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How does a breathalyzer measure blood alcohol content

Carlisle, PA |

Can a BAC be higher than a breath aliyzer?

Attorney Answers 2


A breathalyzer measures deep lung air and provides a rough estimate of the amount of alcohol in your system, based on a scientific formula...which is not very precise. So your Blood Alcohol Content may vary somewhat, either more or less, if it is measured by other means (such as a blood or urine test). I hope this answers your question.

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Dan J. Weisenburger
Attorney at Law

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A BrAC (or breath test derived BAC) is to be distinguished from a true Blood Alcohol Content (BAC).

A BrAC is a Blood Alcohol Concentration expressed through breath. This concentration is expressed in grams of alcohol for 100 milliliters or grams of alcohol per 210 liters of breath.

The top reasons why a BrAC may over-inflate a true BAC...

1. too long exhalation time into analyzer
2. test made during the absorption phase of the blood alcohol curve when arterial venous difference in concentration of alcohol is greatest
3. presence of mouth alcohol when breath sample was analyzed
4. re-breathing of the first exhalation a number of times before analysis of ethanol
5. breath holding (hypoventilation) before sampling breath
6. elevated body temperature hyperthermia
7. non-specific method of analysis and presence of interfering substance

Conversely, the top reasons why a BrAC may under-report a true BAC...

1. hyperventilation immediately before sampling
2. chronic obstructive pulmonary disease COPD
3. analyzing breath too soon after the start of exhalation before the alveolar plateau is reached
4. testing people suffering from hypothermia
5. calculating BAC /BrAC ratios at low BrAC
6. use of low BAC/BrAC calculation factor-partition ratio

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