Attorney at Law


James Parker
February 16, 2022

What Is Extrapolation?

Retrograde extrapolation, or just extrapolation, is a process by which an individual’s blood alcohol content (BAC) can be estimated at a given time by analyzing their BAC at a different time. Retrograde extrapolation can be used in clinical medicine, pharmacology, or toxicology. 

At its core, retrograde extrapolation involves applying a mathematical formula derived from a series of assumed average values. The most accurate retrograde extrapolations are performed by experts with a highly accurate starting BAC, ideally a blood test, who can take into account the various factors that impact the rate of alcohol absorption.

In addition to using an average value for absorption, retrograde extrapolation is most accurate when it is extrapolating back to the peak BAC value, the theoretical maximum BAC of an individual. If, however, an individual has not yet reached their peak BAC for whatever reason, the formula is far less accurate and will give a wide range of possible results.

This means that although retrograde extrapolation can be useful, it is not immune to simple errors like miscalculating when someone last drank alcohol or whether or not their absorption rate has reached its maximum value. These things can be hard to know without multiple samples over a significant period of time or the accurate memory of the subject being examined.

Key Takeaways

  • Retrograde extrapolation is a tool for estimating BAC at a specific time by reverse engineering a BAC taken at a given time. 
  • Retrograde extrapolation uses a set of assumed average values for alcohol absorption and calculates BAC using a mathematical formula.
  • While useful in theoretical situations, retrograde extrapolation can fail to account for the individual factors that may result in it wrongfully implying the guilt of an individual accused of a DUI.
  • If you are facing DUI charges because of retrograde extrapolation, an experienced DUI attorney may be able to improve the outcome of your case by leveraging experience and expert testimony.

Extrapolation and Driving Under the Influence (DUI) Charges

When it is being used clinically, retrograde extrapolation can be useful even if it isn’t perfectly accurate. However, when being used to determine the future of an individual’s freedom, financial status, and right to drive, the cracks in the system can begin to show.

A retrograde extrapolation is simply a mathematical formula. A formula can theoretically be applied without a limit and if the formula is simply a matter of determining how much something increased over time, it could be theorized that a defendant was far more intoxicated than they ever were. 

For example, if a defendant drank only a single beer or glass of wine and their breathalyzer showed they were below the legal limit, the use of retrograde extrapolation could still show mathematically that they could have been far more intoxicated earlier. Since the premise of the use of retrograde extrapolation is to prove that the defendant is lying and that they were more intoxicated before they were pulled over or before they submitted to a BAC test, it can create a false impression. 

Another issue with retrograde extrapolation is that it is based on average rates of absorption. However, small changes can massively impact the rates of absorption and peak BAC as a result. According to the University of Notre Dame, if an individual does not eat before drinking alcohol, they will typically reach maximum BAC levels in 30 - 120 minutes. If, however, that individual has eaten beforehand, that window expands to 1 - 6 hours. This means that if an individual had a drink with their dinner 3 hours ago, they could still have theoretically not fully absorbed the alcohol into their bodies while it could already be mostly broken down if they had not eaten.

These alternatives are not considered by retrograde extrapolation because it is a simple mathematical formula designed to present what could be possible. This potential truth is then submitted to the court as an alternative to show how even though a driver is below the legal limit at the time they were found by an officer, they could have been breaking the law earlier when there was not an officer present. This idea can fundamentally oppose the understood stance of criminal justice that a defendant is presumed innocent until found guilty.

Bottom Line

Retrograde extrapolation is not a wholly useless tool. It can analyze in a theoretical sense, the BAC of a theoretical average person. In reality, however, people are not a collection of average values. Alcohol absorption rates can be affected by fat content, age, height, biological sex, alcohol type, and whether or not the individual ate recently. 

If you are being charged with a DUI on the strength of a retrograde extrapolation, contacting an experienced DUI attorney can help your case. A DUI attorney can use their trial knowledge and expert witnesses to dispute the accuracy of retrograde extrapolation by examining the flaws in its reasoning, establishing a correct timeline, and demonstrating mitigating factors that may have obscured the calculations of the extrapolation.

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