In alcohol-related cases, numbers carry weight. A laboratory report might list a blood alcohol concentration (BAC) to three decimal places. It looks precise. It feels definitive. But one important question often goes unasked:
What type of sample was actually tested?
Was it whole blood?
Or serum?
That distinction can matter more than people realize.
What Are We Actually Comparing
When most people hear “blood alcohol test,” they assume all blood is the same. In reality, laboratories can analyze different components of blood.
Whole blood includes red blood cells, white blood cells, and platelets suspended in plasma – everything in its natural proportions.
Serum is the liquid portion that remains after clotting factors and cells are removed.
Both can be tested for alcohol. Both are scientifically valid specimens. But they are not identical – and because they are not identical, the reported numbers may differ.
Why the Numbers Aren’t the Same
Alcohol distributes primarily into the water portion of blood. Serum (and plasma) contains a higher percentage of water than whole blood. As a result, alcohol concentration measured in serum is typically higher than in whole blood drawn at the same time.
In practical terms, a serum alcohol result is often about 15–20% higher than a whole blood result from the same individual at the same time. That does not mean one test is wrong. It means they are measuring slightly different specimens.
Where This Issue Commonly Appears
This distinction often arises in hospital settings. Many emergency departments perform alcohol testing on serum because it fits routine chemistry workflows. That is appropriate for medical treatment. However, statutory BAC limits are typically based on whole blood concentrations. If a hospital serum result is compared directly to a statutory whole blood threshold without adjustment, the comparison may not be apples-to-apples. And that is where confusion can begin.
Questions Worth Clarifying
When reviewing a laboratory report, several basic questions can help:
- Was the result based on whole blood or serum?
- Was the sample collected for medical care or forensic purposes?
- Was any conversion factor applied?
- If so, what factor was used?
Two numbers that look similar on paper may not represent equivalent measurements if they are derived from different specimen types.
Converting Serum to Whole Blood
In many cases, a serum alcohol result can be converted to an estimated whole blood equivalent for comparison to statutory limits.
A commonly used approach divides the serum value by the ethanol distribution ratio (range 1.13 – 1.19).[1] To be conservative, using a higher ratio will reduces the risk of overestimating the blood alcohol concentration. That approach is not arbitrary. It is derived from published research examining the average relationship between serum and whole blood alcohol concentrations. Like many biological measurements, individual variation exists. But this range is widely cited because it reflects observed, reproducible differences between specimen types.
Why This Matters
For law enforcement, knowing the specimen type may determine whether additional testing is needed.
For attorneys, it can clarify whether a reported value truly aligns with statutory language.
For courts, it helps ensure that equivalent measurements are being compared.
The isn’t about challenging laboratory science. It is about understanding what was actually measured.
The Broader Point
A laboratory number does not stand alone. It reflects:
- The type of sample tested
- The method used
- The context in which the sample was collected
Serum and whole blood are both valid specimens. But they are not interchangeable. When alcohol concentration plays a central role in a case, identifying the specimen type is a small step that can make a meaningful difference in interpretation.
Disclaimer: All views expressed here are my own and do not reflect the views of my employer or any affiliated organizations or partners.
