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Is retrograde extrapolation credible?

Garden Grove, CA |

The officer pulled me over and said I was driving too fast. I tell him I had a few drinks hours ago and I end up do the FST. I believe I did very well but was still arrested for CVC 23152 (a). My blood is taken two hours after the officer stopped me. I have only been served a notice to appear in court but not officially charged of anything. What evidence do they have against me? Can they use retrograde extrapolation to guess my BAC at the time of driving and how credible is it in court?

Thank you for answering my question so far. I was asked to take a breathalyzer test after finishing the FST and asked the officer why I had to take a breathalyzer test. At this point he places me under arrest and tells me I am under arrest for suspicion of driving under the influence. Once I am in the back of the officer's car he says to me "Your refusal to take the test is admittance of guilt". He searches my car and takes me away. *I never said yes or no to any breathalyzer test*

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Attorney answers 3


Retrograde extrapolation can be used in court usually through the testimony of an expert on how the body eliminates alcohol and at what rate. Many defense lawyers attack the assumptions behind this guess but juries usually buy what the DAs expert has to say UNLESS you want to pay not only for a lawyer but an expert witness to explain why the analysis doesn't necessarily work.

The body does normally eliminate alcohol at the rate of about 1/2 to 1 drink per hour.


I don't think, a lot of science behind forensic toxicology agrees, that retrograde extrapolation is a sketchy and unreliable way of trying to estimate someones BAC at the time of driving. An Attorney can help argue this at pretrial in order to try to attain a better plea bargain, and potentially get the DUI dismissed and reduced to a lesser charge depending on the rest of the circumstances surrounding the case. Should the matter proceed to trial, the prosecutor will put on an expert that says it is credible, and the Attorney can try to illicit testimony from that expert to show it really is a guessing game and not enough on it's own to convict beyond a reasonable doubt. Depending on what your BAC level comes back at will make the argument stronger or weaker. I would advise consulting an Attorney in person to discuss your case in more detail. Did you end up doing a P.A.S. test at the scene? Also, police tend to report FST's worse than how the licensee feels they did....another issue to be debated at pretrial and potentially at trial if it goes that far.

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Thanks for your really excellent question.

Because they can't take blood at the time of driving, all blood tests use retrograde extrapolation. It's credible, but of course, it's based on the factors that go into the calculations. What you had to eat, your weight, the amount of water in your system, your sex, metabolism, etc., can all be factors, as can be the timing.

A defense expert and good attorney can show the discrepancies in various types of extrapolation and show reasonable doubt.

All the best to you.