Unit inconsistency in forensic blood and breath alcohol reporting

Abstract

An inconsistency exists between statutory definitions of blood alcohol concentration as a percent by weight, a mass/mass concentration, and modern forensic laboratory reporting based on mass/volume concentration. Percent by weight (mass/mass) and grams per deciliter (mass/volume) are not equivalent units for whole blood because blood has a density of approximately 1.055–1.06 g/mL at 37 °C. When g/dL values are treated as equivalent to percent by weight, BAC is overreported by approximately 5.5–6%. The same inconsistency affects breath alcohol values when units are not properly reconciled.

Highlights

• •Percent by weight BAC differs from g/dL due to whole blood density.
• •Unit mismatch arises from historical statutes and modern lab practices.
• •Breath-to-blood conversions inherit the same density-based discrepancy.
• •Blood density causes ∼5.5–6% difference between mass/mass and mass/volume BAC.

1. Introduction

Several state statutes, such as those in Massachusetts, New York, and South Dakota, define blood alcohol concentration (BAC) using language that describes the per se blood alcohol driving limit in terms of percent by weight, a mass/mass unit of concentration [[1], [2], [3]]. In this manuscript, mass/mass refers specifically to percent by weight as defined in statutory language (grams of ethanol per 100 g of whole blood), whereas mass/volume refers to laboratory reporting in grams of ethanol per unit volume of whole blood (e.g., g/dL).

For example, South Dakota state law defines blood alcohol concentration as a percentage by weight. Statute 32-23-1 states:

“No person may drive or be in actual physical control of any vehicle while:

• (1)There is 0.08 percent or more by weight of alcohol in that person's blood as shown by chemical analysis of that person's breath, blood, or other bodily substance; ”

Most forensic laboratories, however, calibrate their instruments using reference material denominated in mass/volume (i.e., g/dL) [[4], [5], [6], [7], [8], [9]]. Whole blood has a density of approximately 1.055–1.06 g/mL [[10], [11], [12], [13], [14]] at 37 °C and varies modestly with temperature, hematocrit, and other blood components [15,16]. This results in systematic overreporting of BAC when percent by weight is treated as equivalent to g/dL. For example, a BAC of 0.080 g/dL corresponds to approximately 0.075% by weight.

2. An illustrative example: BAC in g/dL reported as percent by weight

Consider a state such as South Dakota, whose statute defines BAC in terms of “percent by weight”. The method of analysis uses calibrators denominated in g/dL. For an unknown blood sample, the instrument output reads: 0.08 g/dL. If 0.08 g/dL is reported as 0.08% by weight without considering blood density, BAC will be overreported by ∼5.5–6%.

The actual percent by weight concentration is:

$$0.08\mathrm{g}/100\text{mL}÷1.06\mathrm{g}/\text{mL}\times 100\approx \mathbf{0.075}\%\mathbf{by}\mathbf{weight}$$

The lack of unit reconciliation in the above example is not merely theoretical. One forensic laboratory operating in South Dakota incorrectly states in its procedures that: “% by weight is equivalent to % alcohol and g/dL alcohol” [17].

3. BAC unit errors extend to breath alcohol testing

The same unit conversion applies to the interpretation of breath alcohol concentration (BrAC) [11,13]. In jurisdictions such as Massachusetts and New York, laws addressing per se BAC driving limits require BrAC results to be converted to BAC as a “percentage by weight” [2,3].

Breath alcohol analyzers, however, are calibrated using reference material denominated in mass/volume (i.e., g/210 L). To convert BrAC to BAC, a 2100:1 conversion factor is used. However, no reconciliation between mass/mass and mass/volume units is performed. The analyzer's output of 0.08 g/210 L is reported as equivalent to 0.08% by weight BAC.

For example, a breath alcohol result of 0.08 g/210 L corresponds, under the assumed 2100:1 blood-to-breath ratio, to 0.08 g/100 mL of blood. Because 100 mL of whole blood weighs approximately 106 g at a density of ∼1.06 g/mL, 0.08 g per 100 mL equals 0.08 g per 106 g of blood. This equals a BAC of approximately 0.075% by weight (0.08 g ÷ 106 g × 100 ≈ 0.075% by weight).

The discrepancy described does not arise from the calibration method, but rather from interpreting mass/volume concentrations as equivalent to percent by weight BAC without reconciling units.

Table 1.

Illustrates equivalent alcohol concentrations across commonly used forensic units in the United States, assuming a whole blood density of 1.06 g/mL and a blood-to-breath ratio of 2100:1 [13].

BAC % by weight (m/m)	BAC g/dL (m/v)	BrAC g/210 L (m/v)
∼0.075	0.080	0.080

Table 1. Comparison of mass/mass (m/m) and mass/volume (m/v) alcohol concentrations. BAC values refer to alcohol in whole blood. BrAC values refer to alcohol in breath. Values assume a whole blood density of 1.06 g/mL and a 2100:1 blood-to-breath ratio.

4. Conclusions

Forensic blood and breath alcohol testing in some jurisdictions in the United States currently contains inconsistencies arising from the use of “percent by weight” statutory language coupled with mass/volume calibration of analytical equipment. Because whole blood has a density of approximately 1.055–1.06 g/mL, these units are not equivalent. Failure to reconcile these units results in BAC and BrAC values being overreported by approximately 5.5–6% in some jurisdictions.

Data availability statement

Data is available in the article.

Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work, the author used ChatGPT to assist with language editing, organization, drafting, and text readability. After using this tool, the author reviewed and edited the content and takes full responsibility for the publication's content.

Funding

The author received no financial support for the research, authorship, or publication of this article.

Declaration of competing interest

The authors declare the following financial interests/personal relationships, which may be considered as potential competing interests: Aaron Olson serves as an expert witness in forensic toxicology cases through ARO Consulting LLC.