Retrograde Extrapolation

The prosecutor or defense attorney might seek to introduce expert testimony regarding retrograde extrapolation in a DUI-related case.

Retrograde extrapolation is a methodology of determining a person’s blood alcohol level (BAL) at an earlier time by multiplying the average rate at which alcohol is eliminated from the body by the amount of time between the driving and the blood test and adding that product to the person’s known BAL. See Vitiello v. State, 281 So. 3d 554, 558 (Fla. 5th DCA 2019).

In this way, retrograde extrapolation can be used to estimate the blood alcohol concentration at the time of driving (as opposed to the time the blood sample was obtained).

Because the BAL will rise or fall over time, expert testimony on this topic often influences the verdict at trial.

Our attorneys have experience in using the services of experts to present evidence about retrograde extrapolation.

We are also experienced in showing problems with the state’s evidence when it falls short of scientific reliability because of unreasonable and unjustifiable assumptions.

Attorney for Retrograde Extrapolation in DUI Cases

If the prosecutor in your case is attempting to admit evidence of retrograde extrapolation in your DUI case in Tampa, FL, contact an experienced DUI defense attorney at Sammis Law Firm.

Our attorneys fight DUI cases in Tampa, FL, and the surrounding areas of Tampa Bay.

Our main office is in downtown Tampa in Hillsborough County. We also have offices in New Port Richey in Pasco County and Clearwater in Pinellas County, FL.

We represent clients charged with DUI after a blood test, including both the medical blood test to determine the BAC or a legal blood test after the sample was drawn at the request of a law enforcement officer.

Contact us to discuss the best ways to fight these complex DUI cases.

Call (813) 250-0500.

Way to Attack the Prosecutor’s Evidence about Retrograde Extrapolation

When the State attempts to present expert testimony on the retrograde extrapolation, the Defense Attorney can seek to exclude that testimony by showing that the expert does not have sufficient evidence to provide a scientifically reliable estimate of BAC at a given point in time.

Many experts have found important reasons that retrograde extrapolation is unreliable, including:

  1. Since it is impossible to know whether the driver has fully absorbed the alcohol in their system, the expert’s opinion is limited because the subject might not be in the pure elimination phase of ethanol kinetics.
  2. Absorption and elimination rates vary for different people based on various factors that might be unknown to the expert.
  3. The elimination rate of alcohol can vary between subjects from as low as .009 to as high as about .40 at the extreme ranges.
  4. The most accurate estimates are based on multiple, timed blood samples to be taken from the subject, which only happens in controlled clinical trials and not during DUI investigations involving only two samples being taken within minutes of each other.
  5. The law enforcement officer might have failed to gather evidence showing when the drinking episode began and ended, what types of alcoholic beverages were consumed, the number of drinks consumed, the amount of alcohol in each drink, and the time each drink was consumed.
  6. The expert might not have sufficient evidence to estimate the absorption time for each drink or the cumulative absorption time for all the drinks.

FDLE’s Review for Retrograde Extrapolation

A recent version of the Toxicology Standard Operating Procedures was issued by FDLE’s Forensic Quality Manager on January 13, 2015, in version TOXSOP15. Unit 3 of the TOXSOP15 deals with technical calculations review and retrograde extrapolation.

The procedures in the Toxicology Standard Operating Procedures apply to retrograde extrapolation calculations performed in the laboratory. The requirements include:

A. Before performing any retrograde extrapolation calculations, the analyst shall collect all required information. The following is the minimum required information to perform a retrograde extrapolation calculation:

1. An FDLE-determined blood alcohol concentration result from a specimen submitted to the FDLE laboratory. Retrograde extrapolation calculations will not be performed on alcohol levels of 0.030 g/100mL or less.

2. The time elapsed between the incident and the blood specimen collection.

3. An estimate of the time of the last drink.

B. If all of the required information above is not provided, a note will be placed in the narrative of the LIMS that the service was requested but could not be performed.

C. If a calculation is to be completed, the analyst should attempt to obtain the following information that may have an impact on calculations and/or interpretations:

1. The drinking history before the incident;

2. The type and number of beverages that were being consumed by the subject;

3. The time and content of the subject’s last meal before the incident; and

4. The age, height, weight, and sex of the subject.

D. The FDLE Retrograde Extrapolation Request Form shall be utilized to document all information collected and shall be retained in the case file.

E. A report shall be issued showing the result of the retrograde extrapolation calculation rounded to two (2) decimal places using conventional rounding. Suppose the required information listed in ‘A’ is provided but deemed inappropriate for performing a retrograde extrapolation calculation. In that case, a report shall be issued stating that the retrograde extrapolation could not be performed and why.

 F. Before the release of retrograde extrapolation results, the calculations, and information used to perform calculations shall go through technical and administrative review. The reviews will cover at least the following:

1. The information used to perform the calculations;

2. The appropriateness of performing the calculation;

3. The appropriateness of elimination rate(s), the volume of distribution(s), and other factors used in calculations;

4. The arithmetic; and

5. The scientific basis of the opinion(s).

G. The case record shall include the following:

1. Who the analyst obtained information from and the date(s) it was obtained;

2. All information listed above that the analyst was able to obtain;

3. Elimination rate(s), the volume of distribution(s), or other factors the analyst used in calculations;

4. Detailed calculations and date(s) that the calculations were performed;

5. A clear indication of the analyst’s opinion(s); and

6. A record of the administrative and technical review, including who conducted the review and the date that the review was done.

Select References:

1. Medicolegal Aspects of Alcohol, James C. Garriott, editor. Lawyers & Judges Publishing Company, Inc.

2. Principles and Application of Medicolegal Alcohol Determination, Widmark. Biomedical Publications.

3. “Pharmacokinetics of Ethanol – Issues of Forensic Importance,” A.W. Jones, Forensic Science Review, V.23 (2), 2011.

4. “Calculation of Blood Alcohol Concentration (BAC) by Sex, Weight, Number of Drinks, and Time,” Fisher et al., Canadian Journal of Public Health, 78, 1987.

A more complete list of requirements can be found in the Toxicology Training Manual.

This article was last updated on Thursday, January 5, 2023.