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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
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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