How bloody is your blood? The hematocrit is the number expressing the cell volume of blood (the size and number of red blood cells in proportion to the rest of the blood). As humans, we all have a natural variation in our hematocrit. Healthy men have a natural variance from 40.7% to 50% and healthy women have a natural variance from 36.1% to 44.3%. The hematocrit level is a moving target. Illness, hydration and stress can cause variations in the hematocrit level.
The hematocrit level affects all breath tests by governing how much alcohol may be contained in the blood and, in turn, how much will pass into the breath. See Nesci, How to Beat a DUI, 2008 ed., p. 57. Under the best of conditions, normal hematocrit ratios will lead to a +/-5% difference in a breath test result. Let’s pause for a moment to contemplate this fact. Would you accept this degree of error from your bank? “We know you asked for $10,000.00, but giving you $9,500 is within our margin of error.”
Breath testing machines do not have built in mechanisms for detecting and stabilizing hematocrit levels. The machine is forced to use an assumed hematocrit level of 45% (averaging the male average of 47% and the female average of 42%). As we have seen above, this builds in a bias against the average female test subject who has a lower hematocrit average and may, in fact have a wildly lower hematocrit level than that assumed by the machine. So what does this mean for women? A person with a lower hematocrit level will have a falsely high BAC reading. See Alcohol Problems and Solutions, David J. Hanson, PhD., Breath Analyzer Accuracy. As one writer observed,
Breath testing, as currently used, is a very inaccurate method for measuring BAC. Even if the breath testing instrument is working perfectly, physiological variables prevent early reasonable accuracy....Breath testing for alcohol using a single test instrument, should not be used for scientific, medical or legal purposes where accuracy is important. Hlastula, M. Physiological errors associated with alcohol breath tests . The Champion, 1985, 9(6). Quoted in Taylor, L. Drunk Driving Defense. New York: Aspen Law and Business, 5th edition, 2000.
Another area that is affected by the machines built-in assumptions are tests done on people suffering from anemia. Anemia is a condition in which your blood has a lower than normal number of red blood cells. If you have anemia, your body doesn't get enough oxygen-rich blood. The body compensates by putting replacing the red blood cells with blood plasma. Alcohol has more of an affinity for the plasma because it is liquid (as opposed to the solid red blood cells). It follows that the higher the ratio of liquid to solids in the blood (called the hematocrit), the higher the amount of alcohol in the blood — and the higher will be the reading on the breathalyzer. Id. See also HERE.
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