Direct measurement
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Blood samples for BAC analysis are typically obtained by taking a venous blood sample from the arm. A variety of methods exist for determining blood-alcohol concentration in a blood sample.[18] Forensic laboratories typically use headspace-gas chromatography combined with mass spectrometry or flame ionization detection,[19] as this method is accurate and efficient.[18] Hospitals typically use enzyme multiplied immunoassay, which measures the co-enzyme NADH. This method is more subject to error but may be performed rapidly in parallel with other blood sample measurements.[20]
In Germany, BAC is determined by measuring the serum level and then converting to whole blood by dividing by the factor 1.236. This calculation underestimates BAC by 4% to 10% compared to other methods.[21]
By breathalyzer
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Joke "Breathalyser 'pint'" beer glass, about 2 inches tall, dating from around the time of the introduction of breathalyzers in the United Kingdom, in 1967
Main article: Breathalyzer
The amount of alcohol on the breath can be measured, without requiring drawing blood, by blowing into a breathalyzer, resulting in a breath alcohol content (BrAC). The BrAC specifically correlates with the concentration of alcohol in arterial blood, satisfying the equation BACarterial = BrAC × 2251 ± 46. Its correlation with the standard BAC found by drawing venous blood is less strong.[22] Jurisdictions vary in the statutory conversion factor from BrAC to BAC, from 2000 to 2400. Many factors may affect the accuracy of a breathalyzer test,[23] but they are the most common method for measuring alcohol concentrations in most jurisdictions.[24]
By intake
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Main article: Pharmacology of ethanol § Modeling
Blood alcohol content can be quickly estimated by a model developed by Swedish professor Erik Widmark in the 1920s.[25] The model corresponds to a pharmacokinetic single-compartment model with instantaneous absorption and zero-order kinetics for elimination. The model is most accurate when used to estimate BAC a few hours after drinking a single dose of alcohol in a fasted state, and can be within 20% CV of the true value.[26][27] It is not at all realistic for the absorption phase, and is not accurate for BAC levels below 0.2 g/L (alcohol is not eliminated as quickly as predicted) and consumption with food (overestimating the peak BAC and time to return to zero).[28][5] The equation varies depending on the units and approximations used, but in its simplest form is given by:[29]
����=���−�×�
where:
EBAC is the estimated blood alcohol concentration (in g/L)
A is the mass of alcohol consumed (g).
T is the amount time during which alcohol was present in the blood (usually time since consumption began), in hours.
β is the rate at which alcohol is eliminated, averaging around 0.15 g/L/hr.[30]
Vd is the volume of distribution (L); typically body weight (kg) multiplied by 0.71 L/kg for men and 0.58 L/kg for women[31][32] although estimation using TBW is more accurate.[33]
Examples:
A 80 kg man drinks 2 US standard drinks (3 oz) of 40% ABV vodka, containing 14 grams of ethanol each (28 g total). After two hours:
����=28/(0.71⋅80)−(0.148⋅2)≈0.197g/L=0.0197%BAC
A 70 kg woman drinks 63 g of 40% ABV vodka, containing 21 grams of ethanol. After two hours:
����=21/(0.58⋅70)−(0.156⋅2)≈0.205g/L=0.0205%BAC
In terms of fluid ounces of alcohol consumed and weight in pounds, Widmark's formula can be simply approximated as[25]
����=8×fl oz/weight in pounds−�×�
for a man or
����=10×fl oz/weight in pounds−�×�
for a woman, where EBAC and β factors are given as g/dL (% BAC), such as a β factor of 0.015% BAC per hour.[25]
By standard drinks
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Main article: Standard drink
United States standard drinks of beer, malt liquor, wine, and spirits compared. Each contains about 14 grams or 17.7 mL of ethanol.
The examples above define a standard drink as 0.6 fluid ounces (14 g or 17.7 mL) of ethanol, whereas other definitions exist, for example 10 grams of ethanol.