It may not surprise you to find out that alcoholics arrested for DUI will generally have higher blood-alcohol readings. It may surprise you, however, to learn that alcoholics will generally have higher blood-alcohol readings because they are alcoholics….. That’s right. It’s because the physiology of alcoholics is different in some important respects.
One of those is that their bodies produce more acetaldehyde — far more. Acetaldehyde? That’s a compound produced in the liver in small amounts as a by-product in the metabolism of alcohol. Unfortunately, alcohol in the lungs has been found to metabolize there as well as in the liver — and to produce acetaldehyde there.
The amount of acetaldehyde produced in the lungs (to then be breathed into the breathalyzer) varies from person to person. "Origin of Breath Acetaldehyde During Ethanol Oxidation: Effect of Long-Term Cigarette Smoking", 100 Journal of Laboratory Clinical Medicine 908. But in a study focusing on alcoholics, researchers discovered that the amount of acetaldehyde in the breath and blood of alcoholics was 5 to 55 times higher than that in nonalcoholics. "Elevated Blood Acetaldehyde in Alcoholics and Accelerated Ethanol Elimination", 13 (Supp 1) Pharmacology, Biochemistry and Behavior 119.
End result: since breathalyzers can’t tell the difference between alcohol and acetaldehyde (see earlier post, "Why Breathalyzers Don’t Measure Alcohol"), alcoholics will usually have falsely higher blood-alcohol readings.