I’ve written in the past about how most so-called "breathalyzers" do not measure alcohol: they actually measure the presence of a molecular group in chemical compounds. Ethyl alcohol (aka ethanol) contains the group, and so when the machine detects its presence (or, more accurately, infrared energy is absorbed by it), it simply assumes that the detected compound must be ethyl alcohol.

Problem: there are thousands of compounds containing the molecular group — of which well over one hundred have been found on the human breath.

Breathing gasoline or paint fumes, for example, or merely absorbing the fumes through the skin, can create false breath test results for days afterwards. And I’ve posted in the past that the problem is particularly acute when the suspect happens to be a diabetic, as diabetics often have high levels of acetone in their breath — a compound which contains the group in its molecular structure.

However, you do not need to be a diabetic to have high levels of acetone. Scientific research has established that acetone can exist in perfectly normal individuals at levels sufficient to cause false high breath-alcohol test readings.  See "Excretion of Low-Molecular Weight Volatile Substances in Human Breath: Focus on Endogenous Ethanol", 9 Journal of Analytical Toxicology 246 (1985).

Fasting or radical dieting, such as with the Atkins diet, can also cause significantly elevated acetone. Studies have concluded that fasting, for example, can increase acetone in the body sufficient to obtain breathalyzer readings of .06%.  This is cumulative — that is, the .06% will be added by the machine to any levels actually caused by alcohol or other compounds.  Thus, a true breath alcohol of .03%, for example, would be reported by the machine as .09%. "The Likelihood of Acetone Interference in Breath Alcohol Measurement", 3 Alcohol, Drugs and Driving 1 (1987).  And low-carbohydrate diets have long been associated with high levels of acetone production.

Of course, for many years law enforcement denied that any such problem existed, just as they denied that "mouth alcohol" and radio frequency interference caused false test results — until manufacturers started adding acetone detectors, mouth alcohol detectors and RFI detectors to their machines (none of which, unfortunately, have proven reliable.)

How reliable are breathalyzers? Not very.  See "How Breathalyzers Work — and Why They Don’t" and "Close enough for government work". As I’ve posted in the past, there appears to be a growing trend toward letting officers draw blood themselves at the scene of arrest. Given the reassurances about these machines so often expressed publicly by law enforcement, one has to wonder why they are increasingly turning to the involved process of hypodermic needles, preservatives, anticoagulents, refrigeration and delayed laboratory analysis….