermentation.
A blood alcohol result tells you how much ethanol is present. It doesn’t tell you where the ethanol came from. One overlooked molecule does.
CH3CHO · the intermediate microbes make ethanol from · the intermediate a living body clears in seconds
Same ethanol peak. The difference is the earlier peak — and what it means.
Fermentation runs through acetaldehyde.
Microbial ethanol production is not one step. Glucose is broken to pyruvate; pyruvate (or acetyl‑CoA) is converted to acetaldehyde; acetaldehyde is then reduced to ethanol by alcohol dehydrogenase. Acetaldehyde is the obligatory, immediate precursor of the ethanol.
Zhu, Gonzalez & Bobik (2011) showed the dependency directly. Whenever alcohol dehydrogenase activity was present in their engineered E. coli, acetaldehyde was promptly pulled onward to ethanol — they had to knock out the native pathways and select for reduced alcohol dehydrogenase before acetaldehyde would build up rather than disappear. In a fermenting system, acetaldehyde and ethanol sit in equilibrium, one enzyme step apart, with CO2 and hydrogen as coproducts.
Consumed alcohol leaves no detectable acetaldehyde.
When a person drinks, their own alcohol dehydrogenase does make acetaldehyde. But it does not accumulate: aldehyde dehydrogenase oxidizes it to acetate almost instantly, and much of the rest binds tightly to hemoglobin and plasma proteins.
Eriksson’s review is blunt — decades of “high blood acetaldehyde” reports turned out to be artefactual, formed during the analysis itself, mainly at protein precipitation. With immediate saline/perchloric precipitation and proper controls, no detectable acetaldehyde was found in the venous blood of enzyme-normal subjects after drinking. Jones (1995) confirms it from the bench: free acetaldehyde in peripheral blood sits below the detection limit of headspace GC.
So the two peaks together point away from drinking.
A living person who drinks strips acetaldehyde out faster than it forms — a correct specimen shows ethanol without it. A fermenting tube has no such clearance system: it builds ethanol out of acetaldehyde and leaves that acetaldehyde behind. When a specimen shows ethanol and appreciable acetaldehyde, the pairing is more consistent with fermentation than with consumption.
§Two directions through the same molecule
The whole distinction lives in which way the arrow runs — and whether anything is standing by to remove the intermediate.
Find the intermediate riding along with the product in a specimen the body was supposed to have cleared — and you have a reason to ask whether that ethanol was ever consumed at all.
!Where the inference has limits
Honest use means stating the boundaries. Acetaldehyde is powerful evidence and a treacherous analyte in the same breath.
It can be created during analysis.
Eriksson’s central lesson: if ethanol is present and the workup is careless, acetaldehyde forms artefactually before it’s ever measured. Any claim needs a method that suppresses this — immediate precipitation, trapping agents, and ideally ethanol-spiked control blood run in parallel to quantify the artifact.
A trace endogenous level exists.
Normal intermediary metabolism and gut microbes produce small amounts, but hepatic first-pass keeps what reaches peripheral blood negligible. The relevant contrast is trace versus clearly detectable.
Some real states raise it during genuine drinking.
Inactive ALDH2 (common in East Asian populations), aldehyde-dehydrogenase inhibitors like disulfiram and calcium carbimide, and heavy chronic use all elevate acetaldehyde. Jones documents breath levels into the hundreds — even above 1,000 nmol/l — in exactly these groups. Read the number against genetics, medications, and clinical picture before attributing it to fermentation.
§Why it matters at the bench and in court
The practical value is that acetaldehyde is an independent line of evidence about the origin of ethanol, not just its amount. Preservative failure, delayed refrigeration, contaminated tubes, and long postmortem intervals all invite fermentation — and the number on the report looks identical whether the ethanol was swallowed or synthesized by microbes.
Read carefully, with proper analytical controls and an eye on the fermentation coproducts, acetaldehyde helps separate the two stories. Microbes make ethanol by way of acetaldehyde. That is the throughline, and it is the reason a peak that should never have survived in a living body is worth pausing over.
