Retrograde Extrapolation in DUI cases


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Tag Archives: blood testing

Retrograde Extrapolation in DUI cases

Retrograde Extrapolation in DUI cases

Retrograde Extrapolation in a DUI case

What is retrograde extrapolation?

Retrograde Extrapolation in DUI cases is a very inexact method in science.  It is the method of using existing known details to retroactively in time estimate what a blood alcohol level was at a specific time in the past.

As stated above by one of the eminent experts in alcohol issues, Dr. Dubowski has stated that retrograde extrapolation can only be accurate when done “by a person who is properly qualified by education, experience, expertise, and competence, if sufficient relevant and material information concerning the subject and the events in issue exists and is available.  Whether these conditions are satisfied in a given instance is a question of fact, which cannot be generalized.”

How is retrograde extrapolation in DUI cases used?

Retrograde extrapolation is used mainly by the prosecution in a DUI case to guess, using whatever data points exist after the fact, what the blood alcohol level for a particular person was at the time of driving.  Because the time of driving is the only relevant time where the alcohol level matters, a prosecutor has to show that a person was either impaired, or above a certain alcohol level, at that time.  It’s not illegal to be drunk, or even a high blood alcohol level, in a private space (like a police station), only when driving a motor vehicle.

Even more speculatively, the prosecution sometimes uses retrograde extrapolation to estimate the amount of drinks that the driver or defendant had before getting in the car.

What are the problems with retrograde extrapolation in DUI cases?

Blood Alcohol DUI Defenses

As you might expect, estimating blood alcohol levels depend on a number of variables, and each of those variables have their own measurement problems, and margins of error.

In the human body, blood alcohol levels do not rise and fall in a steady fashion, but instead demonstrate (through scientific testing studies), spikes or steeping of the curve towards and away from peak alcohol levels.

In the downward part of the curve, returning back to sobriety, blood alcohol elimination is often assumed to be an average of 0.015 percent per hour.  However, scientific studies show great variance in the general population from 0.008 to 0.030 percent per hour, and may vary even more than that in certain individuals.

Retrograde Extrapolation in DUI cases – Factors affecting results

Experts estimating retrograde extrapolation may not have all the information they need to make an accurate calculation.  A proper estimation, at least, would have to have all the factors below:

  1. The person at issue must be in the post peak (dissipation) of alcohol. That requires that the timing of each drink is known.
  2. The calculation must know the person’s gender.
  3. We have to know the person’s weight at the time of driving.
  4. The calculation has to know the person’s lean body mass (to obtain the volume of distribution information).
  5. You must know the person’s age.
  6. The calculation should factor for the person’s true height.
  7. The calculations performed must account for the person’s mental state.
  8. The expert must know the type of alcohol that was introduced into the body.
  9. The calculation must account for the amount of food that is in the person’s stomach, and the timing of eating, before or after drinking.
  10. We have to know the type of food in the person’s stomach.
  11. The expert should know the starting time of the drinking episode.
  12. The calculation should account for the ending time of the drinking episode.
  13. The math must account for pace of the drinking episode.
  14. The retrograde extrapolation should know whether the drinks consumed were carbonated or not;
  15. The calculations done should account for the timing between last drink and time of breath test.
  16. The expert should know the number of samples taken, and the timing of each.
  17. The model used in calculation must know the latest time of any driving.
  18. The calculation should, but in almost every case doesn’t know, the particular absorptive rate for this person.
  19. The calculation should, but in almost every case doesn’t know, the particular elimination rate for this person.
  20. The expert should know of the existence of any sort of gastro-intestinal or other diseases or conditions (lap band, GERD, acid reflux, diverticulitis or interactions with any drugs used by the person).

As you can see, the calculation is complicated.  Using retrograde extrapolation to estimate a defendant’s BAC at the time of driving has been criticized by many experts in the field. This method requires the toxicologist to make a number of assumptions; namely that the alcohol was completely absorbed at the time of testing, that the elimination of alcohol in the DUI suspect occurred at the “average” rate and that the driver’s blood-alcohol curve can be charted with accuracy.

When does the defense contradict retrograde extrapolation in DUI cases?

During the pretrial stage of a case, before trial, the defense can present any reports from forensic alcohol expert witnesses experienced with DUI cases as part of the negotiations towards a plea bargain with the prosecutor.  That may end up in a dismissal of DUI charges, reduction of charges below a DUI, or plea bargain in the DUI case.

At trial, any retrograde extrapolation done by the defense expert witness in a DUI case can also be presented to the jury.  Because the prosecutor cannot interview the DUI defendant and get all the factors above, the defense retrograde extrapolation has to be more accurate than the defense’s calculations.

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DUI blood tests – the gas chromatograph

A gas chromatograph machine is what is used to test for alcohol in DUI cases where there is a blood test taken.

gas chromatograph dui blood testing

The word “chromatograph” sounds like a mouthful, but it comes from the greek words Chromato, meaning color, and graphy, meaning representation.

Chromatographs are the visual representation of the raw gas chromatograph (GC) data which is in itself inaccessible for use in determining the outcomes of the analyses. The software employed by the analytical equipment takes the raw data and make it possible to visualize the peaks, their area and height and relationship to other peaks in the samples in the form of a chromatogram. X and Y axis give the dimensions for time and response. The Forensic toxicologist is reviewing the chromatograms to report the ID of observed peaks and relative amount of the compounds peaks IDd in each samples chromatogram. The types of chromatograms that are required for every analytical run are as follows: QC, Test mixtures, Method Blanks, Matrix Blanks, absolute/solvent/gas blanks and washes. Not to mention the chromatograms associated with the development of the standard curve and calibrations used in the calculation of the AMOUNT of ethanol over a range ~ca from zero to over 0.5% . Furthermore- you must have the “batch sequence list”- the list of all samples run in that batch showing their sequence, timing and intervals for each type of NAMED injection relative to your clients sample.

QC: quality control: measures the ability of the analytical equipment to do its job to defined qualifications of sensitivity, accuracy and precision. Usually is chromatograms showing the ability of the analytical equipment (GC) to resolve known reference standards of defined amounts of Ethanol over 6 or more injections with +/- a defined value of certainty. Usually a low, 0.8 and high amount of ethanol reference standard is run at the beginning and end of each batch run.

TEST MIXTURES: Chromatography demonstrating the ability of the analytical equipment to baseline resolve each component of a complex mixture of similar compounds to those being tested in suspect samples. Used to demonstrate the ability for complete separation of known compounds, measured against their expected retention time and peak height/area under defined operational parameters. Is the machine is unable to meet the defined resolution, separation and peak heights of a mixture of known compounds, it cannot be used for analysis of unknowns.

METHOD BLANK Chromatogram: When the internal standard “IS” (usually propanol- n- or 2-) is added or “spiked” into a sample and analyzed by GC- used to demonstrate the IS is clean and free of contamination, elutes as a single baseline resolved peaks at its predicted retention time and that it does NOT interfere with the peak of the interest (ethanol).

MATRIX Blanks: using a qualified source of the matrix ( fluid) from which suspect samples are extracted (in this case blood), this known blood matrix is also spiked with the same IS in the SAME concentration to show that the matrix – blood- does NOT affect the peak retention time, peak height or area nor produce any additional peaks in the chromatogram. We mass spectrometrists usually refer to this as checking for “matrix effects”- a very well documented phenomena whereby the same compound in water/solvent behaves differently when in a “Matrix”- thus indicating there are influences of the matrix on the value of that IS in a regular Method Blank to be used as a measurand for accurate reporting of ID or amount of the unknowns.

ABSOLUTE/TRUE BLANK chromatograms: injection of either the carrier gas, methanol or nothing to show that between the sample vial and the detector there is nothing that would contribute to peaks or bumps in a chromatogram that may interfere with the analysis. Demonstrates the analytical equipment is free of any contamination from injector to detector including the column, syringe needle, spepta etc…

Chromatograms of WASHES: performed at intervals within an analytical run as per the defined frequency by that labs accreditation body. Usually it’s every 8-10 injections: used to show lack of contamination and carryover of compounds from one sample to the next when running many suspect samples sequentially in a “batch Run”. A wash is best performed as a methanol or solvent only injection- many labs however include the IS in their “washes” to show the IS peak retention time and height of the IS is consistent throughout the batch run. The inclusion of the IS peaks in this wash chromatogram however can mask or co-elute with any carryover contamination peak(s) and thus is not the best choice as a true wash.

Thus each chromatogram contains the information necessary to determine all the original intent of the analysis: peaks representing each compound found in the samples, the relative amount of each peaks contents, the time at which each peak is present in the timed sample run which can be equilibrated against other peaks to determine identity, and if the analytical equipment is capable of separation of compounds in BOTH a test mixture and of real samples which-biological in origin ( i.e., blood) are the most complicated. Chromatograms are the very essence of what is produced when blood samples are analyzed by GC. If the chromatogram is not present for an analyst to review, there can be no determination of the presence and amount of BA nor the certainty the analytical equipment is capable of such determinations.

For questions, see our Orange County DUI Survival Guide.