Retrograde extrapolation in North Carolina DWI cases
How an expert may estimate an earlier alcohol concentration—and why timing, absorption and assumptions control the reliability of the opinion.
What retrograde extrapolation is
Retrograde extrapolation is an expert method used to estimate a person’s alcohol concentration at an earlier time from a later breath or blood result. The calculation works backward using an assumed or supported alcohol-elimination rate and a timeline of driving, drinking and testing.
The method is not automatically valid or invalid. Its usefulness depends on whether the person was in the post-absorption elimination phase, the quality of the timeline, the range of elimination rates considered, the reliability of the chemical result and how transparently the expert explains uncertainty.
Absorption complicates backward estimates
If alcohol was still being absorbed at the relevant time, a later result can be higher than the concentration while driving. Food, drink type, consumption pattern, physiology and medical conditions may affect absorption. An expert who lacks reliable inputs should identify a range or limitation rather than present false precision.
How the opinion is tested
Counsel may examine the expert’s qualifications, source data, assumptions, calculation method, alternative scenarios, error ranges and whether the opinion fits the facts actually in evidence. Cross-examination should focus on scientific foundations rather than labels or rhetoric.
Frequently asked questions
Does a later .08 automatically prove a .08 while driving?
No. The State must connect the result to the legally relevant time and theory; timing and absorption may matter.
Can an expert give a range instead of one number?
Yes. A range may better reflect variation in elimination rates and uncertain inputs.
Is drinking history always reliable?
No. It must be evaluated against receipts, video, witnesses, timing and other available evidence.