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Walking Back the Dog....

A retrograde extrapolation is a "scientific" method in which the State attempts to determine a Blood or Breath Alcohol Concentration (BAC) at some earlier time when a blood or breath test is taken at a later time. Knowing, for example that a person's blood was drawn 2 hours and 15 minutes after driving, and that the BAC is 0.10 grams/100mL of blood, it is theoretically possible to determine what the BAC was AT THE TIME OF DRIVING. I say theoretically, because first, lab technicians are notoriously inept at retrograde extrapolation, and second, the "science" which underpins the process is sketchy and open to attack.

Generally, the attempt to do a retrograde extrapolation is made after only one sample of blood or breath is obtained and for whatever reason, the sample is taken outside the 2 hour window permitted under Nevada law for a per se illegal status. The normal scenario goes something like this: A motorist is discovered at the scene of an accident in a rurual area of the county and after determining that he is not injured, he is transported to the jail for evidentiary testing. The motorist requests a blood test, and because the phlebotomist is not onscene at the jail, he is transported to a local hospital for blood testing. Despite the officer's best efforts, the blood sample is drawn 2 hours and 5 minutes after the motorist was discovered in his vehicle, in actual physical control of it.

The State wants to have the lab use the value of the BAC obtained from the blood draw used to establish a theoretical BAC which occurred at the time of driving. Now, if the blood had been obtained within the 2 hour window, there would be no problem with using the BAC because Nevada law permits a conviction if a person measures over 0.08 grams/100mL of blood 2 hours after driving. Here, however, the sample is obtained beyond that 2 hour mark, the only way to use a BAC is to manufacture one at an earlier time. The thinking is that if the BAC was illegal at time A, then it is possible to show that it was also illegal at time B. This thinking is actually flawed in most instances.

In order to make an accurate retrograde extrapolation, there are certain assumptions or facts that are required:

1. The measured BAC must be assumed to be accurate;
2. No alcohol was consumed between the time of the blood draw and the time being extrapolated back to (in this example, 2 hours and 5 minutes earlier);
3. No alcohol is being absorbed between time of the blood draw and the time being extrapolated back to; and
4. The rate of alcohol metabolism is same as members of the normal population and is consistent over the time period being extrapolated back to.

As a DUI defense attorney, I know that these conditions and assumptions are just not realistic in most cases and in a case where the State wants to "walk back the dog" to the time of driving, they will face an uphill battle.