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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