Two blood samples do not establish a universal alcohol-elimination rate.

AI-assisted literature summary. Primary supplied source: Simic and Tasic, 2007. No personal review or adoption of this article by Okorie Okorocha is represented. Editorial policy.

A calculation can be arithmetically correct and still answer the wrong question. If two blood samples show a decrease in ethanol concentration, the difference divided by the time between collection gives an apparent rate of decline during that interval. It does not, by itself, directly measure the concentration at an earlier event.

What the supplied study examined

Simic and Tasic reviewed records from the Institute of Forensic Medicine in Novi Sad, Serbia, for 1991 through 2005. Their 2007 paper reports 2,023 cases involving two consecutive blood specimens and 1,198 cases retained for analysis after exclusions. The laboratory used headspace gas chromatography. The collection protocol used fluoride-containing tubes and recorded the collection times.

The methods describe excluding cases with no alcohol concentration and cases sampled within one hour after the event. Most sample pairs were separated by an hour; other pairs were collected 30 to 59 minutes apart and the difference was scaled to an hourly rate. That distinction matters: a concentration difference is not automatically an hourly elimination rate.

The authors grouped the initial blood concentrations and compared the apparent rates within and between groups. They reported higher mean apparent rates in higher concentration groups and proposed a concentration-dependent model. These are findings from a historical, selected case series, not a universal rule for every person or every forensic calculation. Read the original publication.

What two samples can calculate

Apparent decline per hour =
(first concentration – second concentration) / elapsed hours

For an illustration, not a real case: a first result of 0.100 g/dL and a second result of 0.085 g/dL collected exactly one hour later produce an apparent decline of 0.015 g/dL per hour. If those same numerical results were separated by only half an hour, the calculated hourly decline would instead be 0.030 g/dL per hour.

Neither example establishes that the person was eliminating alcohol at that rate before the first sample. Neither supplies an earlier blood alcohol concentration without further assumptions. The illustration also assumes comparable measurements and accurate times; it is not an uncertainty calculation.

The limitations are part of the result

Collection times must be reliable. The original paper identifies the accuracy of recorded sampling times as a major limitation of the two-sample method. Ask for the collection record, not merely the laboratory’s receipt time or the time the instrument ran.

Continued absorption can affect the interval. The authors acknowledged that they could not exclude continued absorption or a plateau in some subjects, even after applying their selection rules. An observed decline does not prove that absorption was completely finished throughout the earlier period being reconstructed.

The studied population matters. These were selected forensic cases from one setting, not a controlled dosing study representing all drivers or all patients. The paper reports that women made up only about 0.5% of the sample. Its results do not establish a sex-specific rule for a new person.

A group pattern is not an individual measurement. The paper describes variability within concentration bands. Choosing a group’s average does not turn that average into a measured personal elimination rate. A proposed model must be evaluated for its intended use rather than adopted because its equation produces a precise-looking number.

Why later literature still matters

A later study by Høiseth, Wiik, and Kristoffersen examined low concentrations using consecutive samples from suspected impaired drivers. It reported a reduction in mean elimination rates at low blood alcohol concentrations and again identified incomplete information about alcohol-ingestion time as a limitation. The concentration range and study population belong in the comparison. A finding at low concentrations should not be silently generalized to a different part of the concentration-time curve. Read the later primary study.

Questions to put beside the laboratory packet

1. Which times are recorded: the event, last reported drink, actual collection, laboratory receipt, and analysis?
2. Were both measurements made in the same specimen type, with comparable methods and the same units?
3. What collection, preservation, quality-control, and measurement-uncertainty information accompanies each result?
4. What evidence supports the claim that absorption was complete during the period being reconstructed?
5. Is the opinion based on the observed interval, a population range, or a selected mathematical model?
6. How does the conclusion change when the uncertain inputs or the chosen rate change?

The practical boundary

Keep the measured concentrations, the calculated interval slope, and the estimated earlier concentration separate. State the evidence supporting each step. When the record cannot resolve an assumption, the arithmetic does not resolve it either.

For the broader calculation framework, read the companion retrograde BAC extrapolation guide. To organize the supporting records, use the laboratory-packet review guide.

Sources and scope

• Simic M, Tasic M. The relationship between alcohol elimination rate and increasing blood alcohol concentration: calculated from two consecutive blood specimens. Forensic Science International. 2007;172(1):28-32. doi:10.1016/j.forsciint.2006.11.008. Supplied five-page paper reviewed for this brief.
• Høiseth G, Wiik E, Kristoffersen L. Ethanol elimination rates at low concentrations based on two consecutive blood samples. Publisher’s study page. Used for the limited comparison above, not as a replacement for a case-specific analysis.

This is an educational research brief, not a calculator, an opinion about an individual, or advice on when it is safe or lawful to drive. The supplied paper remains private working material; its PDF, figures, and tables are not reproduced here.