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How do they prove BAC at time of driving? (California DUI)

The BAC taken when the officer pulls someone over and administers the test may be different than the BAC at the time of driving (which may be hours earlier). So how do they prove that the BAC at time it was taken is the BAC at the time of driving for a for DUI case? (First read here for a basic introduction of DUI common offenses). Let’s start first with what is BAC.

What is BAC

BAC is the blood alcohol concentration. It is the measure of alcohol in one’s blood. The measurement is converted to a percentage of a person’s blood. Ethyl alcohol is the alcohol we drink. It is absorbed and distributed by the blood. For a chart of the progressive effect of alcohol, including when one reaches mild euphoria, see here.

How does the officer get your BAC? There are 3 ways that law enforcement get the BAC: by breath, blood, or urine. A blood sample will be the direct way of getting BAC (it is a test of alcohol in the blood, afterall). A sample by breath or urine will have to be converted with a formula to estimate the BAC. Breath test is the most common way to get BAC, but it is not the most direct way to get BAC. It is the most common because it is the portable and convenient option. Thus, the use and accuracy of the breath sample to calculate the blood alcohol can be attacked in the defense.


Alcohol Concentration Curve

Generally, alcohol absorption is described with a curve. There are 3 phases: absorptive phase, peak phase, and post-absorptive phase. The absorptive phase is where the alcohol concentration goes up. Peak phase is where it hits peak level. In Post-absorptive phase, the alcohol concentration goes downwards. An argument can be made as to when the BAC sample actually represents. Does the BAC sample represent the absorptive, peak, or post-absorptive phase? Also, many factors affect absorption of alcohol, an important one is if there was food in digestion, and what type of food and how much food.

How to show drinking “while” driving: Retrograde extrapolation

Unless the officer has a Breathalyzer on the driver while he was driving, there is technically no proof of what the driver’s BAC actually was while he was driving. When the officer takes the BAC after pulling someone over, the BAC taken is not the BAC at the time of driving.  The BAC taken must be related back to the time of driving. This is done by retrograde extrapolation. It determines, based on the time of the test and the result of the test, what the BAC of the driver should have been while he was driving. Retrograde extrapolation is a formula or ratio that takes in certain assumptions about the test subject. Retrograde extrapolation shows that based on a BAC of XX% taken YY minutes after the driver was pulled over, the driver should have had a BAC of ZZ% while driving.

Under VC § 23152(b), it is a rebuttable presumption that if a person had .08% or more BAC within 3 hours of driving, he must have had .08% or more BAC at time of the driving. This is a rebuttable presumption, meaning it is taken as true until the defense refutes it.

 VC § 23152(b) …In any prosecution under this subdivision, it is a rebuttable presumption that the person had 0.08 percent or more, by weight, of alcohol in his or her blood at the time of driving the vehicle if the person had 0.08 percent or more, by weight, of alcohol in his or her blood at the time of the performance of a chemical test within three hours after the driving.

Under this code, the prosecution is able to link the BAC test result to time of the driving without much effort. A driver who tested .08% or more BAC is presumed to have had .08% or more at the time of driving (if the test was taken within 3 hours of driving). The burden then shifts to the defense to refute this presumption. 


See a video on Youtube on How Breathalyzers Work:




About Charlotte Lin

Charlotte is a trusted lawyer by day, and legal ninja by night. In her spare time she enjoys slashing tickets with nunchucks.