BAC
Definition
Blood Alcohol Concentration. Mass of ethanol per unit volume of whole blood. Indian legal cut-off is 30 mg per 100 mL (0.03 g per 100 mL) under Section 185 MVA 1988.
- Full name
- Blood Alcohol Concentration
- Unit
- Mass ethanol per volume whole blood
- India legal limit
- 30 mg per 100 mL under Section 185, MVA 1988
- Sample type
- Whole blood, not plasma or serum
Common questions
Why does BAC differ between whole blood, plasma and breath results?+
Plasma has a higher water content than whole blood, so plasma alcohol readings run higher than whole-blood readings for the same person, typically by a factor of roughly 1.1 to 1.2. Breath analysers estimate blood alcohol from a fixed blood-to-breath partition ratio, which introduces its own source of variation between individuals.
Does BAC keep rising after a person stops drinking?+
Yes, alcohol continues to absorb from the stomach and intestine for a period after the last drink, so a sample taken during this absorption phase can show a higher BAC than the level at the time of the incident being investigated. This is why the timing of sample collection relative to the last drink matters in legal cases.
How is BAC different from a field breath screening result?+
A field breathalyser gives a rapid presumptive estimate used to decide whether to detain a suspect, while an evidentiary BAC is typically obtained from a blood draw analysed by headspace gas chromatography in a laboratory, which is the figure used in prosecution under drink-driving statutes.
Related terms
- ADH Assay
- Alcohol dehydrogenase enzymatic assay. Ethanol plus NAD+ gives acetaldehyde plus NADH; NADH absorbance at 340 nm is proportional to ethanol concentration. Fast...
- Fuel-Cell Breath Analyser
- Electrochemical sensor: ethanol oxidises at a platinum electrode generating a current proportional to BrAC. Specific for ethanol, used in Drager Alcotest and...
- Headspace GC-FID
- Gas chromatography with flame-ionisation detection run on the vapour above a sealed sample. The standard forensic method for methanol and ethanol in...
- IR Breath Analyser
- Infrared absorption at about 3.4 micrometres (C-H stretch) or 9.5 micrometres (C-O stretch). Specific and quantitative; used in Intoxilyzer 8000 and evidential...
- Putrefactive Ethanol
- Ethanol produced post-mortem by microbial fermentation of glucose, lactate and amino acids. Can give false-positive BAC; vitreous humor is the matrix of...
- Wet-Bath Simulator
- Calibration accessory holding a 0.10 g per L aqueous ethanol solution at 34 degrees Celsius. The vapour above gives a known reference...
- Widmark Formula
- A = r × W × C, where A is total alcohol in body (g), W is body weight (kg), C is...