What is a Full Blood Count?

The Full Blood Count (FBC) is one of the commonest blood tests performed by doctors. It provides an enormous amount of information about your state of health, your immune system, nutritional status and the clotting ability of your blood.

When a full blood count is performed, your blood is analysed to count the number of each type of cell within the blood, and then various measurements are made of those cells from which conclusions can be made about a wide range of medical conditions. 

There are three main cell types in the blood.

• Red blood cells
• White blood cells
• Platelets

Red Blood Cells - 

These are small, disc-shaped cells that contain the molecule haemoglobin. Haemoglobin is the iron-containing molecule that transports oxygen around the body and gives blood its red colour.

White Blood Cells - 

These are components of the immune system and are further divided into different types of white blood cells, each of which has slightly different roles in combatting infection.

For example, neutrophils are mainly involved in fighting bacterial infections, lymphocytes are involved in fighting viral infections, and eosinophils are involved in allergic responses and fighting parasites, and may also be raised in conditions like eczema or asthma.

Platelets - 

These are very small cells that look like fragments of other cells. Their role is in making the blood clot following an injury. You need adequate numbers of platelets for the blood to clot properly. 

A full blood count blood test can tell you a lot about your health including:

• General health : providing information on your general health.
• Diagnose conditions : Help diagnose conditions like anemia, leukemia, or infections.
• Monitor treatment : Help monitor how well you are responding to treatment for conditions.
• Screen for cancer : Help screen for cancers like leukemia.
• Find cause of symptoms : The full blood count test will help identify the cause of symptoms like fatigue, weakness, Pain, fever, swelling, bruising, or bleeding.

• Cell counts: number of different types of blood cells
• Cell size: average size of red blood cells, which can indicate certain conditions
• Platelet size: the full blood count blood test also measures the size of platelets, which can indicate how many platelets your bone marrow is producing.

Red Blood Cells

In a full blood count, the number of red blood cells in your blood is counted, the size of the red blood cells is checked (this is known as the ‘mean cell volume’ or MCV) and the amount of haemoglobin in the red blood cells is analysed.

These different parameters tell us a lot about your health. Low levels of red blood cells, and low levels of haemoglobin, is known as anaemia. Anaemia results when there is blood loss (such as bleeding) or when the body is not able to produce enough red blood cells which may be due to a variety of medical problems. The symptoms of anaemia include tiredness, shortness of breath, feeling light-headed and, in severe cases, chest pain.

The size of the red blood cells (the MCV) will also vary depending on various medical conditions. For example, if the red blood cells are bigger than expected then this can be due to deficiencies in Vitamin B12 or folic acid, an underactive thyroid, certain medications, or alcohol excess. Smaller than expected red blood cells may be due to iron deficiency, inherited blood conditions such as sickle cell anaemia or thalassemia or, rarely, lead poisoning. If someone is found to be anaemic, checking the MCV therefore gives us clues as to the cause of the anaemia.

White Blood Cells

Raised levels of these cells, identified in a full blood count blood test, are very common and usually suggest that the body is responding to an infection of some kind. The cause of the rise in white blood cells is usually obvious although, if it isn’t, the full blood count blood test can be repeated after a period of weeks to ensure that it has gone back to normal.

Sometimes, abnormal cell numbers may be due to certain blood disorders including sinister diagnoses like leukaemia. If such a sinister cause is suspected, the white blood cells can be closely examined under the microscope and further investigations may be required including procedures such as a bone marrow biopsy.

Platelets

Platelets may be low due to certain medications, certain blood disorders, after a viral illness (particularly in children) or sometimes due to an autoimmune condition which results in the destruction of platelets. Very low platelet numbers can result in a higher risk of bleeding and bruising.

Platelet numbers can rise in response to inflammation, either due to infection or other inflammatory conditions, and you may see high platelet numbers in these contexts. As with white blood cells, if platelets are found to be high, the full blood count test is usually repeated after a few weeks to make sure it has gone back to normal. Very high platelet numbers can be due to an underlying blood disorder which can make the blood more likely to clot. This is a risk for other problems like heart attacks and strokes and the condition needs to be treated under the guidance of a haematologist

As you can see from the above, the full blood count is an extremely useful test and it is taken to assess a wide array of symptoms, ranging from tiredness to easy bruising. We can derive a tremendous amount of information from this one, straightforward test. Minor abnormalities in the Full blood count are extremely common and usually do not signify any worrying, underlying cause, but abnormalities should be discussed with your doctor to ensure that the right follow up is planned.