Retrograde Extrapolation: Guessing Backward in Time

There is almost always a gap between the time you were driving and the time your blood was drawn. It takes time to stop a car, conduct an investigation, make an arrest, and get to a place where blood can be taken. The number the lab reports describes your blood at the moment of the draw, not the moment you were behind the wheel. To bridge that gap, the State sometimes estimates backward in time, a technique called retrograde extrapolation. It rests on assumptions, and the assumptions often do not hold.

The move sounds scientific, and in the right circumstances it can be done. But it requires knowing things about your body and your drinking that the State usually does not know, and a single wrong assumption can flip the conclusion. This is a place where a confident-sounding estimate can be far shakier than it appears.

If you were still absorbing alcohol while driving, your level at the wheel could have been lower than at the test, the opposite of what a simple backward estimate assumes.

The Test Happens Later

Alcohol does not stay at one level. After you drink, the alcohol is absorbed into the bloodstream, the level rises to a peak, and then it slowly comes down as the body eliminates it. A blood test captures one point on that changing curve. The question in a DUI case is not where the curve was at the test. It is where the curve was when you were driving, which can be a very different place.

Extrapolation Assumes You Were Eliminating

To estimate backward, the State typically assumes you had already passed your peak and were in the elimination phase at the time of driving, so that your level was higher earlier and has been falling at a steady rate ever since. If that assumption is wrong, the estimate is wrong in the defendant’s favor being erased. If you were still absorbing while driving, your level at the wheel was lower than at the peak, and projecting backward with a falling-rate assumption overstates what you were at the time that matters. Whether you were absorbing or eliminating depends on when you drank, how much, and what and when you ate, which the State rarely knows with any precision.

The absorption problem, and why it cuts your way

Here is the flaw at the center of a backward calculation. To run it, the State picks an elimination rate, usually around 0.015 per hour, even though the real rate across people runs anywhere from about 0.010 to 0.025. Then it assumes your alcohol level was already falling when you drove. But alcohol takes time to absorb, and if you were still absorbing at the time of the stop, your level was rising, not falling. Run the clock backward on a rising level and you do not recover a higher past number, you invent one, because your true level while driving was lower than the reading taken later. The only way to know which phase you were in is to know when you had your last drink, what you ate, and how you were drinking that night, and those are exactly the facts the State usually cannot supply. Without them, a backward calculation is a guess wearing a decimal point.

The Rate Is Not the Same for Everyone

Even when elimination is the right phase, the rate at which a person eliminates alcohol varies. Commonly cited average rates span a wide band from person to person, and an individual’s rate depends on factors that are not captured by a single textbook number. Multiplying an assumed rate by the hours that passed builds any error in that rate into the final estimate, and the longer the gap, the larger the built-in error becomes.

Each assumption has to be true for the estimate to mean anything. The State often cannot establish even one of them with confidence.

The Missing Information

A sound extrapolation needs facts: when the last drink was, how much was consumed and over what period, whether and when food was eaten, and reliable timing for the draw. In most cases the State has incomplete answers at best, and it fills the gaps with assumptions favorable to the prosecution. When a defense puts those missing pieces in front of the court, the estimate is revealed for what it is, a projection built on a foundation the evidence does not support.

What We Do

We pin down the actual timeline, the time of driving, the time of the draw, and everything known about the drinking pattern, and we test whether an extrapolation can reliably be done at all. We read it together with the measurement uncertainty in the underlying result, because an estimate built on an uncertain number inherits that uncertainty too. Where the assumptions cannot be supported, the backward estimate should carry little weight.

Why This Matters

The level the law cares about is the level at the time of driving, and that number is not measured. It is inferred. When the inference rests on assumptions the evidence cannot support, a result that seemed to settle the case becomes an open question about what your level really was when it counted.

When the State reaches backward in time to a number it never measured, I make it show its work. I ask which elimination rate it used and why, and whether it can prove you were past absorption when you drove, because if your level was still rising, the honest math runs the other way and your driving-time level was lower. I know this science, so I do not let an assumption dressed as a calculation put alcohol in your blood at a time and amount no one measured.

Questions About Retrograde Extrapolation

What is retrograde extrapolation?

It is an estimate of what your blood alcohol level was at the time of driving, calculated backward from a test taken later. It assumes a drinking and elimination pattern, and the estimate is only as good as those assumptions.

Why is the gap between driving and testing a problem?

Because alcohol levels change over time, rising during absorption and falling during elimination. The test captures one point on that curve, but the law cares about a different point, where the curve was when you were driving.

Could my level have been lower while I was driving?

Yes. If you were still absorbing alcohol at the time of driving, your level at the wheel could have been lower than at your peak. A backward estimate that assumes a falling level can overstate what you were when it mattered.

Isn’t the elimination rate a known number?

There is an average, but real rates vary widely from person to person and depend on individual factors. Applying one textbook rate over several hours builds any error in that rate into the final estimate.

What information does a sound estimate need?

The timing and amount of drinking, whether and when food was eaten, the phase you were in, and reliable timing for the draw. The State often lacks this and fills the gaps with assumptions that favor the prosecution.

How do you challenge an extrapolation?

We establish the real timeline and what is known about the drinking, test whether the assumptions hold, and combine it with the uncertainty in the underlying result. Where the assumptions cannot be supported, the estimate should carry little weight.

Related: serum versus whole blood, measurement uncertainty, drugs and impairment, how we challenge a blood test, and the blood test overview.

Can the State calculate what my blood alcohol was while I was driving?

Only by assumption, and the assumptions can fail. Back-calculation uses an elimination rate of about 0.015 per hour, though real rates range from roughly 0.010 to 0.025, and it assumes your level was already falling. If you were still absorbing, your level was rising, so your true level while driving was lower than the later reading, not higher.

This page is general information, not legal advice, and it does not create an attorney-client relationship. Blood testing in Florida is governed by Fla. Stat. 316.193, 316.1932, and 316.1933 and the Florida Administrative Code chapter 11D-8. Procedures and rules change, and every case turns on its own facts. Past results do not guarantee a similar outcome.