In South Carolina, under certain circumstances, an arresting officer can obtain a blood sample from a DUI suspect. For a DUI defense attorney and his or her client, an arrest based on a blood sample raises a number of different issues than cases involving an arrest based on a breath sample.
The Blood Test Kit
Law enforcement use Blood Test Kits for obtaining blood samples in DUI cases. Among other things, these kits contain Vacutainer™ test tubes. These kits have an expiration date, after which the vacuum in tube is no longer warranted. This is important because the precise vacuum in each tube ensures that the right amount of blood (10 ml.) will be drawn in proportion to the preservatives and the anticoagulant that mix with the blood in the tube. If there is too much chemical and not enough blood, your test results can be affected. Further, if a Vacutainer™ leaks, microorganisms from room air can enter the blood sample and cause the blood to ferment. Because alcohol is a byproduct of fermentation, it can affect to blood test results. Finally, the blood sample must be refrigerated to avoid further contamination.
Drawing the Blood
The medical personnel who actually draws the blood from a DUI suspect must be trained to do venipuncture (blood withdrawal). In drawing the sample, hospitals sometimes use isopropyl alcohol to clean the suspect's skin which can contaminate the blood sample.
Testing the Blood
Depending upon whether a test was conducted upon whole blood or plasma (serum), the results of the analysis can be affected:
Typically, a blood alcohol content analysis of plasma or serum will result in approximately a 16% higher alcohol content than a test of whole blood.
Additionally, a suspect's hematocrit level can affect the blood alcohol test. Hematocrit is the percentage of your whole blood that is composed of cellular material. If you have a hematocrit of 47 (which is average for males), then 47% of your blood is cellular material and 53% is plasma (mostly water). However, if you have a higher hematocrit, you are going to get a higher blood alcohol content (BAC) reading because you have less liquid in your blood.
Finally, IV fluids given before blood draw will result in a higher BAC reading because alcohol tends to follow/bond with water in the blood, and the water tends to draw alcohol out of your body tissues and into your blood stream.
Testing Procedure - Gas Chromatography
Typically, blood is analyzed using gas chromatography (GC). GC is a method of (1) identifying a substance and (2) determining the concentration of that substance. There are two types of GC: (1) gas chromatography which directly measures the blood sample and (2) gas head space chromatography which tests the gas or vapor above the liquid--not the liquid itself. The head space is the space above the liquid. In this test, the alcohol evaporates (at a rate of speed determined by temperature) from the liquid to the gas in the head space above the liquid. The alcohol evaporates until it reaches the point of equilibrium, which is determined by temperature. The higher the temperature, the more alcohol in the gas above the liquid. With this process, the lab must heat the blood sample (mixed with internal standards), draw off the vapor, and inject the vapor into the chromatograph for analysis. The process assumes there is a relationship between the alcohol in this vapor and the actual alcohol in the blood. This relationship can be affected if the temperature is not properly regulated. Further, the gas Chromatography analysis depends on the validity and accuracy of the standards that are used to calibrate the chromatograph.
Chain of Custody
As with all evidence collected by law enforcement, the prosecutor must show a proper chain of evidence. So, the prosecutor must show that the blood that was tested was the same blood that left the DUI suspect's arm.