Retrograde extrapolation is a method used by law enforcement to estimate the blood alcohol content (BAC) of a person suspected of DWI. As is often the case when a highly complicated scientific process is being performed by the police, it is often done inaccurately and is founded on shaky scientific principles. 

We’ve been ringing this bell for a long time but it bears repeating: you cannot trust that the police or the court have gotten the science right, and the bad science doesn’t stop at non-standardized sobriety tests or breathalyzers. 

Mark Thiessen of Thiessen Law Firm isn’t just the only lawyer in America who is triple Board Certified* in DWI, but he is also an ACS-CHAL Forensic-Lawyer Scientist. Mark is here to talk to you about the science (or lack thereof) behind retrograde extrapolation, and why you should never let junk science lead to a conviction for DWI. 

*Mark Thiessen’s board certifications include:

  1. Criminal Law by the Texas Board of Legal Specialization
  2. DUI Defense by the National College for DUI Defense as approved through the American Bar Association
  3. DUI Law by the DUI Defense Lawyers Association
  4. Board Certified Advocate Criminal Trial Law by the NBTA Foundation

What is retrograde extrapolation?

Retrograde extrapolation is a method used by law enforcement to determine what a DWI suspect’s BAC was before it was actually tested. Here’s an example:

Say you’re driving home at 11 PM and you are pulled over under suspicion of DWI. The police believe that you have been drinking and give you a sobriety test but can’t get you in for a breath or blood test until well after midnight, by which time your BAC will have likely dipped under the legal limit. What happens then, you walk free right? Nope. The police chemists will use the retrograde extrapolation formula to attempt to estimate what your BAC was at the time of the traffic stop.

If you can’t already tell, this stuff can be a circus. These BAC estimations are allowed as evidence in DWI cases in Texas with the addition of so-called expert testimony from a police chemist. 

How does retrograde extrapolation work?

The retrograde extrapolation formula essentially starts with your blood or breath test and adds points to your BAC based on: 

  • The time of your last drink
  • The type and number of beverages you had prior to the incident
  • The time and content of your last meal prior to the incident
  • Your age, height, weight, and sex

There is no magic formula to calculate back your BAC.  If you are still absorbing alcohol then the expert cannot retrograde extrapolate at all. Alcohol absorption can take anywhere between 18 min and 2 hours and 14 minutes. It really depends on the person, how much food they ate, their metabolism, and what they were drinking.  

Once BAC begins to fall, it declines at a rate of between 0.01 to 0.03 per hour, with the average person losing 0.015 – 0.02 per hour. As you can see the process is slow, and the rate at which you sober up varies immensely from person to person. It takes the average person at the legal limit anywhere from 4 to 10 hours to completely sober up. It just depends on how much you had to drink.

How accurate is retrograde extrapolation? 

There is no magic retrograde extrapolation calculator that adds all of this information accurately and gives a perfect readout of what a suspect’s BAC was at what time, but the larger problem is that these rely on information from the human that they are supposing is too intoxicated to be driving. The human is, however, completely capable of giving an accurate, lucid log of what they ate and drank and at what times. 

This presents a serious logical inconsistency: either the suspect is essentially not intoxicated or the police are operating on unreliable information. 

So, when should a retrograde extrapolation not be used for determining a prior BAC? We argue that retrograde extrapolation should essentially never be used because varying absorption rates and lack of verifiable information make the technique far too inaccurate. 

What is the most accurate BAC calculator? Blood tests are far more accurate than breath tests, although both present many problems and take a well-trained operation to get right. Either way, DWI costs are far too high to stake the results of your case on the police scientist. You need a lawyer who can talk toxicology, no matter the test. 

Continue reading: Can breathalyzers be wrong?

Problems with retrograde extrapolation calculation

Experts have been calling into question the accuracy of BAC as a measure of intoxication as well as the ability to accurately measure BAC for decades, so why should an estimation be any different? Some major problems with retrograde extrapolation calculations include:

  1. Absorption and elimination rates vary from as low as .008/hr to as high as .40/hr depending on the subject and are based on factors immeasurable by the experts.
  2. It is impossible to know whether or not the suspect has fully absorbed the alcohol in their system, and therefore might not be in the elimination stage of ethanol kinetics.
  3. In order to establish an accurate elimination rate, the experts would need to take timed BAC tests every hour, not just one test hour after the arrest. 
  4. The law enforcement officers have no way of knowing how many beverages were consumed by the suspect and when, nor how much they ate and when. 

This list could go on and on, but this should be enough to show you that any attempts to calculate BAC hours after the fact are generally bogus — but in order to prove that, you’ll need the best DWI lawyer in Texas that you can find. 

Booked for retrograde extrapolation intoxication? Call Thiessen Law Firm for help.

Don’t let an inaccurate retrograde extrapolation calculation wreck your case. If you or a loved one registered a BAC test that was under the legal limit, you may not be out of the woods yet as they might still try to raise it. If you have been arrested under suspicion of DWI and you don’t have an attorney, get one before it’s too late. 

Mark Thiessen of Thiessen Law Firm fights for his clients’ rights using expert knowledge of the law and science. Call Thiessen Law Firm today at (713) 864-9000 or contact us online for a free case evaluation.

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Mark Thiessen is an aggressive trial lawyer best known for his devotion to justice for his clients and high rank as a DWI Super Lawyer in Texas.