Full blood count

What is being analysed?
The full blood count is an automated test that counts the cells in the blood. It provides information on the white blood cell, red blood cell and platelet populations, including the number, type, size, shape and some physical characteristics of the cells. In a few minutes haematology analysers (the instruments used for this test) can measure thousands of red blood cells, white blood cells and platelets and compare them with a series of preset parameters. Any abnormalities found are flagged, and experienced laboratory staff assess whether to accept the automated result and/or send the sample for further tests.
In most cases the automated count is very accurate and the test is complete at this point. If there are significant abnormalities in one or more cell populations, a peripheral blood smear is made. In this test a drop of blood is placed on a glass slide and spread into a thin layer, left to dry and then stained with a specific stain. A specialist examines the stained slide under the microscope and assesses the cell populations present. This procedure adds some further information to the automated count.

Blood is made up of cells suspended in a fluid called plasma. These cells (red blood cells, white blood cells and platelets) are produced and go through their first stage of maturation in the bone marrow. Normally these cells are released into the bloodstream as they are needed.

White blood cells
There are five types of white blood cells that the body uses to fight infections and other diseases. These subpopulations (neutrophils, lymphocytes, basophils, eosinophils and monocytes) are present in fairly stable proportions, which can rise or fall temporarily depending on what is happening in the body. For example, during an infection there may be an increase in neutrophils (left shift). With an allergy the number of eosinophils may increase, and with leukaemia there may be an increase in a single type of cell, for example lymphocytes. In this case the subpopulation may be present in large numbers, both in mature form and at various stages of maturation. The full blood count shows, for example, whether there are enough white blood cells to fight an infection, shows when there are more than expected, and gives the percentage and number of each subpopulation.

Red blood cells
Red blood cells are lightly coloured red and shaped like a doughnut, coloured at the edge and pale in the centre. Inside is haemoglobin, a protein that carries oxygen. The full blood count shows whether there are enough red blood cells and whether they look normal. Red blood cells are normally uniform in shape and size; variations may be present with vitamin B12 and folate deficiencies, iron deficiency and other conditions. If there are too few normal red blood cells, the patient is said to be anaemic and may have associated symptoms such as tiredness and weakness. Less often, there may be too many red blood cells (erythrocytosis, polycythaemia). In extreme cases this can interfere with blood flow.

Platelets
Platelets are fragments of specialised cells that play a key role in the clotting process. If a patient does not have enough platelets, they may be at high risk of bleeding. In certain conditions and in some people there may be giant platelets or platelet clumps, which are particularly difficult to analyse with haematology analysers. In these cases a peripheral blood smear on a slide is needed

How is it used?
When is it requested?
What does the test result mean?

How is it used?
The full blood count is used to screen for a wide range of blood disorders such as anaemia, infection and many other diseases. It currently consists of a panel of tests that check different parts of the blood:

The white blood cell count is the number of white blood cells per volume of blood. Both increases and decreases are highly significant.

The differential count gives information on the white blood cell subpopulations. There are five different subpopulations of white blood cells, each with its own role in defending the body against infections. The differential count assigns the white blood cells to their classes: neutrophils (also called polymorphonuclear cells or granulocytes), lymphocytes, monocytes, eosinophils and basophils.

The red blood cell count is the number of red blood cells. Both increases and decreases can be present in abnormal conditions.

Haemoglobin measures the concentration of the oxygen-carrying protein.

The haematocrit measures the percentage of cells relative to the liquid part of the blood.

The platelet count is the number of platelets in a given volume of blood. Both increases and decreases are linked to clotting disorders. The mean platelet volume (MPV) is a parameter calculated by the analyser that represents the average volume of all the platelets. Young platelets are larger, so the mean platelet volume increases when new platelets enter the circulation. This parameter gives the doctor information on platelet production by the bone marrow.

The mean corpuscular volume (MCV) is the average volume of the circulating red blood cells. The MCV rises when red blood cells are larger than normal (macrocytosis), for example in anaemia due to vitamin B12 deficiency. A low MCV means the red blood cells are smaller (microcytosis), as seen in iron deficiency anaemia or thalassaemia.

The mean corpuscular haemoglobin (MCH) is a calculation of the average haemoglobin in each red blood cell. In macrocytosis the red blood cells tend to be larger than in normal cells or in microcytosis, so people with these changes often have a higher MCH as well.

When is it requested?
The full blood count used to be very frequently requested during routine check-ups. Although it is not requested quite as often today, it is still used to detect or monitor a wide range of conditions. In many patients the full blood count is used to assess general health. If they are healthy and the cell populations are within normal limits, they do not need to repeat the full blood count until their general health changes or until the doctor considers it appropriate.
If a patient has symptoms associated with anaemia, such as tiredness or weakness, or has an infection, inflammation, bruising or bleeding, the doctor may request a full blood count to find the cause. Significant increases in white blood cells can help confirm an infection and suggest further tests to identify its cause. Changes in the size and shape of red blood cells can help determine whether a fall in red blood cells is due to reduced production, increased loss or increased destruction. A low or extremely high platelet count can confirm the cause of excessive bleeding or excessive clotting.

Many conditions can cause increases or decreases in the various cell populations. Some of these conditions need treatment, while others resolve on their own. Some diseases, such as cancer (and chemotherapy), can interfere with cell production by the bone marrow, increasing the production of one cell line at the expense of the others or reducing the bone marrow’s overall production. Some medicines can lower the white blood cell count, and some vitamin and mineral deficiencies can cause anaemia. The doctor may request a full blood count to monitor these conditions or drug treatments.

What does the test result mean?
There is no single reference range for this test. Reference values may differ between laboratories, as they depend on many factors, including the patient’s age and sex, the reference population and the analytical methods. Your laboratory report should include specific reference ranges for each test.

The table below explains what increases or decreases in the individual components of the full blood count mean: