is the preferred method, since results are available almost instantaneously. A measurement in excess of the specific threshold level, such as a BAC of 0.08% (8 basis points), defines the criminal offense with no need to prove impairment.

In some jurisdictions, there is an aggravated category of the offense at a higher BAC level, such as 0.12%, 0.15%, or 0.25%. In many jurisdictions, police officers can conduct field tests of suspects to look for signs of intoxication.