Breath Tests for Blood Alcohol Determination: Partition Ratio

by Srikumaran K. Melethil, Ph.D, Professor of Pharmacology, University of Missouri at Kansas City

The author is a professor in the UMKC School of Pharmacology, and also a graduate of the UMKC School of Law. The following is a condensation of a paper written in fulfillment of a research and writing requirement in the course of Scientific Evidence and Opinion Testimony.  In relevant part:


1. Introduction:

In cases involving drunk driving, the prosecution has to prove that the defendants blood alcohol concentration (BAC) at the time of the offense is at or above a statutory concentration. In the majority of jurisdictions it is 0.10% [i.e., 0.1 gram of alcohol per 100 milliliters of blood]. In some jurisdictions, it is 0.08%, see People v. Ireland, 39 Cal. Rptr. 2d. 870 (Cal. Ct. App. 1995). In this connection, there is an ongoing national debate to reduce this value to 0.08% nationwide. In order to provide proof of BAC it is necessary to obtain a suitable biological sample (i.e., blood, urine, expired air) from the defendant at the time of arrest. Determination of BAC by use of a breath test, is by far the most popular scientific test for drunk driving. The breath test involves the measurement of alcohol in an appropriate sample of breath, expired alveolar air. (Alveolar air is that part of the expired air, which is in equilibrium with blood; usually this is taken as the terminal portion of expired air. One likely reason for the high variability observed in partition ratios is the difficulty in obtaining true alveolar, or deep lung air for analysis). This breath alcohol concentration is then multiplied by a factor called the partition ratio to convert the concentration measured in the breath to the corresponding alcohol concentration in the blood. In most jurisdictions, a value of 2100 is used for this ratio by statutory mandate. However, this partition ratio of 2100 can differ from individual to individual or differ in a given individual from time to time. Therefore, while it is quite simple to perform, the use of breath tests to determine BAC suffers from a major and fundamental weakness in that it is an indirect method.

For that reason, the conversion (extrapolation) of the directly measured concentration of alcohol in the expired air to obtain its concentration in the blood has been the subject of much litigation. Understandably, this conversion is fraught with problems of variability (uncertainties) introduced by the theoretical assumptions underlying the method. As was pointed out by one of the leading researchers in this area, The most trying forensic difficulties were consequent to what now appears to some to be an error in policy made by the pioneers in breath testing. This was in deciding to calculate the blood concentration from a quantity of alcohol found in the breath. Mason & Dubowski, Traffic & Chemical Testing in the United States: a Resume & Some Remaining Problems, 20 Clinical Chemistry 126, 128 (1974). The following section will present the scientific basis for the statutory decision to select a partition ratio of 2100 and the variability, both inter-subject and intra-subject, to be expected in this ratio.

2. Basic assumptions:

A direct correlation is assumed between the concentration of alcohol in the alveolar air and concentration of alcohol in the blood, more precisely, ethanol. This assumption is based on Henry’s Law which states that, at constant temperature, the concentration of gas dissolved in a liquid is proportional to its concentration in the air directly above the liquid. Brent and Stiller, Handling Drunk Driving Cases, ‘ 7 (Breath Tests) (1985). As applied to determination of BACs, this means that the concentration in the expired alveolar air is directly proportional to the concentration in the blood (i.e., the greater concentration of alcohol in the blood, the greater its concentration in the expired alveolar air). It is at the alveoli, commonly called air sacs (of which there are about 700 million in an average adult), where exchange of gases occur between blood and the expired alveolar air. Alcohol is a volatile liquid and assumed to freely diffuse (i.e., readily pass) across the membranes of the alveoli. Due to the latter assumption, it is also assumed that the exhaled alveolar air is in equilibrium with the blood. Equilibrium can be best explained as a condition where the ratio of concentrations of alcohol in blood and expired alveolar air has achieved a constant value. Therefore, in principle, its concentration in blood can be estimated by measuring its concentration in the expired alveolar air.

The commonly used partition-ratio of 2100 can be expressed as follows:

In principle, this ratio is determined by simultaneously (or as close to simultaneous as experimentally possible) measuring the concentration of alcohol in the blood and expired alveolar air of test subjects administered alcohol under controlled conditions. While values in the scientific literature for this ratio range from 1900 to 2400, an international panel chose, in 1972 Essentially by fiat, the currently accepted value of 2100. Brent, supra at 133.

3. Factors that affect the partition ratio.

Some factors that affect the partition-ratio, such as the effect of temperature, may be obvious, even to a non-scientist. There are others that are not so apparent. These factors can either increase or decrease the actual BAC.

a. Effect of Temperature: The widely used partition blood-to-air partition ratio of 2100 is based on a normal body temperature of 98.6 0F. A higher body temperature of the individual will overestimate the actual BAC because of the higher volatility (or vapor pressure) of liquids like alcohol at a higher temperature. An elevation in body temperature of 1 0C (1.8 0F) results in a 7% higher value in the result. Therefore, a person with a body temperature of 100.4 0F and with an actual blood alcohol of 0.0935 % will register a value of 0.10 % by the breath test. As can be seen from this hypothetical example, a small difference in body temperature can make the difference of guilt or innocence of drunk driving in defendants with a BAC close to the legal limit. This widely accepted ratio is also based on the assumption that the average temperature of exhaled air is 93.20 F.

b. Atmospheric Pressure: There is little evidence to support the belief that the partition ratio is affected by atmospheric (barometric) pressure. Breathalyzer tests conducted at altitudes of 5000 feet and 10000 feet essentially gave the same results. This is expected based on scientific principles of gases.

c. Cellular Composition of Blood: Blood contains suspended cells (e.g. red and white cells) and proteins, and is therefore only a partial liquid. The partition ratio of 2100 is based on a average hematocrit (the cell volume of blood) of 47%; hematocrit values range from 42 to 52 % in males and 37 to 47 % in females. Therefore, a person with a lower hematocrit will have falsely elevated blood alcohol based on a breath test; this variability has been estimated to be relatively small, ranging from – 2 to + 5 %.

Since alcohol freely diffuses into cells but not into cellular membranes, the subtle point to be aware of is the variability in volume of the cell debris (i.e. volume of cell membranes after cells are analyzed), and not the actual hematocrit that is responsible for the reported variability. Understandably, a higher hematocrit value represents a higher value of cell debris. The mean value from several studies show that debris can account for about 16% of the volume of blood. For example, 0. 119 mg % (in serum) is equivalent to 0.10% of BAC. Fitzgerald and Hume, Intoxication Test Evidence: Criminal and Civil, 4:26 at 152 (1987).

d. Physical Activity and hyperventilation: Exercise can underestimate blood alcohol values. In one study BACs of subjects before and after running up a flight of stairs decreased 11 to 14 % after one trip and 22-25 % after two such trips. In a another study, a 15% decrease in blood alcohol was reported in subjects following vigorous exercise or hyperventilation.

e. Changes in water content of expired air: Water, present in the form of vapor, in expired air will condense into the liquid form with a lowering of temperature. Air exhaled into the tubes of a breath test device, such as the Breathalyzer, is assumed to be saturated with water at about 93.2F . Decreases in this temperature can result in an underestimation of reported BAC due to condensation of water and the subsequent removal of alcohol from the expired air. One study showed that when the mouthpiece of the breath test instrument was kept at 23 0C, there was an average decrease in temperature of exhaled air by 1.6 0C.

f. Radio Frequency Interference (RFI): Andre Moenssens, et al., Scientific Evidence in Civil and Criminal Cases  3.09 at 204 (4th ed. 1995). This interference describes the effect of an electronic instrument on a radio wave or current that it is not designed to pick up. If a particular Breathalyzer as an electronic instrument were susceptible to RFI, then the measurement of light distance obtained when the operator balances the meter might not be an accurate indication of the amount of alcohol in the breath sample. Instead, the light distance might reflect, in part, a deflection in the meter needle caused by a stray current induced by radio waves in the surrounding environment.

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