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Frequently Asked Questions

What is blood made up of?

Plasma, the liquid part of blood, makes up about 60% of the blood’s volume. Plasma is mainly made from water, but contains many different proteins and other chemicals such as hormones, antibodies, enzymes, glucose, fat particles, salts, etc.
Blood cells, which can be seen under a microscope, make up about 40% of the blood’s volume. Blood cells are made in the bone marrow by blood ‘stem’ cells. Blood cells are divided into three main types:
Red cells (erythrocytes). These make blood a red colour. One drop of blood contains about five million red cells. A constant new supply of red blood cells is needed to replace old cells that break down. Millions are released into the bloodstream from the bone marrow each day. Red cells contain a chemical called haemoglobin. Haemoglobin is attracted to oxygen and the two substances can bind together. This allows oxygen to be transported by red blood cells from the lungs to all parts of the body.
White cells (leucocytes). There are different types such as neutrophils (polymorphs), lymphocytes, eosinophils, monocytes, basophils. They are a part of the immune system and are mainly involved in combating infection.
Platelets. These are tiny and help the blood to clot if we cut ourselves.

In order constantly to make blood cells, haemoglobin, and the constituents of plasma, you need a healthy bone marrow and nutrients from food including iron and certain vitamins.

When blood spills from your body (or a blood sample is taken into a plain glass tube) the cells and certain plasma proteins clump together to form a clot. The remaining clear fluid is called serum.

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How is a blood test normally done?

The vein used for blood sampling is usually on the inside of your elbow or the back of your wrist.
A tourniquet (tight band) is usually placed around your upper arm. This makes the vein fill with blood and makes it easier for the blood sample to be taken.
The skin over the vein is cleaned with an antiseptic wipe.
A needle is then inserted into the vein through the cleaned skin. The needle is connected either to a syringe, or directly to blood sample bottles.
When the required amount of blood is taken, the needle is removed. The small wound is pressed on with cotton wool for a few minutes to stop the bleeding and prevent bruising. A sticky plaster may be put on. The blood is placed in bottles.

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What are the variations of blood taking?

Some blood tests require several samples taken over a period of time. For example, they may be done to check how you respond to something. If you require repeated samples fairly close to each other (over the following few hours or so) a doctor may insert a ‘butterfly’ needle into the vein, which can be taped to the skin. Samples of blood can then be taken without using a needle each time.
If only a small amount of blood is needed then a few drops of blood can be squeezed out from a small prick in the tip of the finger or earlobe. For example, only a small amount of blood is needed for checking the blood sugar (glucose) level using a test strip of paper.
Some blood tests are taken from an artery in the wrist. For example, to measure the level of oxygen in the artery. This is usually only done in hospital in certain circumstances.
You may be told to not eat for a time before certain tests. For example, a test of blood glucose is commonly done first thing in the morning before you have anything to eat.

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Are there any complications from blood taking?

Sometimes a bruise develops where the needle was inserted. This is much less likely to happen if you press over the site with cotton wool for several minutes with your arm left straight (not bent).
As with any wound, an infection may develop where the needle was inserted. See your doctor if the wound site becomes red and inflamed.
Rarely, some people feel faint during a blood test. Tell the person doing the test if you feel faint as you should immediately lie down to prevent fainting.

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Why are several blood samples taken?

Blood can be tested for many different things. The person who requests the blood test will write on the form which tests they want the ‘lab’ to do. Different blood bottles are used for different tests. For example, for some tests the blood needs to clot and the test is looking for something in serum. For some tests, the blood is added to some chemicals to prevent it from clotting. If the blood glucose is being measured, then the blood is added to a special preservative, etc. This is why you may see your blood added to blood bottles of different sizes and colours.

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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.

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What does a test of red blood cells (RBC) indicate?

To test how much haemoglobin there is in the blood.
The mean corpuscular volume or MCV test, measures the size of the red blood cells. If you are suffering from anaemia the 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. Almost all types of anaemia will cause a low red blood 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 which can happen 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 for parasites for example in the case of sleeping sickness or malaria and also for bacteria in the case of food poisoning.

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What do the white blood cells (WBC) indicate?

A raised white blood cells count 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.

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What do the platelets indicate?

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.

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Why are blood tests taken?

Blood tests are taken for many different reasons. For example:
Help diagnose certain conditions, or to rule them out if symptoms suggest possible conditions.
Monitor the activity and severity of certain conditions. For example, a blood test may help to see if a condition is responding to treatment.
Check the body’s functions such as liver and kidney function when you are taking certain medicines which may cause side-effects.
Check your blood group before receiving a blood transfusion.

The most common blood tests are:
Full blood count – checks for anaemia, and other conditions which affect the blood cells
Kidney function
Liver function
Blood glucose (sugar) level
Blood clotting tests
Tests for inflammation
Blood cholesterol level
Immunology – such as checking for antibodies to certain viruses and bacteria
Blood grouping
Thyroid function

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What are blood coagulation examinations?

If a patient is found to be suffering from a blood coagulation disorder so that either their blood does not clot properly, or if it clots too well then a blood coagulation examination will be necessary.
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 do not 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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