Retrograde Extrapolation

Using Retrograde Extrapolation is Unfair in a DUI Trial

Prosecutors often invoke retrograde extrapolation in the course of trying a drunk driving case in Florida. This mathematical process is used to argue that a suspect’s blood alcohol concentration was higher at the time of arrest than it was at the time of blood or breath testing.

While based on scientific principles, retrograde extrapolation does not comprehensively take into consideration how different bodies process alcohol at varying rates. Therefore, it is not a 100 percent accurate process.

When does retrograde extrapolation come into play? 

Retrograde extrapolation is relevant particularly in cases where a DUI suspect’s blood or breath test registered below the level of legal intoxication in Florida. The level at which a person is legally intoxicated in Florida is when his or her blood alcohol level (BAL) or blood alcohol concentration (BAC) measures 0.08 or higher.

A prosecutor may try to argue retrograde extrapolation in the instance of a DUI suspect who was not issued a breath or blood test until several hours after arrest. This assumes that a person’s BAC level was presumably higher at the time of arrest.

An Explanation of Retrograde Extrapolation

Retrograde extrapolation uses a mathematical equation to calculate an educated guess about a person’s BAC at the time of arrest. The prosecution often will enlist state experts to provide the jury with a retrograde extrapolation based upon factors that include: 

  • BAC at the time of breath or blood test;
  • an average hourly rate of alcohol elimination (or how quickly/slowly a person’s body eliminates alcohol from the blood); and
  • time elapsed between arrest and test.

Other factors impact the potential accuracy of retrograde extrapolation, including the volume of alcohol consumed, the person’s body weight/composition, how long the alcohol was consumed, and patterns of alcohol consumption.

Potential Flaws in Using Retrograde Extrapolation in the Prosecution of DUI Defendants 

It is important to note that retrograde extrapolation is based upon an average of the general population. By prosecutors’ and scientists’ own admission, these formulas do not take personal variations into account.

Researchers for the College of Public Health out of the University of South Florida concluded that retrograde extrapolation is based on sound scientific principles and can produce reasonable assumptions about a person’s prior BAC. That said, the researchers stressed that the extrapolations were accurate when applied in a forensic setting under controlled circumstances. This is typically not the case when a DUI suspect is tested for BAC.

In order to accurately determine retrograde extrapolation, it would be necessary to know exactly: 

  • how much alcohol the DUI suspect consumed;
  • what was consumed and the rate of consumption;
  • the time of consumption of each and every drink; and
  • the individual’s personal rate of alcohol elimination.

Police reports for DUI do not typically include such detailed information about a suspect’s circumstances prior to arrest. Therefore, the state’s evidence of retrograde extrapolation typically is flawed in principle.

Defending Against the Use of Retrograde Extrapolation 

A DUI defense may assert that the potential flaws and unreliability of retrograde extrapolation render it useless in a trial. The defense may also argue that only the blood test itself is admissible evidence, though it has been shown that even these tests can have critical flaws.

If you have been charged with DUI and the prosecutor has argued using retrograde extrapolation, contact a defense attorney. Call 877-663-5110.