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Can you beat the machine?

There is plenty of disagreement about the reliability of breath-testing instruments. Prosecutors argue that they are almost infallible, while defense attorneys say lots can go wrong.

But they tend to concur on one key point: It is very difficult to intentionally fool the machine.

Involuntary impacts

To whatever extent a measure of blood-alcohol concentration can be thrown off by an individual, it is generally involuntary - and it usually raises the reading.

Because the instruments detect alcohol, defense lawyers say, they can react to someone who works around ethanol (as in aircraft maintenance); to a diabetic in a dangerous state of ketoacidosis (the body may produce isopropanol); or to a person with acid reflux (alcohol in the stomach is far more concentrated than the blood level that is measured on the breath).

Prosecutors maintain that filters in the instrument flag such interfering substances.

 Intentional manipulation

Pennies under the tongue and many other claims are urban myths.

One trick that could lower the reading - at least in theory - is hyperventilating, which gets rid of some of the alcohol in your lungs, before you blow into the machine. (Holding your breath could raise it.)

But police are supposed to observe suspects for 20 minutes before the test, and then have them blow twice.

Plus, intoxication makes it harder to pull off. "I could tell a client, 'If you are arrested, do this,' but if they are drunk, they are not going to think to do these things," said Thomas E. Workman, a DUI lawyer in Massachusetts.

- Don Sapatkin