The Complex World of DUI Prosecution: The Controversy of Retrograde Extrapolation

Driving under the influence (DUI) cases often involve intricate scientific methods to determine a suspect’s blood alcohol content (BAC). One such method that is widely used by law enforcement is retrograde extrapolation. This complex technique involves estimating a person’s past BAC based on the assumption of a constant alcohol metabolism rate. While it is a standard tool in DUI cases, it raises questions about the reliability of such estimates and whether they align with the legal principle of proving guilt “beyond a reasonable doubt.”

The consensus in the field is that the human body metabolizes alcohol at a rate of approximately 0.015 per hour. This rate is the foundation for retrograde extrapolation, a process used when a suspect has not undergone blood testing but is suspected of DUI. In such cases, law enforcement relies on this technique to estimate the individual’s BAC at the time of the alleged offense.

Consider a scenario where a driver refuses testing during a DUI stop and is subsequently taken to the police station for testing two hours later. The recorded BAC at the station is 0.06. Utilizing retrograde extrapolation, law enforcement may assert that, at the initial stop, the driver’s BAC exceeded the legal limit at 0.08.

However, this seemingly straightforward process becomes far more complex when one delves into the various factors influencing alcohol metabolism. To accurately extrapolate a person’s past impairment, several crucial variables must be considered:

1. Accuracy of Initial Results: The reliability of the initial BAC results, often obtained through breathalyzer tests, can be compromised by substances that interfere with the testing process.
2. Individual Characteristics: Factors such as gender, weight, lean body mass, age, accurate height, and mental state play a significant role in how quickly an individual metabolizes alcohol.
3. Type of Alcohol Consumed: Different alcoholic beverages have varying alcohol content, affecting how quickly the body processes the substance.
4. Consumption Conditions: Whether the person had food in their stomach, the amount of food consumed, the start and end times of drinking, the drinking pace, and whether the drinks were mixed all contribute to the complexity of retrograde extrapolation.
5. Health Conditions: Gastrointestinal conditions can impact how food and drink are absorbed, further complicating the accuracy of retrograde extrapolation.