This leaflet gives general information about blood tests. There are separate leaflets that describe various specific types of blood test.
What is blood made up 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 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 of red blood cells are made each day. Red cells contain a chemical called haemoglobin. This binds to oxygen and takes 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 specialised 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.
- Platelets. 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, 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.
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 not to 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.
- Children who need a blood test may be given cream to put on at home before the test to numb the skin.