Carolina Criminal & DUI Lawyer BlogPolice and prosecutors in North Carolina regularly use breath or blood test results taken after someone has been driving to prove 
impairment. Questions can arise about the accuracy of the reported BAC if testing was delayed. Retrograde extrapolation attempts to estimate prior blood alcohol concentration by “back tracking,” using a combination of math and science.
Retrograde extrapolation rests on understanding the processes of how alcohol is absorbed and later eliminated by the human body – Bill Powers, DUI Defense Attorney in North Carolina
This post breaks down the basic science behind these calculations, examines how they work, and looks at what makes them both potentially reliable and questionable in different situations.
If you have specific questions about your legal matter, you may TEXT or call Bill Powers at 704-342-4357. The DWI lawyers at the Powers Law Firm help people in the Charlotte-Metro region, including legal representation for DUI charges in Iredell, Union, Gaston, Rowan, and Lincoln County, North Carolina.
Understanding the Basics of BAC
How alcohol affects each person varies based on numerous factors – from basic body composition to what and when someone last ate.
Most people understand that as a person drinks, alcohol enters the bloodstream.
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At some point, the body reaches its peak alcohol concentration.
By determining the BAC (blood alcohol concentration) or BrAC (Breath Alcohol Concentration) and applying scientific formulas, toxicologists attempt to determine what those levels were at an earlier time.
What is Retrograde Extrapolation?
DUI charges may involve breath or blood tests taken after a traffic stop or wreck.
In North Carolina, the DUI law allows test results of 0.08 or higher “at any relevant time after driving” as evidence of impairment.
But what happens when test results are below 0.08?
What happens if testing took place at some time after the stop or accident to such extent that the reported BAC may not accurately reflect the blood alcohol concentration?
This is where retrograde extrapolation can enter the picture, potentially complicating the process.
What is the Rising BAC Defense?
Think of retrograde extrapolation like rewinding a movie.
The process uses math and science to work backward through time from a known test result.
Prosecutors turn to this method when they believe alcohol levels were higher at the time someone was driving than when testing occurred later.
The basic concept follows how alcohol moves through the body.
Blood alcohol levels rise as drinks enter the system, reach a peak, then fall as the liver processes alcohol out.
A breath test or blood draw captures one point on this timeline.
Retrograde extrapolation takes that number and attempts to calculate backward, suggesting what the levels might have been earlier – Bill Powers, DWI Defense Attorney in North Carolina
The human body processes alcohol based on several factors.
Food in the stomach affects how alcohol enters the blood. The liver breaks down alcohol at different rates.
Body composition changes how alcohol spreads through the system.
Medical conditions impact processing times.
Retrograde extrapolation tries to account for these factors through scientific formulas.
How Does Alcohol Move Through the Body?
The path alcohol takes through the human body resembles a river system. 
When someone takes a drink, the alcohol flows first into the stomach.
A small amount passes directly through the stomach walls into the blood, but most alcohol travels down into the small intestine.
The small intestine acts like a major tributary, where alcohol pours into the bloodstream.
Think of the bloodstream as the main channel carrying alcohol throughout the body.
The blood carries alcohol to the brain, which affects judgment, coordination, and reaction times.
The same blood flows through the liver – the body’s natural filtration system.
The liver breaks down alcohol at its own pace, turning it into water and carbon dioxide.
This river of alcohol rises and falls based on timing.
The flow rises as new drinks add more alcohol into the bloodstream.
At some point after the last drink, the river crests at its highest level.
Then the liver’s filtering process takes over, and the alcohol level starts dropping as the liver continues breaking down alcohol into water.
Food can change how fast alcohol enters this river system.
A meal in the stomach works like a dam, slowing down how quickly alcohol moves into the small intestine.
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This explains why drinking on an empty stomach can lead to faster effects – the alcohol rushes straight through with nothing to slow it down.
The type of drink matters too.
Beer and wine contain other substances that can slow absorption compared to straight liquor.
The Science Behind Back-Tracking BAC Levels
- Snapshot in Time: Both blood and breath tests provide a snapshot of your blood alcohol concentration (BAC) at a specific moment. This is analogous to taking a photo of a moving car – it captures a single moment in time, but doesn’t reflect the car’s speed or direction at other points.
- Measurement Units:
- Blood Test: Measures BAC in grams of alcohol per 100 milliliters of blood. For example, a BAC of 0.08% means there are 0.08 grams of alcohol in 100 milliliters of blood.
- Breath Test: Measures BrAC (Breath Alcohol Concentration) in grams of alcohol per 210 liters of breath. This is because there’s an estimated ratio between the amount of alcohol in the blood and the breath.
In essence, both tests provide a snapshot of your BAC at a specific moment, and the results might be used to determine if you are legally impaired.
Alcohol Elimination and Back-Calculation
The human body eliminates alcohol at a relatively constant rate, typically around 0.015% per hour.
However, this rate can vary depending on factors like body weight, metabolism, and liver function.
How Do Breath Test Machines Work?
It’s important to note that this rate is not linear, and the body’s ability to process alcohol can be affected by various factors, including food intake and overall health.
Back-Calculation: An Imperfect Science 
Retrograde Extrapolation, or what some refer to as “back-calculation,” is a method used to estimate a person’s blood alcohol concentration (BAC) at a specific time in the past, often the time of driving.
It involves taking a measured BAC and working backward to determine the estimated BAC at an earlier time.
While back-calculation can be a useful tool in some instances, it’s important to understand its limitations. It relies on several assumptions:
- Complete Absorption: The calculation assumes that all alcohol has been fully absorbed into the bloodstream.
- Constant Elimination Rate: It assumes a steady rate of alcohol elimination over time, which may not always be accurate.
- Linear Decline: The calculation often assumes a linear decrease in BAC over time, which might not perfectly reflect the body’s complex metabolism of alcohol.
These assumptions can introduce inaccuracies into the back-calculation, especially in cases where the person’s drinking pattern was irregular, or they had consumed alcohol over an extended period.
It’s important to remember that back-calculation is an estimate, not an exact science.
While it can provide valuable insights, it should be interpreted with caution and considered alongside other evidence of impairment.
Factors Affecting Blood Alcohol Concentration (BAC)
Factors such as food consumption, body composition, metabolism, and drinking patterns can significantly influence blood alcohol concentration (BAC) levels.
The rate at which the liver metabolizes alcohol varies from person to person.
Factors such as genetics, liver health, and overall metabolism can influence this rate.
Certain medications and underlying health conditions can also affect the body’s ability to process alcohol.
The Science Behind DUI Testing and Retrograde Extrapolation
Understanding the science behind DUI testing and retrograde extrapolation is crucial in navigating the complexities of DUI cases. While breath and blood tests provide snapshots of a driver’s blood alcohol concentration (BAC) at a specific moment in time, the accuracy of these tests and subsequent calculations can be influenced by various factors.
Factors such as body weight, metabolism, and the presence of food in the stomach can significantly impact how quickly alcohol is absorbed and metabolized. Additionally, the timing of the test relative to the time of driving can affect the accuracy of back-calculation estimates.
It’s important to note that retrograde extrapolation, while a useful tool, is not without limitations. The accuracy of the calculations relies on several assumptions, including a constant rate of alcohol elimination and complete absorption of alcohol into the bloodstream. However, individual variations in these factors can influence the results.
Therefore, while scientific methods and calculations can provide valuable insights, it’s essential to consider the limitations and potential inaccuracies associated with retrograde extrapolation. Ultimately, each DUI case is unique, and a thorough understanding of the scientific principles involved is important.
If you have specific questions about your legal matter, you may TEXT or call Bill Powers at 704-342-4357.
The DWI lawyers at the Powers Law Firm help people in the Charlotte-Metro region, including legal representation for DUI charges in Iredell, Union, Gaston, Rowan, and Lincoln County, North Carolina.