Full Blood Count and Blood Smear

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A full blood count is a commonly done test. It can detect anaemia and various other blood problems. A blood smear is a test in which blood cells are looked at under a microscope.

Blood cells

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:

  • Hormones.
  • Antibodies.
  • Enzymes.
  • Glucose.
  • Fat particles.
  • Salts.

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.

An 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 millilitre 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 - this means that you have fewer red blood cells than normal, or have less haemoglobin than normal in each red blood cell. The most common reason for an FBC 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 in the UK is a lack of iron. (Iron is needed to make haemoglobin.) With this type of anaemia, the average size of the red blood cells is smaller than normal.
  • Too many red cells - this is called polycythaemia and can be due to various causes.
  • Too few white cells - this is called leukopenia. 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 - this is called leukocytosis. Depending on which type of white cell is increased it is called neutrophilia, lymphocytosis, eosinophilia, monocytosis or basophilia. There are various causes - for example:
    • Various infections can cause an increase of white blood cells.
    • Certain allergies can cause an eosinophilia.
    • Leukaemia is a type of blood cancer where there is a large number of abnormal cells, usually white blood cells. The type of leukaemia depends on the type of white cell affected.
  • Too few platelets - this is called thrombocytopenia. This may make you bruise or bleed easily. There are various causes.
  • Too many platelets - this is called thrombocythaemia (or thrombocytosis). This is due to disorders which affect cells in the bone marrow which make platelets.

This is a thin film of blood which 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 the characteristic 'sickle' shape of the red blood cells which occur in sickle cell anaemia. Also, infecting germs such as the malaria parasite can be seen in a blood smear.

Lots of different conditions can cause an abnormality in a blood count. Therefore, if an abnormality is found, you often need further tests to find the cause. For example, anaemia is the most common abnormality. 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 these tests are normal 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 to help to find the cause of the abnormality.

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

Original Author:
Dr Tim Kenny
Current Version:
Peer Reviewer:
Dr John Cox
Document ID:
4747 (v40)
Last Checked:
Next Review:
The Information Standard - certified member
Now read about Thrombocytopenia and Platelet Function Disorders

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