What is a full blood count and why is it done?

A full blood count (FBC) is a very common clinical procedure and often the “starting point” for most medical investigations. An FBC not only tests for disorders and abnormalities of the blood but, as blood travels throughout the whole body, it can give an indication of disease present in other organs.
An FBC, as the name suggests, is used to obtain a count of the blood cells in the sample of blood taken. The counts from this small sample are used to estimate the levels of different blood cells within your body’s blood system.
Blood is made up from three main types of blood cell: red blood cells, white blood cells and platelets. The number of cells present, the size and proportions of these cells, and the haemoglobin level are measured in an FBC. Haemoglobin is the oxygen carrying component of red blood cells.


How is an FBC performed?

A blood test can be carried out by your doctor, or you will be sent to a pathology centre where other health care professionals, such as a nurse, will perform the procedure. The full blood count is a very quick procedure, usually only 5 minutes!

Preparing for an FBC

There is no preparation necessary for a full blood count. It can be done at any time of the day.
However, if the blood test will be used not only for a blood count but for other reasons (e.g. glucose or cholesterol monitoring), you will be required to fast beforehand. Therefore, these tests are better done in the morning.

What happens during an FBC?


You are required to sit upright in a chair and rest your arm face up on an elevated arm rest. The clinician will tie a strap (tourniquet) around the top of your arm to temporarily restrict the blood flow from your arm back to your heart. This will make the vein inside your elbow “pop out”, and therefore easier to find. The area where the needle will be inserted is wiped with a sterile alcohol wipe to reduce any risk of infection. A needle is inserted into the vein and a small amount of blood is drawn into the vial attached to the needle.
If seeing blood makes you queasy, it is best to look away. You may like to bring some jelly beans with you to suck on after the procedure in case you feel faint. Usually the clinician will ask you questions as they take the blood to distract you from the procedure. It is done in no time! If there is any pain at all, it is minimal. More often than not, you will only feel a slight discomfort.
After the procedure, the clinician will press a small wad of cotton on the entry point to stop the flow of blood. The cotton wad is strapped on with a bandaid. This only needs to remain on for a couple of hours. Sometimes a little bruise is left in this spot, but this is nothing to worry about.

The FBC for children

There is very little difference between children’s veins and adult veins. Therefore the procedure is very similar. Children can be quite scared of needles, though, so it is important that the parent and medical practitioner both encourage the child to remain calm and relaxed. You may wish to explain to the child that it only stings for a very short time, and offer them a treat afterwards.

Blood sample analysis

The analysis of the collected blood is carried out automatically by a machine in a pathology laboratory. The laboratory will send the results to your doctor. You will be able to call up for your results after approximately 5 days.

Results from an FBC

Full blood countMost FBCs come back with cell numbers in the normal range. About 5% of people that are tested will have a minor blood count abnormality. If there are symptoms associated with this abnormality, they can usually be treated quite easily. Sometimes, however, a full blood count can present abnormal results that are indicative of more serious diseases. In this case your doctor will refer you to a blood specialist for further tests.
Abnormal numbers of a specific type of blood cell can be indicative of specific problems. The information provided here must be used as a guide only. If you have an abnormal blood count, your doctor will determine what treatment is necessary.


These are the cell responsible for the clotting of the blood to stop wounds bleeding. The platelets stick together at the site of an open wound to form a temporary barrier from the external area.


The normal platelet count is 150–400 billion platelets per litre of blood (or 150-400 x 109/L). As you can imagine, this would make cells very small!


A low blood platelet count can be due to a condition called thrombocytopenia. Some drugs can cause a low platelet count. If this is the case, platelet levels will return to normal after stopping the drug. Symptoms can include excessive bleeding or bruising.


A high platelet count can be due to a condition called thrombocytosis.
This condition can predispose a patient to thrombosis in certain situations. People diagnosed with this should be very wary of situations that require them to remain in the same position for a long time as the blood can get “stuck”. If you work in an office environment, make sure you get up at least once an hour: stretch your legs, take a walk to the kitchen and make yourself a cup of tea! Plane rides are another risk for people with thrombocytosis. There are special blood circulation clothing that can be purchased for these situations.

Red blood cells (erythrocytes)

The major function of the red blood cell is to transport oxygen to all parts of the body. Red blood cells are made continuously in the bone marrow, and are released into the blood stream to replace the old circulating blood cells.


As haemoglobin is a chemical in a cell rather than a cell itself, it is measured by weight. The normal haemoglobin content of red blood cells is 115–150 grams per litre. This equates to approximately 27–34 picograms per red blood cell in one litre of blood. A picogram is one trillionth of a gram, or 1 x 10-12 grams! The normal volume of red blood cells in one litre of blood is 77-98 femtolitres. A femtolitre is even smaller than a picolitre (1 x 10-15 litres).


Low haemoglobin content is the most common abnormality found in routine FBCs. This is a condition known as anaemia. Anaemia can be present with or without an iron deficiency. Your doctor will likely assess your diet and history of drug use in order to determine the appropriate treatment.


Polycythaemia is associated with an abnormally high haemoglobin concentration in the blood, and is an indication that red blood cell numbers are also too high. This could be due to respiratory or circulatory disorders or, in some cases, to a tumour. Sometimes a high red blood cell count is due to dehydration.

White blood cells (leukocytes)

White blood cells function as part of the body’s immune system. They help to protect the body from infection and disease. There are several different types of white blood cell:


The combined white cell count is normally 4–10 billion cells per litre of blood.


A low neutrophil count can be associated with infection. This condition is named neutropenia. Low white blood cells can also be indicative of a bone marrow disorder.


The primary diagnosis for a high white blood cell count is leukocytosis. It is possible that a person with leukocytosis may have lymphoma or leukaemia. Therefore, your doctor will need to assess your history and take further blood tests. If the count returns high again, they will refer you to a specialist.
Raised eosinophil counts are most commonly the result of allergies or asthma.
If your blood cell counts are abnormal, it is important that you have frequent check ups to make sure serious problems do not develop.


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