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| Blood Analysis |
| Blood analysis
is a simple test which can be useful for many cases involving a blood
stained crime scene and in the verification/identification of a unknown
victim's identity. |
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Choose one of the topics below for more information:
--> Verification
--> Blood
types
--> The
test
--> The
rh factor
--> Other
bodily fluids
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| Verification |
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| When a stain is
found at the scene of a crime, the first thing that has to be
determined is whether the stain is blood or some other bodily
fluid. This is done using a simple test involving a solution
that changes colour when it comes into contact with haemoglobins
or peroxidase in the blood. Another type of test commonly
used involves luminal spray, which makes any residue
containing blood, glow in the dark as well as picking up on
traces of blood that may have been scrubbed away. The next step
is confirming whether the bloodstain belongs to a human. Serologists,
people who study blood, place the sample and a testing solution
into small wells on a gel-coated glass plate, and the two will
defuse towards each other. If the sample is human blood,
it will contain human antigens and where the two solutions
meet on the gel-coated plate, a noticeable band forms. |
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Blood particles below a microscope show up like the image above.
Photo courtesy of www.imageafter.com.
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| Blood
Types |
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| Determining which person the bloodstain
belongs to involves an investigation of blood types. The human blood
contains over 100 different antigens, therefore it would be time consuming
and unpractical to test for every single one. Serologists instead
use a number different blood testing techniques, but by far the most
common and effective technique is the ABO system. This system is also
used to determine compatibility for blood donors and recipients. The
ABO blood type system involves checking the surface of the red blood
cells for two antigens known as A and B, with blood type being named
after the type of antigens it contains - A, B, AB and O. By noting
that a blood clump forms when the same type of antigen meets the same
type of antibody, an experiment can be done on the solution
of blood to determine the blood type. |
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| The
Test |
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| The test is done using two solutions
each containing antibodies to type A and type B antigens. The first
solution contains type A antibodies and when mixed with type A blood,
will cause it to form clumps. The same concept is used to test for
B antigens, where a solution of type B antibodies would cause all
type B antigens in the blood to clump together. If blood clumps under
contact with both A and B antibodies, then it is of the blood type
AB, since both antigens are present in the blood. O blood does not
clump with any other blood type and is therefore identified because
it is solitary. |
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| The
RH Factor |
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| For finer results yet, the blood groups
can be assigned either a + or a - figure after it to indicate the
presence (+) or absence (-) of a blood protein known as the Rh
factor (named after the Rhesus Monkey, in which it was first recognized).
Using an antibody solution to the Rh protein, the same concept is
used, where blood clumping determines the absence/presence of this
protein. Thus, the finer blood groups include A+, A-, B+, B-, AB+,
AB-, O+ and O-. |
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| Other
Bodily Fluids |
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| Blood is not the only fluid that is
excreted from the body and tested by serologists. Substances like
saliva, semen, urine and excrement contain DNA, can be compared with
a suspect. In cases concerning rape, investigators need to be sure
that the swab taken or the stain found is semen and this is confirmed
using a test that changes colour on contact with SAP (seminal acid
phosphatase), spermine and choline. Microscopes are also used
to see individual sperm, but this technique is not accurate, as a
rapist who has had a vasectomy or is sterile will not show
sperm under a microscope, even if they committed the rape. However,
blood, semen and urine samples all contain DNA, which is slightly
more accurate (and expensive) in singling out the criminal. |
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