Blood Test – General

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.

Different blood samples

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.

Blood tests are taken for many different reasons. For example, to:

  • 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

For details on each of the above tests see separate leaflets called ‘Blood Test – Blood Count and Smear’, ‘Blood Test – Kidney Function’, ‘Blood Test – Liver Function Tests’, ‘Blood Test – Glucose’, ‘Blood Tests – Clotting Tests’, ‘Blood Test – Detecting Inflammation’, ‘Cholesterol’, ‘Antibody and Antigen Tests’, ‘Blood Test – Blood Grouping’, and ‘Blood Test – Thyroid Function’.


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