Drug and Criminal Defense in San Diego
Free Case Evaluations and 24/7 Availability - Call Today! 619.202.5583

What is retrograde extrapolation?

Retrograde extrapolation is fancy legalese used in DUI cases when the expert, typically the prosecutor's expert, tries to establish that at the time the suspect was driving, his or her blood alcohol content (BAC) was actually higher than it was when they provided the chemical test. That is, they use the BAC sample and extrapolate backwards to try and scientifically establish the BAC at the time the suspect was driving.

Here is an example. A DUI suspect is stopped at 7:00p.m. During the detention, the suspect chooses to perform field sobriety tests. The suspect then chooses a chemical test (VC 23612) and provides a sample of .07 BAC at 8:00p.m. At trial, the prosecutor's expert will try to extrapolate backwards from the .07 sample (to 7:00p.m.) and establish that at that time, the suspect's BAC was over .07.

The issue is what information does the expert need to accurately extrapolate and determine the correct BAC? Specifically, there is a "simple part" of the retrograde extrapolation calculation by adding between 0.01 and 0.02 per hour to the result of suspect's breath test. However, this assumes the suspect has already completely absorbed the alcohol and is in the post absorption phase – which means the suspect was eliminating the alcohol at the time of the test.

Courts have looked at various factors to determine the reliability of retrograde extrapolation.

  1. Gender
  2. Weight
  3. Age
  4. Height
  5. Mental state
  6. Type and amount of food in the stomach
  7. Type and amount of alcohol consumed
  8. Time of last drink
  9. Drinking pattern of the suspect on date of arrest
  10. Time elapsed between last drink and blood draws
  11. Number of samples taken
  12. Time between offense and blood draws
  13. Average alcohol absorption rate
  14. Average elimination rate

Retrograde extrapolation requires many variables to reliably calculate the BAC of a suspect. If you have been arrested for a DUI, contact Kern Law, APC today and receive a free consultation. I have a decade of helping those faced with DUI charges.