Law enforcement's primary method for determining if a person is driving under the influence of alcohol is a chemical test. That is, a police officer will take a sample of a susect's blood or breath. The chemical test assumes that the composition of everyone's blood is the same. Specifically, the test assumes that all people have the same hematocrit level. However, this assumption is incorrect.
The hematocrit level, or packed cell volume, is a measure of the proportion of blood volume that is composed by solids. Whole blood is composed of solid particles in liquid. the solid portion of whole blood contains: (1) white blood cells; (2) red blood cells; and (3) platelets. The liquid portion of the blood is known as plasma.
In this manner, if a man has a hematocrit level of .51, then his whole blood consists of 51% solids and 49% liquids (plasma). This solid to liquid ratio will effect the outcome of a blood alcohol concentration test. The reason is the liquid portion of the whole blood, the plasma, contains water. Alcohol is more susceptible of being dissolved in water than is the solid portion. Consequently, the liquid portion of the whole blood will have a higher concentration of alcohol than the solid portion.
Stated another way, the higher the hematocrit level (thus the less liquid) in the blood, the greater the concentration of alcohol in the liquid portion of the blood. Ultimately this means several people with the same amount of alcohol in their body, but different hematocrit levels, will have different test results.
Men and women have different average hematocrit levels. A normal hematocrit for a man is 45 (plus or minus 7%). Women have a normal hematocrit level of 42 (plus or minus 5%). There are numerous other factors that can effect a hematocrit level. However, a person's hematocrit is not proportional to body size.
Hematocrit ranges primarily effect breath alcohol testing. This is because in blood testing the blood is mixed with an internal standard (such as N-Propranol which has similar structure to Ethanol, but has a different number of carbons). In sum, varying hematocrit levels expand the range of accuracy in blood alcohol concentration testing. This expanded range of accuracy can be a valuable defense for a person accused of driving under the influence alcohol (DUI).