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BLOOD ANALYSIS IN DUI CASES
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:
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Whole blood consists
of serum plus cellular material and fibrinogen (clotting
agent).
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Serum or plasma is
blood which has been filtered to remove the cellular
material and fibrinogen. |
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.
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If you or someone you know needs a DUI
defense attorney in South Carolina, call the Futeral Law
Firm locally at (843) 284-5500, toll free at (877) 913-5500,
or email
info@charlestonlaw.net.
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