Blood Alcohol Concentration (BAC) Scale
The BAC scale measures the concentration of alcohol in blood, expressed as grams of alcohol per 100 mL of blood (g/dL) or as a percentage, with 0.08 g/dL (0.08%) representing the legal threshold for intoxication and driving impairment in the United States. 1
Legal and Regulatory Thresholds
The National Institute on Alcohol Abuse and Alcoholism defines the critical BAC threshold at 0.08 g/dL (equivalent to 0.08% or 80 mg/dL), which serves dual purposes: 1
- Legal intoxication limit for driving in the United States 1
- Definition of binge drinking when this level is reached after consuming 5 drinks (men) or 4 drinks (women) within approximately 2 hours 2, 1
This 0.08% threshold is used as a benchmark in driving studies evaluating drug impairment, providing a standardized reference point for assessing clinically meaningful driving impairment 2
Measurement and Conversion
BAC can be determined through multiple methods: 3, 4
- Direct blood analysis via gas chromatography, considered the gold standard 5
- Breath-alcohol concentration (BrAC) converted using a blood/breath ratio, typically 2100:1 in most jurisdictions (though ratios of 2000:1,2300:1, and 2400:1 are used in different countries) 4
Important caveat: Blood samples typically show 11.3% higher alcohol concentrations than corresponding breath values, and arterial-venous differences continuously change during absorption, distribution, and elimination phases 5, 4
Clinical Interpretation Ranges
Beyond the legal threshold, BAC levels have distinct clinical significance: 1, 6
- Any detectable level (>0.00 g/dL): Indicates recent alcohol consumption 1
- 0.08 g/dL: Legal intoxication and driving impairment threshold 1
- 143-182 mg/dL (1.43-1.82 g/dL): Used to define "intoxicated" groups in trauma and medical research settings 1
- 355 mg/dL (3.55 g/dL): Mean BAC in fatal acute alcohol intoxication cases, though this is lower than many textbooks suggest 6
Correlation with Alcohol Consumption
To convert alcohol consumption to BAC context: 2
- Standard drink definitions vary globally: 8-13.6 g of alcohol per drink depending on region 2
- Volume to weight conversion: Multiply alcohol percentage by volume by 0.79 g/mL (specific gravity of alcohol) 2
- Moderate consumption (20-60 g/day): Correlates with PEth levels of 20-200 ng/mL, relevant for metabolic dysfunction and alcohol-related liver disease assessment 2, 1
Preanalytical Considerations
For accurate BAC measurement, critical factors include: 3
- Non-alcoholic disinfectants must be used for venipuncture to avoid false elevation 3
- Modern pre-evacuated sterile collection tubes control non-enzymatic oxidation and microbial contamination 3
- Chain of custody is the primary distinction between diagnostic and forensic BAC testing; preanalytical procedures are otherwise identical 3