What is Retrograde Extrapolation? What You Need to Know

Understanding the Science of Retrograde Extrapolation

Retrograde extrapolation is a scientific method used to estimate a suspect’s blood alcohol concentration (BAC) at a specific time in the past, typically when they were driving, based on their current BAC and various influencing factors [1.].

This process involves a formula that accounts for the alcohol elimination rate, the hours since the individual stopped drinking, and their current BAC to backtrack and determine their BAC at the time of driving.

“The aim of retrograde extrapolation in DUI or DWI cases is to determine the suspect’s BAC at the time of driving, rather than when the blood sample was taken. A variety of factors contribute significantly to this calculation, including the timing and quantity of the last drink, the period of alcohol consumption, and the person’s alcohol metabolic rate.”

The formula considers the BAC decline rate, typically between 0.015 to 0.02 percent per hour, which is crucial for assessing whether the individual was over the legal limit while driving.

However, the accuracy of this method can be affected by factors such as the person’s metabolic rate, the timing and amount of alcohol consumed, and how recently they had their last drink, making it essential to consider these variables for a reliable estimate.

When is Retrograde Extrapolation Used in DUI Cases?

Retrograde extrapolation is commonly employed when a BAC test result falls below the legal limit of 0.08 percent, which alone does not constitute evidence of a per se DUI.

If the BAC test is conducted well after the traffic stop, law enforcement may use retrograde extrapolation to argue that, although the suspect’s BAC was below the legal limit at the time of the test, it was likely above the legal limit while they were driving.

This is crucial because a BAC of 0.08 percent or higher is presumed to indicate impairment, making driving with such a BAC a direct violation of the law.

When a suspect’s BAC is below the legal limit, prosecutors must rely on additional evidence to prove impairment, which can make their case more challenging.

Problems with the Accuracy of Retrograde Extrapolation Calculations

The accuracy of using BAC to measure intoxication has long been debated, and estimating BAC through retrograde extrapolation introduces even more challenges [2.]. Key issues with this method include:

• Variable Absorption and Elimination Rates: BAC absorption and elimination rates can range from as low as 0.008/hr to as high as 0.40/hr, varying greatly between individuals due to factors that experts cannot precisely measure.
• Uncertain Alcohol Absorption Stage: It’s impossible to determine if the suspect has fully absorbed the alcohol in their system, meaning they may not yet be in the elimination phase, which is crucial for accurate extrapolation.
• Lack of Multiple BAC Tests: Accurate elimination rates would require BAC measurements taken at regular intervals, yet typically only one test is conducted, often an hour after the arrest, leading to potential inaccuracies.
• Unknown Consumption Details: Law enforcement cannot accurately determine the number of drinks consumed, the timing of consumption, or the amount and timing of food intake, all of which significantly impact BAC levels.

Challenging Expert Testimony on Retrograde Extrapolation in DUI Cases

When the prosecution attempts to introduce expert testimony on retrograde extrapolation, a skilled defense attorney can argue for its exclusion by demonstrating that the expert lacks sufficient evidence to provide a scientifically reliable estimate of the defendant’s BAC at a specific time [3.].

Key reasons why retrograde extrapolation is often deemed unreliable include:

• Uncertainty in Alcohol Absorption: It’s impossible to confirm whether the driver has fully absorbed the alcohol in their system, limiting the expert’s opinion since the individual may not be in the pure elimination phase of ethanol metabolism.
• Variable Absorption and Elimination Rates: These rates can differ significantly among individuals based on various factors that might be unknown or unmeasurable by the expert, with alcohol elimination rates ranging from as low as 0.009 to as high as 0.40.
• Lack of Multiple Timed Samples: The most accurate estimates require multiple timed blood samples, a procedure typically reserved for controlled clinical trials and not standard DUI investigations, where only two samples are taken within minutes of each other.
• Incomplete Evidence Collection: Law enforcement may fail to gather critical information such as when the drinking episode began and ended, the types and amounts of alcoholic beverages consumed, and the timing of each drink, all of which are crucial for accurate BAC estimation.
• Insufficient Data on Absorption Time: The expert might not have enough evidence to accurately estimate the absorption time for each drink or the cumulative absorption time for all drinks consumed, further undermining the reliability of the retrograde extrapolation.

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If you or a loved one was involved with these matters, you should contact our law firm immediately for a free case evaluation. You may be entitled to a settlement by filing a suit and we can help.

References:
1. https://versustexas.com/blog/retrograde-extrapolation/
2. https://www.thetexastrialattorney.com/blog/science-of-retrograde-extrapolation/#:~:text=Problems%20with%20retrograde%20extrapolation%20calculation&text=Some%20major%20problems%20with%20retrograde,factors%20immeasurable%20by%20the%20experts
3. https://criminaldefenseattorneytampa.com/dui/chemical-testing/blood-test/retrograde-extrapolation/