Serum/Plasma Ethanol vs Whole-Blood BAC: Forensic Conversion

Technical review: Reviewed by Okorie Okorocha, M.S., M.S., Esq., attorney and forensic toxicologist. Professional forensic toxicology profile. This reference is educational and does not substitute for case-specific expert analysis.

Direct answer: A hospital serum or plasma ethanol result is not automatically the same as a forensic whole-blood BAC. Serum and plasma generally contain more water than whole blood, so ethanol concentrations in those matrices are usually higher. Published studies often place the average serum- or plasma-to-whole-blood relationship near 1.1 to 1.2, but individual ratios vary. Forensic conversion should therefore identify the specimen matrix, analytical method, timing, and uncertainty instead of silently dividing by one universal number.

Why serum and plasma ethanol are usually higher than whole-blood ethanol

Ethanol distributes primarily in body water. Plasma and serum contain a greater proportion of water than whole blood because whole blood also contains cellular material, principally red blood cells. A 2022 review of ethanol distribution reported typical water contents of approximately 90% to 92% by weight for plasma and serum versus approximately 78% to 80% for whole blood. On that basis, the expected average plasma- or serum-to-whole-blood ethanol ratio is about 1.15:1, which is broadly consistent with empirical studies.

This is why a clinical laboratory result reported from serum or plasma can be numerically higher than a forensic whole-blood result collected at essentially the same time even when both analyses are analytically sound.

What have paired-sample studies found?

Published paired-sample studies show both a central tendency and meaningful individual variation. Winek and Carfagna reported average serum/whole-blood and plasma/whole-blood ratios of approximately 1.12, with observed serum/whole-blood ratios from about 1.09 to 1.18 in their dataset. Charlebois and colleagues later studied 235 subjects and reported serum-to-whole-blood ratios ranging from 1.04 to 1.26, with a mean of 1.14.

Rainey examined 211 patients and found a much broader observed serum-to-whole-blood ratio distribution, with a median of 1.15. That work emphasized probabilistic interpretation rather than treating one ratio as biologically fixed. The practical lesson is not that any single historical dataset supplies the correct conversion for every case. It is that matrix conversion carries biological variability that should be acknowledged when the converted value is compared with a legal threshold.

Why a single conversion factor can be misleading

It is common to see a serum or plasma ethanol result converted to a whole-blood equivalent by dividing the result by 1.10, 1.12, 1.15, 1.18, or another selected factor. The arithmetic is simple, but the apparent precision can be misleading.

A 2007 trauma-center study compared hospital serum ethanol measured enzymatically with simultaneously collected whole blood analyzed by forensic headspace gas chromatography. The authors reported that the serum/whole-blood relationship varied with concentration in their dataset and concluded that one generally applicable conversion factor was not appropriate. They instead modeled the relationship using regression and prediction intervals.

Forensic interpretation should therefore distinguish between an average relationship and an individual case conversion. A numerical conversion can be useful, but the opinion should make clear which factor or model was used, why it was selected, and what uncertainty accompanies it.

Clinical enzymatic ethanol testing is not the same as forensic headspace GC

Hospitals commonly measure serum or plasma ethanol with an alcohol dehydrogenase-based enzymatic assay because it is rapid and clinically useful. Forensic laboratories commonly use headspace gas chromatography, often GC-FID, for evidentiary alcohol testing. These are different analytical systems with different validation, interference, quality-control, and reporting considerations.

A 2025 review of enzymatic serum/plasma ethanol testing emphasized that hospital ADH methods are not inherently forensic assays. The review noted potential interferences, including effects associated with lactate dehydrogenase/lactic acid and hemolysis, and reiterated that serum/plasma ethanol concentrations are typically higher than whole-blood concentrations. That does not make a hospital result invalid; it means the laboratory method and its limitations should be understood before the number is used for a legal conclusion.

For the forensic analytical framework, see Blood Alcohol Testing by Headspace GC-FID and Measurement Uncertainty in Forensic Toxicology.

Serum, plasma, and whole blood should be identified explicitly

The words blood alcohol are often used loosely in medical records. A chart may display an “ethanol” result without making the specimen matrix obvious on the face of the result. Before converting or comparing a hospital number with a statutory whole-blood threshold, the reviewer should determine whether the tested material was:

• whole blood;
• serum;
• plasma;
• another matrix; or
• a result whose specimen type is unclear from the report and must be confirmed from laboratory records.

Serum and plasma generally behave similarly for ethanol because both are cell-free fluid matrices, but they are not merely synonyms for whole blood. Broader matrix principles are discussed in Biological Matrices in Forensic Toxicology.

Can you simply divide a serum ethanol result by 1.15?

Not as a universally exact conversion. Dividing by 1.15 can provide a rough central estimate in some contexts because approximately 1.15 is close to the expected average distribution ratio. But the published literature shows person-to-person variation, method-related considerations, and study-to-study differences. When the converted value is close to an evidentiary cutoff, a point estimate without an uncertainty analysis can be particularly misleading.

A defensible opinion should identify whether the purpose is a rough clinical-to-forensic comparison, a conservative threshold assessment, a statistical prediction, or an individualized expert opinion. Those are different tasks.

Why the issue matters near a legal threshold

Matrix differences matter most when a clinical serum or plasma result is compared directly with a legal limit expressed in whole blood. A serum ethanol concentration of 100 mg/dL, for example, should not simply be relabeled as a whole-blood BAC of 0.100 g/dL without addressing the matrix difference. Depending on the conversion approach and uncertainty, the estimated whole-blood equivalent may be materially lower.

At the same time, an expert should avoid using an artificially extreme conversion solely to force the estimate below or above a threshold. The scientifically appropriate approach is to present the evidence-supported relationship, its variability, and the consequences of reasonable assumptions.

Timing and retrograde extrapolation are separate questions

Matrix conversion answers one question: how a serum or plasma concentration relates to whole-blood concentration at approximately the same time. Retrograde extrapolation answers a different question: what an earlier concentration may have been based on a later measurement. The two analyses should not be collapsed into one opaque calculation.

If a hospital serum sample was collected after an incident and counsel wants an estimate of the earlier whole-blood BAC, the analysis may require at least two distinct steps: first address serum/plasma versus whole-blood comparability; then address absorption status, elapsed time, and elimination. See Retrograde BAC Extrapolation: Assumptions, Absorption, and Uncertainty.

What records should be reviewed?

Before relying on a hospital ethanol result in litigation or expert testimony, consider obtaining:

• the original laboratory report and specimen identifier;
• documentation identifying the specimen as serum, plasma, or whole blood;
• collection and analysis times;
• the assay platform and methodology;
• calibration and quality-control information where relevant;
• the laboratory’s reportable range and uncertainty or precision data;
• hemolysis or other specimen-quality flags;
• medical records bearing on treatment, fluid administration, transfusion, and timing;
• any contemporaneous forensic whole-blood sample; and
• the factual timeline if retrograde extrapolation is contemplated.

For a broader evidence-review framework, see the Toxicology Laboratory Records: Evidence Review Checklist.

What should an expert say about the conversion?

A transparent report should state the specimen matrix, the reported concentration and units, the analytical method if known, the conversion approach, the scientific source for that approach, and the uncertainty or range that matters to the opinion. If the source record is ambiguous about whether the hospital tested serum, plasma, or whole blood, that ambiguity should be resolved before offering a precise converted BAC.

The expert should also separate three distinct sources of uncertainty: analytical uncertainty in the laboratory result, biological variability in the serum/plasma-to-whole-blood relationship, and pharmacokinetic uncertainty if an earlier time is being estimated.

Frequently asked questions

Is serum ethanol the same as whole-blood BAC?

No. Serum generally contains a higher proportion of water, so its ethanol concentration is usually higher than the simultaneous whole-blood concentration.

What is the typical serum-to-whole-blood alcohol ratio?

Many studies place the average near approximately 1.1 to 1.2, often around 1.15, but individual ratios vary and the precise relationship depends on the population, concentration range, analytical methods, and study design.

Can a hospital ethanol result be used in a forensic case?

Potentially, yes. But the reviewer should identify the matrix and method, assess analytical limitations, and avoid treating a clinical result as though it were automatically equivalent to a forensic whole-blood headspace-GC result.

Does converting serum to whole blood require retrograde extrapolation?

No. Matrix conversion and retrograde extrapolation are separate analyses. One addresses specimen comparability; the other estimates concentration at an earlier time.

Is dividing by 1.15 always conservative?

No. Whether a selected factor is conservative depends on the scientific question, the actual individual ratio, the comparison being made, and the evidentiary purpose. A single factor should not be presented as a biologically exact constant.

Selected scientific sources

1. Jones AW. Distribution ratios of ethanol and water between whole blood, plasma, serum, and erythrocytes: Recommendations for interpreting clinical laboratory results in a legal context. 2022.
2. Rainey PM. Relation between serum and whole-blood ethanol concentrations. Clinical Chemistry. 1993;39(11):2288-2292.
3. Winek CL, Carfagna M. Comparison of plasma, serum, and whole blood ethanol concentrations. Journal of Analytical Toxicology. 1987;11(6):267-268.
4. Comparison of hospital laboratory serum alcohol levels obtained by an enzymatic method with whole blood levels forensically determined by gas chromatography. Journal of Analytical Toxicology. 2007;31(1):23-30.
5. The enzymatic analysis of alcohol (ethanol) in serum and plasma with the alcohol dehydrogenase reagent: focus on intra-analytical and post-analytical aspects. 2025.

Editorial note: This page is educational and is not a case-specific expert opinion. Interpretation requires the complete record. See our Editorial Standards & Scientific Review.