What happens if a DWI suspect submits to a blood alcohol concentration (BAC) test but that test is completed several hours after arrest? Whereas Minnesota law criminalizes a motorist who is driving with a BAC of .08 or more at the time of driving—”or as measured within two hours of the time of driving, operating, or being in physical control” of a vehicle—it is still possible to obtain a conviction if the test was conducted after this two hour window.
Whereas many people—including some DWI defense attorneys—mistakenly believe that a person cannot be charged with DWI if police obtain a breath, blood, or urine sample beyond this two-hour stipulation, in some cases, prosecutors can calculate a motorist’s BAC at the time of driving through retrograde extrapolation. Retrograde extrapolation is a scientific guessing of sorts of someone’s BAC at an earlier point in time.
Reasons for delays in obtaining BAC measurements
Even though two hours seems to be a long time, most DWI BAC tests are conducted more than two hours after the motorist was stopped. Reasons for these delays include:
- The time needed to conduct field sobriety tests and preliminary breath test
- The time for the initial stop
- The time taken to wait for a tow truck for the suspect’s vehicle, if necessary
- The time taken to make contact with a suspect who has already arrived at another location
- The time needed to transport the suspect to the police department, hospital, or other testing location
- The time necessary to complete a search warrant application and obtain said warrant
- Extra time in cases where the breath test machine malfunctions
The science behind retrograde extrapolation
Whereas there is some scientific evidence when retrograde extrapolation is applied to alcohol consumption by utilizing known absorption and elimination rates, there are numerous other variables and considerations that come into play when trying to determine someone’s BAC level at an earlier time.
There is no standard time frame wherein a person will reach his/her peak blood alcohol concentration as it varies considerably from person to person. Many variables are necessary in order to use retrograde extrapolation correctly:
- Time since first drink
- Time since last drink
- Drinking pattern
- Type of alcoholic beverage(s)
- Whether there was food in the stomach
If these variables are known, one can reasonably estimate BAC from an earlier time. In most cases, however, police officers do not obtain this critical information, and unknown variables result in unreliable calculations.
Defenses to unreliable retrograde extrapolation
Generally, expert testimony is useful if said testimony has a reasonable basis, is relevant, assists the trier of fact, and has a probative value that outweighs any prejudice. Because police officers fail to obtain all of the aforementioned important variables, then any useful retrograde extrapolation is rendered irrelevant. Further, if the person providing the evidence is some type of forensic authority, then the potential for unfair prejudice to the defendant arises, and the evidence can be challenged and, ultimately, inadmissible in court. In one court case out of Illinois, the state court of appeals found that expert testimony relating to retrograde extrapolation, in that particular case, was not reliable.