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How is Retrograde Extrapolation Used in DUI Cases?

Posted by Dmitry Gorin | Jan 19, 2024

If you're arrested on suspicion of DUI in California, you're required to submit to either a breath test or a blood test. However, the police know that between the time you're arrested and the time you're tested, your Blood Alcohol Content (BAC) may drop—possibly even to within acceptable levels. 

To charge you with a crime, law enforcement needs a way to calculate your BAC when you were stopped by police—especially if you declined to be tested at the scene. 

Retrograde Extrapolation in California DUI Cases

That's why California, along with many other states, allows law enforcement to use a system called retrograde extrapolation to estimate how much alcohol was in your blood at the time you were driving. Under this system, it is possible to test below the 0.08 BAC threshold and still be charged with DUI.

Simply put, retrograde extrapolation is a way to estimate a DUI suspect's blood alcohol content (BAC). Police will use this method to calculate somebody's BAC hours before they are tested. It is used in DUI cases where the test was delayed until hours after the person was pulled over.

Retrograde extrapolation is significant because it is frequently inaccurate. If you are accused of Vehicle Code 23152 VC driving under the influence, you can challenge this process and defend against the charge.

Retrograde extrapolation is not always accurate as the formula relies on a standard rate at which the human body eliminates alcohol. Most people's bodies will closely follow this rate, but not everybody. Also, the formula relies on the accuracy of the breath test or blood test itself, and both tests have margins of error and are susceptible to human error.

Thus, the primary criticism of this method is that it often yields inaccurate results—and for this reason, it's often one of the more common elements that a skilled criminal defense attorney will challenge when defending you against DUI charges.

How Does Retrograde Extrapolation Work?

Once a person stops consuming alcohol, their BAC will peak, level off, and begin to drop. On average, the rate of decrease is about 0.015 percent per hour, but it varies by individual in a range between 0.008 and 0.02 percent.  This is enough of a range for someone who had a BAC of 0.08 at the time of their arrest to drop as low as 0.065 or 0.06 within just one hour of their arrest.

How Does Retrograde Extrapolation Work?

Retrograde extrapolation is a scientifically based method that involves calculating backward from the Blood Alcohol Content (BAC) level measured at the time of testing to estimate what the BAC would have been at the time of the alleged offense. 

This process hinges on understanding the physiological and metabolic processes involved in alcohol absorption and elimination. 

By utilizing established rates of alcohol metabolism, along with the time elapsed between the incident and the test, law enforcement will extrapolate an estimated BAC at the time of arrest. If this estimate meets or exceeds 0.08 BAC, they may proceed with DUI charges.

Simply put, when someone drinks alcohol, their BAC rises. When they stop drinking, their BAC begins to level off and starts to fall. This happens slowly over the course of several hours.

Once the BAC starts to decline, it normally falls at a certain rate, meaning someone who reaches the legal BAC limit of 0.08 percent will be sober in four to ten hours. Retrograde extrapolation uses this steady rate of decline to estimate BAC in the past through a breath or blood test.

Are There Any Accuracy Concerns?

While retrograde extrapolation may sound like a precise science, it frequently yields inaccurate results, which may then result in someone being inappropriately charged with DUI. A variety of factors can influence the accuracy of these calculations, but two of the most common are:

  • Differences in the rate of alcohol absorption and elimination. Everyone absorbs alcohol at different rates, depending on numerous variables, including weight, gender, genetic factors, and even how much food the individual consumes while drinking. Standardized rates used in retrograde extrapolation may not accurately reflect an individual's unique absorption rate, leading to skewed results.
  • Differences in peak BAC. Retrograde extrapolation is often based on the assumption that all individuals hit peak BAC levels simultaneously. However, this is simply not true. Some people might reach peak BAC levels within an hour of drinking, while others might take up to three hours. Given these variations, retrograde extrapolation can sometimes produce inaccurate estimates of a person's BAC at the time of driving.
  • Timing is a crucial factor. Retrograde extrapolation presumes someone's BAC was dropping when they were arrested. Often, the suspect was drinking alcohol right before the traffic stop. Their BAC level could have risen when they were given a BAC test. Retrograde extrapolation does not consider this factor and might further inflate these numbers.

Can You Challenge Extrapolated Results?

If you're facing a DUI charge in California based on retrograde extrapolation, you can challenge these results if you believe they are inaccurate. An experienced attorney can employ several strategies to question the accuracy and reliability of the test. Some common challenges raised include, but are not necessarily limited to:

  • Calibration of the testing equipment. Your attorney may ask to have the equipment used to test your BAC evaluated. If the equipment was faulty or not properly calibrated, it could have produced inaccurate results, which would, by extension, lead to an inaccurate extrapolation.
  • Timing and procedures. Your attorney may question the timing and procedures followed during the testing. Any deviation from standard protocol could impact the results and provide grounds for dismissing the charges.
  • Expert witnesses. Your attorney may bring in expert witnesses who can testify about the inherent inaccuracies of retrograde extrapolation. They can explain the many factors that can affect alcohol absorption and metabolism, thereby casting doubt on the accuracy of the calculated BAC at the time of the alleged offense. 

Challenges to extrapolated results may be particularly effective when BAC levels are calculated near the legal limit because of the margin of error. 

When the BAC test occurs long after the traffic stop, police will use retrograde extrapolation. This allows them to argue that while you were under the legal limit when arrested, you were over the legal limit while driving.

This matters because drivers with a BAC of 0.08 percent or above are presumed to be under the influence. If your BAC is below the legal limit, prosecutors must prove you were impaired with other evidence, which makes their case difficult.

If your attorney prevails in such a challenge, the DUI charges may be dropped because the extrapolated results cannot prove beyond a reasonable doubt that your BAC was above the legal limit at the time of your arrest. Contact us for a case review. Eisner Gorin LLP has offices in Los Angeles, CA.

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About the Author

Dmitry Gorin

Dmitry Gorin is a licensed attorney, who has been involved in criminal trial work and pretrial litigation since 1994. Before becoming partner in Eisner Gorin LLP, Mr. Gorin was a Senior Deputy District Attorney in Los Angeles Courts for more than ten years. As a criminal tri...

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