Blood Alcohol Concentration (BAC)
Definition
Mass of ethanol per unit volume of whole blood, expressed as mg per 100 mL (equivalent to mg/dL or mg%), or as per cent w/v. In the US it is most often expressed as g/dL (so 0.08 g/dL = 80 mg/100 mL).
- Definition
- Mass of ethanol per unit volume of whole blood
- Common units
- mg/100 mL, mg/dL, or % w/v
- US convention
- g/dL (0.08 g/dL equals 80 mg/100 mL)
- Measured by
- GC-headspace analysis of blood samples
Common questions
Why do BAC unit conventions differ between countries and does it affect comparisons?+
Different jurisdictions historically adopted different reporting conventions, mg/100 mL in much of Europe and parts of Asia, g/dL in the US, and % w/v in some older statutes. The underlying quantity is identical once converted, so a legal limit of 0.08 g/dL and 80 mg/100 mL describe the same concentration, but analysts must convert carefully when comparing results across reports.
Does BAC measured from blood match a breath-alcohol reading exactly?+
No, breath testing estimates blood alcohol using a fixed partition ratio between breath and blood alcohol, commonly assumed near 2100:1, but this ratio varies between individuals and over the course of the alcohol absorption and elimination curve. Blood analysis by GC-headspace is generally treated as the more direct and accurate measurement for evidentiary purposes.
Related terms
- 2100:1 Partition Ratio
- The conventional blood/breath partition assumption used by breath alcohol analysers worldwide: 2100 mL of venous blood contains the same mass of ethanol...
- Beta Elimination Rate (Beta)
- The rate at which BAC falls during the post-absorptive phase of alcohol metabolism, expressed as mg/100 mL/hour. Population mean approximately 15 mg/100...
- DB-ALC1 / DB-ALC2
- A matched pair of Agilent fused-silica GC columns used for dual-column orthogonal confirmation of blood ethanol. DB-ALC1 uses a polyethylene glycol stationary...
- Internal Standard (IS)
- A compound chemically near-identical to the analyte, often a deuterated isotopologue (d3-morphine, d5-EDDP, d5-alprazolam, pyrene-d10), spiked into every sample at a known...
- NIST SRM 1828
- A series of NIST-certified reference material aqueous ethanol standards (0.02 to 0.40 per cent w/v) used to calibrate forensic blood ethanol analytical...
- Partition Coefficient (K)
- The ratio of analyte concentration in the liquid phase to concentration in the gas phase at equilibrium. For ethanol in blood at...
- Sodium Fluoride / Potassium Oxalate (Grey-Top Tube)
- The standard blood collection tube for forensic ethanol analysis. Sodium fluoride inhibits microbial fermentation (which would produce artefactual ethanol post-collection); potassium oxalate...
- Static Headspace
- A sample preparation technique in which a sealed vial is heated until volatile analytes reach thermodynamic equilibrium between the liquid matrix and...
- Widmark Factor (Rho / R)
- A dimensionless pharmacokinetic constant that represents the distribution of ethanol across body water. Averages approximately 0.68 for adult males and 0.55 for...