Full Blood Count and Blood Smear

Post your experience
See others (1 there)

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. This binds to oxygen, and takes oxygen from the lungs to all parts of the body.
  • White cells (leucocytes). There are different types of white cells 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

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.

What is a full blood count?

A full blood count (FBC) is one of the most common blood tests done. A blood sample is taken which is prevented from clotting by using a preservative in the blood bottle. The sample is put into a machine which automatically:

• counts the number of red cells, white cells, and platelets per ml of blood.
• measures the size of the red blood cells and calculates their average (mean) size.
• calculates the proportion of blood made up from red blood cells (the haematocrit).
• measures the amount of haemoglobin in the red blood cells.

The main abnormalities which can be detected are:

• Anaemia which means that you have less red blood cells than normal, or have less haemoglobin than normal in each red blood cell. The most common reason for a full blood count to be done is to check for anaemia. There are many causes of anaemia. The average size of the red cells can give a clue as to the cause of some anaemias. For example, the most common cause of anaemia is a lack of iron. With this type of anaemia the average size of the red blood cells is smaller than normal.
• Too many red cells which is called polycythaemia and can be due to various causes.
• Too few white cells which is called leucopenia. Depending on which type of white cell is reduced it can be called neutropenia, lymphopenia, or eosinopenia. There are various causes.
• Too many white blood cells which is called leucocytosis. Depending on which type of white cell is increased it is called neutrophilia, lymphocytosis, eosinophilia, monocytosis, basophilia. There are various causes. For example:
  • various infections can cause an increase of white blood cells.
  • certain allergies can cause an eosinophilia.
  • leukaemia causes a large increase in the number of white blood cells. The type of leukaemia depends on the type of white cell affected.
• Too few platelets which is called thrombocytopenia. This may make you bruise or bleed easily. There are various causes.
• Too many platelets which is called thrombocythaemia. This is due to a disorders which affect cells in the bone marrow which make platelets.

What is a blood smear?

This is when a thin film of blood is examined under a microscope. This is used to look for abnormal shapes of cells which cannot be detected by the automated machine. For example, to detect he characteristic 'sickle' shape of the red blood cells which occur in sickle cell anaemia. Also, infecting parasites such as the malaria parasite can be seen in a blood smear.

Do I need further tests?

An abnormality in a blood count can often be caused by various conditions. Therefore, if an abnormality is found you often need further tests to clarify the cause. For example, the most common abnormality is anaemia. If you are found to have anaemia you may be advised to have another blood test to check on the level of iron or certain vitamins in your blood. And, if further blood tests do not clarify the cause then other tests may be needed.

A bone marrow biopsy is sometimes needed to find the cause of anaemia and other blood cell problems. The bone marrow is where the blood cells are made from blood 'stem' cells. A sample (biopsy) of bone marrow can be examined under the microscope and tested in other ways to help to find the cause of the abnormality.

Various other tests may be advised, depending on the abnormality found in the blood count.

© EMIS and PIP 2006   Updated: February 2006

The authors and editors of this article are employed to create accurate and up to date content reflecting reliable research evidence, guidance and best clinical practice. They are free from any commercial conflicts of interest. Find out more about updating.

Patient UK Hearing Impairment Survey

Patient UK are grateful to the 550 people who took part in this survey.
To see the results click here.
If you'd like to leave your feedback, please go to our interactive forum.