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Retrograde Extrapolation: The Rising Alcohol Defense

Dayton DUI Attorney Charles Rowland > DUI Articles  > Retrograde Extrapolation: The Rising Alcohol Defense

Retrograde Extrapolation: The Rising Alcohol Defense

retrograde extrapolation

Retrograde Extrapolation: The Rising Alcohol Defense

Retrograde extrapolation is the scientific and mathematical process used by chemists and toxicologists to estimate what a person’s blood alcohol content was at a specific time based on test results obtained at a later period of time. For the DUI case, it is used to determine whether or not a driver had a BAC of 0.08 or higher at the actual time of driving based on what the BAC was at the time of testing.

Typically, we encounter retrograde extrapolation in cases where the collection of blood evidence is collected hours after the act of driving was completed. It may be due to a drivers refusal to take a chemical test. Often, in serious car crash cases, it will  take some time to assess and care for the injured.  Whatever the source of the delay, prosecutors rely on retrograde extrapolation to estimate what a driver’s blood alcohol concentration was at the time of driving. Via their expert witness, they project backwards from the information in a chemical test conducted at a later time.

The Problems With Retrograde Extrapolation

Retrograde extrapolation has many problems. The method requires the toxicologist to make a number of assumptions. In addition, they must assume that the alcohol was completely absorbed at the time of testing. In addition, that the elimination of alcohol in the DUI suspect occurred at the “average” rate. Lastly, they must assume that the driver’s blood-alcohol curve can be charted with accuracy.

DUI defenders know that the rate of alcohol absorption and elimination can be affected by a number of factors. These include the:

  • body temperature,
  • type of alcoholic beverage consumed,
  • amount and type of food consumed, and
  • weight and sex of the test subject.

Extrapolation is calculated using five factors and a general alcohol elimination rate of 0.015/hour.  In fighting your DUI charge, we may need to obtain an independent toxicologist as an expert witness. The expert will show that the blood or breath alcohol test may actually indicate that the driver had a blood alcohol level below 0.08 percent at the time of driving. The higher test results demonstrate a rising blood alcohol level. In addition, a toxicologist may be called to rebut the State’s expert testimony regarding retrograde extrapolation. In addition, your toxicologist can explain how retrograde extrapolation may be a flawed method of estimating and calculating a driver’s true BAC. This is commonly referred to as the “rising alcohol defense.”


Charles Rowland


Charles M. Rowland II has been representing the accused drunk driver for over 20 years. Contact him at (937) 318-1384 if you find yourself facing a DUI (now called OVI) charge.

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