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What is a
blood test?
Blood
tests are a very useful diagnostic tool.
Blood is made up of several different
kinds of cells and other compounds,
including various salts and certain
proteins.
The liquid portion of the blood is
called plasma. When blood clots outside
the body, the blood cells and some of
the proteins become solid. The remaining
liquid is called serum, which can be
used in chemical tests and in tests to
find out how the immune system fights
diseases.
Doctors can take blood samples and grow
the infectious organisms that cause an
illness to see exactly what they are
through a microscope.
How
is a blood test carried out?
Blood
samples for testing can be taken either
from a vein (which carries blood to the
heart) or from an artery (which takes
blood away from the heart).
If only a few drops of blood are needed
(for monitoring blood sugar in diabetes,
for example) it is enough to make a
small prick in the tip of the finger and
then squeeze the blood out.
Most blood tests are taken from a vein,
commonly from those around the elbow.
First a cord (tourniquet) is tied around
the upper arm to make the vein
prominent. It may be a bit tight, but
this makes it much easier to take the
test.
The site of the injection is then
cleaned with spirit and then a needle is
put into the vein. The needle will be
attached either to a low pressure blood
test bottle, or to a syringe where the
plunger is pulled back to create low
pressure. When the necessary amount of
blood has been extracted, the needle is
removed and a little ball of cotton wool
is held over the wound. This should be
pressed for one to two minutes before
applying a sticking plaster.
If blood is taken from an artery, it is
usually extracted from the wrist where
there is an artery that is very close to
the skin. This may be slightly
uncomfortable, as the artery wall has
more pain nerves in it than the vein
wall.
After taking blood from an artery it may
be necessary to hold a ball of cotton
wool on the place where the injection
was made for about five minutes to stop
any bleeding.
Some people are very sensitive to
needles and the sight of their own blood
and may feel faint when a blood sample
is taken. This is not uncommon and can
be reduced by sitting or lying down
while the sample is taken. If you feel
faint or think that you might feel
faint, immediately tell the person who
is taking blood.
What do doctors examine in the blood?
Blood
contains two main elements: the fluid
that is called plasma and cells. There
are three kinds of cells: red blood
cells, white blood cells and platelets.
To get the information they need from
the blood, doctors actually do several
tests with the blood sample. These
include measurements of the levels of
the cells and a blood smear. A blood
smear is a film of blood placed on a
slide to allow doctors to look at the
individual cells under a microscope.
These tests are listed below.
Red
blood cells
One
of the most important red blood cell
tests is used to find out how much
haemoglobin there is in the blood.
Haemoglobin carries oxygen around
your body. This is called the
haemoglobin concentration or level.
Another important test, the mean
corpuscular volume or MCV test,
measures the size of the red blood
cells.
If a person suffers from anaemia
their haemoglobin level will always
be less than normal. But the size of
the red blood cells depends on the
type of anaemia you have.
A haematocrit test measures the
total volume that red blood cells
take up in the blood. In practice,
this is done by spinning a test tube
of blood until the red blood cells -
the heaviest part of the blood - go
to the bottom of the tube. Then
their volume is calculated.
Almost all types of anaemia will
cause a low haematocrit (a low red
blood cell volume), as will very
severe bleeding. A high haematocrit
can occur if a person is dehydrated
from not drinking enough fluid or
because they are losing fluid as
happens with diarrhoea, burns and
sometimes surgery.
If the red blood cells are pale, it
can be a sign of iron deficiency
anaemia. If they have a strange
shape, it may be because of sickle
cell anaemia or pernicious anaemia.
Doctors also add stains to the blood
smear to test the blood for
parasites, for example in the case
of sleeping sickness or malaria.
They may also test for bacteria in
the case of blood poisoning.
White
blood cells (WBC)
The
doctor counts the total number of
white blood cells and works out how
many different types of white blood
cells the patient has. This is
called the differential WBC count.
The number of white blood cells may
go up and this may be because of a
bacterial infection, bleeding or a
burn. More rarely the cause of a
raised white count is due to
leukaemia, cancer or malaria.
A person may lose white blood cells
because they have autoimmune
problems - this is where the
antibodies that should fight
diseases attack the body instead.
Other reasons for loss of white
blood cells include viral
infections. More rarely, this can be
a side effect of certain kinds of
medication.
Doctors keep an eye on white blood
cells to work out how a disease is
changing. By monitoring the blood
count in this way they can alter the
patient's treatment as necessary.
Platelets
Platelets are very small cells in
the blood that clump together at
sites of injury to blood vessels.
They form the basis of the blood
clot that would form if you cut
yourself.
Low numbers of platelets can make a
person vulnerable to bleeding,
sometimes even without injury
occurring. Causes of low platelet
counts include autoimmune diseases
where you produce an antibody to
your own platelets, chemotherapy,
leukaemia, viral infections and some
medicines.
High numbers of platelets make a
person more vulnerable to blood
clots. High platelet counts are
found in conditions involving the
bone marrow such as leukaemia and
cancer.
What are
blood coagulation examinations?
More
tests will be needed if a patient is
found to be suffering from a blood
coagulation disorder so that either
their blood doesn't clot properly, or if
it clots too well.
When a vein is damaged, usually a little
blood clot will form on the inside. This
clot is made of blood platelets and
proteins from the blood plasma (called
the coagulation factors).
A person will bleed more than normal if
they have a low number of blood
platelets, if there is a lack of
coagulation factors, or if they don't
work.
If the bleeding disorder is caused by
problems with the coagulation factors
more tests will be needed. Sometimes a
coagulation disorder is passed on in the
family, but it could also be due to a
liver problem, as the liver makes many
of the blood clotting factors.
Coagulation tests will be performed
regularly for people who are on blood
thinning medicines such as warfarin.
Doctors will change the dose of these
medicines depending on the test results.
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