Retrograde extrapolation is a way to estimate a DUI suspect's blood alcohol content (BAC). Police use the method to calculate what someone's BAC was, hours before they were tested. Retrograde extrapolation is significant because it is often inaccurate. Suspects accused of driving under the influence can challenge the process and defend against a DUI charge.
What is retrograde extrapolation?
Retrograde extrapolation is a process used by chemists who work for law enforcement. It aims to determine what a suspect's BAC was, hours before it was tested.
Example: Clyde is arrested for suspected drunk driving at 9pm. By the time he is given a breath test at the police station, it is 10:30pm. Police chemists use the test from 10:30pm. Using retrograde extrapolation, they estimate Clyde's BAC at 9pm, when he was driving.
Nearly all states allow police to use retrograde extrapolation in DUI cases. It typically requires expert testimony from a police chemist. Judges allow juries to consider extrapolation evidence. California is one of these states.1
How does it work?
When people drink alcohol, their BAC rises. When they stop drinking, their BAC begins to level off, then it begins to fall. This all happens slowly and steadily over the course of several hours.
Once BAC has begun to decline, it tends to fall at a rate of between 0.008 to 0.02 percent per hour.2 This means someone who reaches the legal BAC limit of 0.08 percent will be sober in 4 to 10 hours. They would also have a BAC of between 0.06 and 0.072, one hour after peaking at the legal limit.
The process begins with the results of a BAC test. Then it applies an average rate of decline. Finally, it applies the time that passed between the test and the traffic stop. In this way, police can estimate a DUI suspect's BAC level while they were still driving.
When is it used?
Retrograde extrapolation is often used when the results of a BAC test are under the legal limit of 0.08 percent. When the reading is under the legal limit, it is not evidence of a per se DUI.
When the BAC test happened long after the traffic stop, police turn to retrograde extrapolation. This lets them to argue that, while the suspect was under the legal limit at the time of the test, they were actually over the legal limit while driving.
This matters because drivers who have a BAC of 0.08 percent or above are presumed to be under the influence. Driving with a BAC at or over the legal limit is, in itself, a violation of the law.
If a suspect's BAC is below the legal limit, prosecutors have to prove that they were impaired with other evidence, as well. This makes their case far more difficult.
Why is it unreliable?
Retrograde extrapolation is unreliable because it uses lots of assumptions. It also presumes that the DUI suspect eliminates alcohol at a normal rate. These assumptions can lead to inaccurate results if the suspect, for example:
- ate immediately before or while they drank alcohol,
- has a high tolerance for alcohol,
- drinks regularly and eliminates alcohol at a higher rate than normal, or
- drank immediately before the arrest.
The timing is the most important factor. Retrograde extrapolation presumes that a suspect's BAC was dropping at the time of the arrest. In many cases, though, the suspect was drinking alcohol mere moments before the traffic stop. Their BAC level may have still been rising when they were given a BAC test. Retrograde extrapolation does not account for this. Instead, it would inflate these numbers even further.
Example: Mary takes a shot of whiskey before driving. She is quickly pulled over, arrested, and performs a BAC test at the police station. Her BAC is still rising from the shot of whiskey. When police use retrograde extrapolation, her estimated BAC is even higher.