Daily Archives: March 5, 2006
When a blood sample is taken from a DUI suspect for later analysis, it is usually done in one of two ways. The suspect may have the blood drawn at the police station by a technician, using a prepared kit containing a vial, or it may be taken by a nurse or technician at a medical facility. In either event, it is critical that the vial in which the blood is contained is sterile and contains two things: a preservative and an anticoagulant. The preservative, in conjunction with refrigeration, is to prevent the blood from fermenting — and thereby producing alcohol in the vial before it is tested. The anticoagulant is to prevent the blood from coagulating, or clotting.
Why are we concerned about coagulation of the blood?
Blood is made up of a mixture of solid particles supended in a liquid. The solid particles consist of red blood cells, white blood cells and clotting platelets; the liquid portion is called serum or plasma. (The percentage by volume of the solid particles to the liquid is called the hematocrit of the blood: a hematocrit of .47, for example, would indicate that the individual’s blood consists of 47 percent solid particles — cells and platelets — and 53 percent plasma.)
When blood clots, the liquid portion (plasma) separates from the solid portion (blood cells and clotting platelets). This will be seen in the sample vial as a red clump at the bottom (cells) with a yellowish liquid on top (plasma). When this sample is tested at the laboratory, usually days later, it is the plasma that is tested for alcohol content; the clotted cells at the bottom are not included.
So what? Well, alcohol is attracted to water — that is, it is soluble in water. And since plasma is a liquid and contains water, and alcohol is attracted to water, the plasma in the blood sample will contain a higher percentage of alcohol than in the whole blood sample. The higher the percentage of plasma in the sample being tested, the higher will be the blood alcohol concentration (BAC). Put another way, if two subjects have the same BAC in their bodies but the blood sample from one has clotted and so has a higher percentage of plasma, that person’s “sample” will show a higher BAC.
Unfortunately, it is not uncommon for blood samples collected by police agencies or hospital personnel to contain no anticoagulant, or to contain insufficient amounts of the chemical. (And, of course, we have the emerging practice of just letting the police officer himself perform the blood draw and sample preservation out on the highway.)
An additional problem is that the kits used by technicians usually contain a vial already containing a preservative (commonly sodium fluoride) and an anticoagulant (commonly potassium oxalate) in powder form at the bottom. However, when the blood is added to the vial, the technician does not vigourously shake it — and the chemicals are not mixed with the blood. Result: coagulation — and a falsely high blood alcohol result.