Two Blood Samples Do Not Prove a Universal Alcohol Elimination Rate
By THE OKOROCHA FIRM*
Two Consecutive Blood Samples Do Not Automatically Establish a Universal Alcohol Elimination Rate
Two blood samples can produce a slope. They cannot make the assumptions behind that slope disappear.
In alcohol cases, an analyst may subtract the second blood-alcohol concentration from the first and divide by the elapsed time. The calculation estimates the rate at which the measured concentration changed between those two collection events. It is tempting to call that number the person’s elimination rate and carry it backward to an earlier time.
That move is not automatic. It depends on timing, the phase of the alcohol curve, the accuracy of collection records, and the assumption that the observed later slope fairly represents the earlier interval.
A published study by M. Simic and M. Tasic illustrates why the issue deserves more than a default number.
What the two-sample calculation actually measures
Assume the first specimen reports a higher alcohol concentration than the second. The difference between the two results, divided by the time between draws, gives an apparent hourly decline. That is a description of the two measured points.
It does not independently prove:
- When drinking stopped
- Whether absorption was complete at the first draw
- Whether the person was on a stable terminal elimination slope
- Whether the recorded collection times are exact
- Whether both specimens were handled and analyzed without material error
- Whether the same slope applied before the first specimen
Those are factual and scientific predicates, not details to be assumed after the arithmetic is complete.
For the broader role of timing and pharmacokinetic assumptions, see A Toxicology Lawyer Explains Pharmacokinetic Variability and Breath-to-Blood Conversion: Scientific Limits.
What the 2007 study found
Simic and Tasic analyzed 2,023 cases involving two consecutive blood specimens collected from suspected impaired drivers. Their reported first-sample concentrations ranged from 0.1 to 2.99 g/kg. For the 1,198 cases in which the second sample was collected at least one hour later, the overall mean apparent elimination rate was 0.221 g/kg per hour.
The more important finding was variability. When cases were grouped by initial concentration, the mean apparent rate increased across the reported BAC ranges. The paper concluded that the beta-slope depended on BAC and that use of a standard rate versus a BAC-corrected approach produced statistically significant differences in retrograde calculations. Those differences became larger at higher measured concentrations and over longer extrapolation intervals.
The study also reported substantial variation within a narrow BAC range. In the 1.4 to 1.49 g/kg group, for example, apparent rates ranged from 0.14 to 0.353 g/kg per hour. That spread is a warning against presenting an average as though it were an individual biological constant.
An average describes a group. A case-specific opinion concerns one person, one timeline, and one set of specimens.
The absorption-phase problem
The most important limitation is phase identification. Alcohol concentration does not begin declining the moment a person takes the last drink. Absorption may continue, the curve may flatten temporarily, and food can alter the timing and shape of the concentration profile.
Simic and Tasic attempted to reduce this problem by analyzing cases in which at least one hour had passed between reported drinking and blood sampling. Even so, the authors expressly acknowledged that some subjects may not have fully entered the elimination phase at the first draw. Some could have been in a plateau phase.
That admission matters. If the first point is not on the terminal elimination portion of the curve, the slope between the two samples is not a clean measure of terminal elimination. A simple line drawn backward may misdescribe what happened before the first collection.
The question is not whether alcohol generally eliminates over time. The question is whether the evidence establishes which portion of the curve applied during the interval being reconstructed.
Timing and laboratory records control the calculation
The paper also identified the accuracy of recorded sampling times as a major limitation. A two-point calculation is sensitive to time because elapsed time is the denominator. A small timing error can materially alter the calculated hourly rate when the interval between samples is short.
A serious review should obtain the source records rather than rely only on a summary report. Those records may include phlebotomy times, collection labels, accession timestamps, chain-of-custody documents, run sequences, calibrations, quality controls, replicate data, chromatograms, and instrument audit trails.
If the collections came from different facilities or were tested by different methods, comparability must also be addressed. A numerical difference between reports may contain both biological change and measurement uncertainty.
See Forensic Toxicology Case Review Checklist and Toxicology Laboratory Packet Review Guide.
Why retrograde extrapolation needs an explicit foundation
Retrograde extrapolation estimates an earlier alcohol concentration from a later measurement. The result depends on the selected starting concentration, time interval, assumed phase, and elimination rate. Change any of those inputs and the estimate changes.
The 2007 paper supports the proposition that elimination behavior is not captured perfectly by one universal zero-order number across all concentrations and all people. It does not supply a license to substitute the paper’s group equation for individualized evidence. Its data came from a defined case population and its own acknowledged limitations travel with the results.
A transparent opinion should therefore state:
- Which specimens and times were used.
- Why absorption was considered complete.
- Which elimination rate or range was selected.
- Whether measurement uncertainty was included.
- How sensitive the conclusion is to reasonable alternative assumptions.
Without that foundation, the calculation can look more certain than the underlying evidence.
The bottom line
Two consecutive blood samples are valuable evidence. They can show how the reported concentrations differed over the measured interval. They do not automatically prove a universal elimination rate, identify the earlier phase of the alcohol curve, or justify carrying one slope backward without limits.
The arithmetic is the easy part. The scientific work is deciding whether the two points represent the biological process the opinion claims to reconstruct.
Source
M. Simic and M. Tasic, “The relationship between alcohol elimination rate and increasing blood alcohol concentration – calculated from two consecutive blood specimens,” Forensic Science International 172(1) (2007) 28-32. PubMed record. DOI: 10.1016/j.forsciint.2006.11.008.
