Full Blood Count and Blood Smear

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

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 (leukocytes). 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 make blood cells, haemoglobin and the constituents of plasma constantly, you need a healthy bone marrow and nutrients from food including iron and certain vitamins.

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 - 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 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 - 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, 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 - this is called thrombocytopenia. This may make you bruise or bleed easily. There are various causes.
  • Too many platelets - this is called thrombocythaemia. This is due to disorders which affect cells in the bone marrow which make platelets.

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 the 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.

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.

Original Author:
Dr Rachel Hoad-Robson
Current Version:
Peer Reviewer:
Dr Hannah Gronow
Document ID:
4747 (v39)
Last Checked:
13/12/2012
Next Review:
13/12/2015
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