Breath Testing After “One for the Road”

Posted by Lawrence Taylor on December 4th, 2004

It’s a common situation. You’re at a restaurant, it’s been a fine meal, you’ve paid the bill and it’s time to head home. You finish off the glass of wine and head for the car.

Bad move.

A few blocks from the restaurant, you’re stopped for speeding. The officer smells the wine still on your breath and asks you to step out of the car. A few minutes later and you’re on the way to the police station — and a breathalyzer. But you know that you and your wife each had only two glasses of wine from the bottle with dinner. The charts say that at your weight your blood-alcohol level should be around .05%, so you’re well under the .08% legal limit, right?

Wrong: the reading is .10%, your license is confiscated and you are booked for DUI.

What happened?

What happened was what the toxicologists call "absorptive stage analysis". In English, your breath sample was tested while your body was still absorbing the alcohol from the last drink. Any testing during this stage of absorption will result in falsely high blood-alcohol readings.

Explanation…..Your body will continue absorbing alcohol for roughly an hour after drinking, reaching peak blood-alcohol levels sometime before that point; the presence of food in the stomach can delay this for as much as 4 hours.During this one-hour period, the alcohol is passing from the stomach and intestine into the blood, but has not yet reached a stage of "equilibrium" — that is, uniform distribution of alcohol throughout the body.

In other words, some parts of the body will have higher levels of alcohol than others — in some cases, far higher.Since the alcohol is initially passing into the arteries, arterial blood will be much higher in alcohol content than will venous blood.

Where does the alcohol come from that is being measured by the breathalyzer?

That’s right: the arteries.

Arterial blood bathes the alveolar sacs of the lungs, and alcohol diffuses into the lung air — to be breathed out into the breathalyzer mouthpiece. As one of the most noted experts in the field of forensic toxicology has observed: "Breath is not a reliable means of estimating a subject’s BAC (blood alcohol concentration)…

There is a significant likelihood that a given subject will be in the absorptive state when tasted under field conditions. Because of large differences in arterial BAC and venous BAC during absorption, breath tests consistently overestimate the result that would be obtained from a blood test — by as much as 100% or more." Simpson, "Accuracy and Precision of Breath Alcohol Measurements for Subjects in the Absorptive State", 33(6) Clinical Chemistry" 753.

In other words, if you take a breath test within an hour or so after your last drink, you will have a false high reading — possibly twice as high as your true blood-alcohol level.

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