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Blood Alcohol Concentration, Measures of


The first analytical methods for measuring Alcohol (ethanol) in blood and other body fluids were developed in the nineteenth century. Although by modern standards these pioneer efforts were fairly crude, they were sufficiently reliable to establish a quantitative relationship between blood-alcohol concentration (BAC) and the various signs and symptoms of inebriation. A significant advance in methodology came in 1922 when Erik M. P. Widmark published his micro-method for analyzing ethanol in specimens of capillary blood.

Blood was drawn by pricking a fingertip or earlobe. The specimen for analysis, 100-150 milligrams, was collected with specially prepared S-shaped glass capillaries that contained a thin film of potassium oxalate and sodium fluoride on the walls of the tube. In Widmark's day, the small amounts (aliquots) of blood needed for each analysis could be measured more accurately by weight than by volume, since constriction pipettes were not yet available. Widmark therefore weighed the amount of blood required to the nearest milligram (0.001 g) with the aid of a torsion balance. The results of ethanol determinations were then reported in terms of mass per mass units, actually milligram of ethanol per gram of whole blood (mg/g), sometimes referred to as per mille (meaning, parts per thousand). This way of reporting BAC survives today in Scandinavian countries where Widmark's method became widely used for legal purposes.

Widmark's micromethod of blood-ethanol analysis involved the following four steps: (1) separation of ethanol from blood by diffusion in specially blown glassware; (2) oxidation of ethanol with a mixture of potassium dichromate and sulfuric acid; (3) addition of potassium iodide to the reaction mixture after oxidation of ethanol; and (4) back titration of liberated iodine with standard sodium thiosulfate and a starch indicator to detect an endpoint.

Many later modifications to this basic procedure appeared, such as using a different endpoint indicator for the titration (e.g., methyl orange), or another kind of oxidizing agent (e.g., ferrous salts), or separation of ethanol from the biological matrix in a different way. It became common practice to refer to these modified methods with the name of the scientist who first published the reportincluding Harger, Kozelka-Hine, Smith, Southgate-Carter, and Cavette, to name just a few.

Later developments in methods of ethanol analysis (such as gas chromatography) plus the availability of modern clinical laboratory equipment made it more convenient to dispense the aliquots of blood needed for analysis by volume rather than by weight. Micropipettes and more recently diluter-dispenser devices are now widely used for dilution of blood prior to the analysis. The term "concentration" has little meaning when used alone, because it can be expressed in many different ways. The choice of units for reporting BAC differs among countries: for example, milligrams per hundred milliliters (mg/100 ml) in Great Britain (unfortunately often appearing as the ambiguous mg%); gram percent weight per volume (g% w/v) in the United States; and milligrams per milliliter (mg/ml) in many European countries. Other ways of reporting BAC in clinical medicine are milligrams per deciliter (mg/dl), grams per liter (g/liter), or micrograms per liter (μg/liter). When countries outside Scandinavia enacted legal limits of ethanol in the blood of motorists, the concentrations were defined in units of mass of ethanol per unit volume; whether it was grams, milligrams, or micrograms of ethanol in a volume of milliliters, deciliters, or liters seems chosen arbitrarily.

Because the specific gravity of whole blood is greater than water (on average, 1 ml of whole blood weighs 1.055 g), BAC expressed in terms of mass per mass (w/w) is not the same as mass per volume (w/v). In fact, a concentration of 0.10% w/v equals 0.095% w/v. This difference of about 5.5 percent could mean punishment or acquittal in borderline cases of driving while under the influence of alcohol. With the current trend toward "per se" ethanol limits in many U.S. states, great care is needed to ascertain whether w/v or w/w units were intended by the legislature when the statute was drafted. Table 1 gives examples of concentration units commonly used to report BAC for legal purposes. Note that if ethanol were determined in plasma or serum, the concentration would be about 10 to 15 percent higher than for the same volume of whole blood, because there is more water in the sample after the erythrocytes (red blood cells) are removed.

In clinical chemistry laboratories, the Système International d'Unités (SI) has gained worldwide acceptance. According to the SI system, the amount of substance implies "mole" rather than mass. The mole or a submultiple there of replaces mass units such as grams or milligrams. Accordingly, the concentration of a substance of known molecular weight might appear as mole/liter or millimole per liter (mmol/l) or micromole per liter (μmol/liter). Note that liter is the preferred unit of volume when reporting concentrations of a substance in solution in the SI system. The molecular weight of ethanol is 46.06, and therefore a concentration of 1.0 mol/l corresponds to 46.06 g of ethanol in 1 liter of solution. Likewise 1.0 mmol/l contains 46.06 mg; 1.0 μmol/l contains 46.06 μg, and so on. Publications in the field of biomedical alcohol research often report BAC in this way. It follows that 0.1 g% w/v or 100 mg/dl is the same as 21.7 mmol/l.

Statutory limits of BAC existed in several countries before methods of analyzing the breath were developed. It therefore became a standard practice to convert the concentration of ethanol measured in the breath (BrAC) into the presumed concentration in the blood. For this purpose, a conversion factor, usually 2,100:1 was used. Presumably, it was less troublesome to make this conversion than to rewrite the statute to include both BAC and BrAC as evidence of impairment. Accordingly, breath-ethanol analyzers were calibrated in such a way that the readout was obtained directly in terms of the presumed BAC. This conversion of breath to blood ethanol created the dilemma of a constant blood breath ratio existing for all subjects under all conditions of testing. In the United States and elsewhere, a blood/breath factor of 2,100:1 was approved for legal purposes with the understanding that this gives a margin of safety (about 10%) to the accused. Indeed, more recent research suggests that the blood/breath factor should be 2,300:1 for closer agreement between direct BAC and the result derived from BrAC. In the Netherlands and Great Britain, 2,300:1 was chosen to set the legal limit of BrAC when evidential breath-ethanol analyzers were introduced in these countries. Similarly, in some U.S. states, a legal limit of 0.1 g/210 liters in breath is considered equivalent to 0.1 g% w/v in blood for law-enforcement purposes. Table 2 gives the statutory limits of breath-ethanol concentrations in several countries.

Both the prescribed BAC or BrAC limits for motorists and the units of concentration used to differ among countries and even within regions of the same country. The notion of reaching an international agreement about one common BAC or BrAC limit for motorists is an attractive one but hardly attainable.

(See also: Blood Alcohol Content ; Breathalyzer ; Driving Under the Influence ; Drug Testing and Analysis )


Jones, A. W. (1989). Measurement of alcohol in blood and breath for legal purposes. In K. E. Crow & R. D. Batt (Eds.), Human metabolism of alcohol, vol. I. Boca Raton, FL: CRC Press.

Widmark, E. M., P. (1922). Eine Mikromethode zur Bestimmung von Ä thylalkohol im Blut. Biochemical Zeitschrift, 131, 473.

A. W. Jones

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