Everyone's Blood is Not the Same
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.
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