Dieting Causes False Positives on Ignition Interlocks

Posted by Lawrence Taylor on April 2nd, 2007

As I’ve railed repeatedly in the past, breath testing machines are inaccurate and unreliable for a wide number of reasons.  (See "How Breathalyzers Work – and Why They Don’t").  One of these reasons is that they are non-specific for alcohol.  Consider this recent news story:


Diet Driving: Low-Calorie Diet

Produces False Positives for Alcohol

Swedish researchers have discovered that a low-calorie diet can register a false positive on certain in-car ignition interlock devices that disable a vehicle if alcohol is detected on one’s breath.

The anomaly was discovered when a non-drinking airplane pilot reported the incident. Turns out the man was on a very restrictive diet that had him losing weight rapidly, which is what may have caused the false reading. As reported in the latest issue of the International Journal of Obesity, motorists on very low-calorie diets may release certain ketones that could be converted into a secondary alcohol known as isopropanol.

Police officials point out that false positives are eliminated in the field as breathalyzer tests are used in conjunction with secondary tests that focus on the type of alcohol and other factors. No citation for drunk driving would be issued in those situations. However, if you have one of these interlock devices on your car, your low-cal diet could spell the demise of your travel plans.


And "police officials" don’t know what they are talking about.  Most "breathalyzers" have the same problem as ignition interlock devices: they are non-specific for ethyl alcohol — that is, they can’t distinguish between ethyl alcohol and thousands of other chemical compounds, among them ketones.  See my earlier posts, "Why Breathalyzers Don’t Measure Alcohol" and "Dieting Can Cause High Breathalyzer Results".

The same problem is commonly encountered with diabetics, who have elevated levels of ketones on their breath when they are hypoglycemic.  See "Drunk Driver?…or Diabetic?" and "The Diabetic DUI".


(Thanks to Troy McKinney.)

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  • kingkongagn

    Ketones aren’t only released in restrictive low-calorie diets, but are also released during low-carb diets such as the Atkins or South Beach diets. And, as someone who has done the Atkins diet, the level of ketones released can be so high that they can be not only present in the mouth, but tasted. This not only resulting in a false positive on a “breathalyzer,” but also creates a possible mouth alcohol problem. Given how popular low-carb dieting is these days, one has to wonder how many people may have been arrested for driving while dieting.

  • Michael Luckett

    I have had a Interlock for 9 years.  I am terrified to have it removed.  I actually like to have for peasce of mind.  Recently I was called in ( going through a divorce my ex told them I had been drinking- Get Even time.) and told that I had 15 Violation in 9 years.  Today I have benn sober for almost 7 years.  The machine would fail say 0.0476 then 55 sec later say 0.000 and give a pass.  I always figured it was a glitch.  These people say the system can't fail.  I read the article about dieting and the release of ketones.  This made sense.  I am a R and D chemist in a paint lab and we use all solvents,  Aalcohols , Esters,aromatics, add lots of KETONES.  Do you think that I may absorb enough to give a failure from time to time?  We will wipe our hands with acetone or Methyl Ethyl Ketone everyday to remove dried up paint or sticky uncured material.  These are industrial coatings for wood, plastic, and metal.  Mostly solvent based coatings.

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