Blood tests, What does the blood consist of?
These can be seen under a microscope and 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 color, One drop of blood contains about five million red cells.
And a constant new supply of red blood cells is needed to replace old cells that break down
Millions of red blood cells are made each day, and Red cells contain a chemical called hemoglobin.
This binds to oxygen and take oxygen from the lungs to all parts of the body.
White cells (leukocytes)
There are different types of white cells which are called neutrophils (polymorphs), lymphocytes, eosinophils, monocytes and basophils.
They are part of the immune system. Their main role is to defend the body against infection.
Neutrophils engulf germs (bacteria) and destroy them with special chemicals.
Eosinophils and monocytes also work by swallowing up foreign particles in the body.
Basophils help to intensify inflammation.
Inflammation makes blood vessels leaky.
This helps specialized white blood cells get to where they are needed. Lymphocytes have a variety of different functions.
They attack viruses and other germs (pathogens). They also make antibodies which help to destroy pathogens.
These are tiny and help the blood to clot if we cut ourselves
This is the liquid part of blood and makes up about 60% of the blood’s volume. Plasma is mainly made from water but also contains many different proteins and other chemicals, such as
- Fat particles
In order to constantly make blood cells, hemoglobin 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
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
And a tight band (tourniquet) 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 may be cleaned with an antiseptic wipe
A needle is then inserted into the vein through the clean skin, causing a sharp pricking sensation
The needle is connected either to a syringe, or directly to blood sample bottles
The tourniquet is undone
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 sticking plaster may be put on. The blood is placed in bottles.
There may be a slight ache following a blood test
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How much blood is normally taken?
This depends on the sort of tests you are having done. Many tests can be done on the same sample of blood so you do not need to have a single bottle for each one:
The bottles do not necessarily have to be completely filled either
If you need a lot of tests therefore, you could end up having about 30 ml of blood (about six medicine teaspoons) taken out of your arm.
It’s worth saying that the body can well cope with this, as the volume is soon made up by the blood production system in the bone marrow.
Considering that about 500 ml of blood are given by blood donors during each donation.
Fasting blood tests
You may be told not to eat or drink anything except water before certain blood tests
The amount of time you will be requested to fast varies from test to test and even sometimes from doctor to doctor
The fasting time for a blood glucose tests for example is usually 8 to 10 hours.
For a fasting cholesterol test it can be 9 to 12 hours
Fasting tests are usually done in the morning so you can fast through the night
Recent guidelines advise that fasting may not be necessary when having a cholesterol check, but be guided by your doctor
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
So call 997 immediately 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
If you have any of these symptoms, call 997 immediately
Delay may result in serious health complications
Talk to your Doctor, Nurse or Health Education Specialist if you have any questions or concern