Blood alcohol content
Blood alcohol content (BAC), also called blood alcohol concentration or blood alcohol level, is a measurement of alcohol intoxication used for legal or medical purposes.[1]
BAC is expressed as mass of alcohol per volume of blood. In the US and many international publications, BAC levels are written as a percentage such as 0.08%, meaning that there is 0.08 g of alcohol for every 100 mL of blood.[1][2] In different countries, the maximum permitted BAC when driving ranges from the limit of detection (zero tolerance) to 0.08%.[3][2] BAC levels above 0.40% are potentially fatal.[1]
Estimation[edit]
Direct measurement[edit]
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.[17] Forensic laboratories typically use headspace-gas chromatography combined with mass spectrometry or flame ionization detection,[18] as this method is accurate and efficient.[17] 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.[19]
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.[20]
Test assumptions[edit]
Extrapolation[edit]
Retrograde extrapolation is the mathematical process by which someone's blood alcohol concentration at the time of driving is estimated by projecting backwards from a later chemical test. This involves estimating the absorption and elimination of alcohol in the interim between driving and testing. The rate of elimination in the average person is commonly estimated at 0.015 to 0.020 grams per deciliter per hour (g/dL/h),[43] although again this can vary from person to person and in a given person from one moment to another. Metabolism can be affected by numerous factors, including such things as body temperature, the type of alcoholic beverage consumed, and the amount and type of food consumed.
In an increasing number of states, laws have been enacted to facilitate this speculative task: the blood alcohol content at the time of driving is legally presumed to be the same as when later tested. There are usually time limits put on this presumption, commonly two or three hours, and the defendant is permitted to offer evidence to rebut this presumption.
Forward extrapolation can also be attempted. If the amount of alcohol consumed is known, along with such variables as the weight and sex of the subject and period and rate of consumption, the blood alcohol level can be estimated by extrapolating forward. Although subject to the same infirmities as retrograde extrapolation—guessing based upon averages and unknown variables—this can be relevant in estimating BAC when driving and/or corroborating or contradicting the results of a later chemical test.
There have been reported cases of blood alcohol content higher than 1%: