Full Blood Count

A full blood count is the most commonly used form of blood test. The test assesses a number of different components in the blood, including red blood cell count, haemoglobin levels and platelet levels.

The full blood count test is usually used to assess the general health of an individual, rather than a diagnosis tool for specific health conditions; however, the test often gives important clues to specific conditions and this can help doctors to reach a diagnosis.

What does the test measure?

The full blood count is actually a number of tests, which assess a number of different components; these include:

Red blood cells: red blood cells are vital; they carry oxygen around the body and help to remove waste products from the body. The full blood count measures the number of red blood cells in the body. The red blood cell count measures the number of red blood cells within the volume of blood; if the number is particularly high or low this can signify a medical health problem. Red blood cells are red because they contain haemoglobin, which has a deep red colour.

White blood cells: white blood cells are a vital part of the body’s immune system. White blood cells are responsible for fighting off infections and helping to protect the body against harmful bacteria. There are many different types of white blood cells, including lymphocytes, neutrophils, monocytes, basophils and eosinophils. The full blood count can measure the total number of white blood cells in the blood (this is known as the white blood cell count); a white blood cell differential test assesses the different types of white blood cells. White blood cells are colourless because they do not contain haemoglobin.

Platelets: platelets are also known as clotting cells, as they are responsible for clotting. Platelets stick together to stem bleeding; if there are not enough platelets, an individual may experience excessive bleeding, while a high platelet count can contribute to the formation of blood clots. The full blood count contains a platelet count, which can help doctors to diagnose problems with clotting.

Haemoglobin: haemoglobin is a vital component of the blood; it carries the oxygen around the body in the bloodstream. Haemoglobin also gives red blood cells their colour. The full blood count measures the amount of haemoglobin in a volume of blood; this can help doctors to assess the body’s ability to transport oxygen around the body.

Haematocrit: also known as HCT, this is a test which measures the amount of space taken up by the red blood cells in a set volume of blood; the result can either be recorded as a percentage or a proportion (recorded as 0-1).

Mean corpuscular volume: the mean corpuscular volume is a measurement of the average size of the red blood cells; if the cells are too large or too small, this can contribute to problems.

Why would I need a full blood count?

It is very common to have a full blood count; most people will have a number of these tests during their lifetime. Doctors usually advise patients to have full blood count to assess the state of their general health, but it can also be used to screen for conditions such as anaemia and detect infections. In many cases, the test will provide clues about a patient’s health, rather than producing an exact diagnosis; the results of the test will be interpreted by a doctor and they may then recommend that patients have further tests to reach a precise diagnosis.

How is the test performed?

The full blood count is performed by taking a sample of blood from a vein in the arm; usually, blood will be taken from a vein on the inside of the elbow, but samples may also be taken from the wrist. Your doctor will usually use a tourniquet to make the veins in the arm swell, as this will make it easier to take the sample; the tourniquet will be placed around your upper arm and it may feel tight but should not be painful. Once your veins have swollen, the doctor or nurse will insert a needle into the vein; the needle is attached to a syringe, which will collect the blood sample. Once the sample has been collected, it will be bottled, labelled and sent to the laboratory. The doctor will stem any bleeding using a cotton wool pad and then place a plaster over the wound so that it can heal.

If you have a fear of needles and injections, you should talk to your GP or nurse before you have the test; they can then try to put your mind at ease and make the experience as stress-free as possible.

When will I get my results?

The time it takes to get the results back will vary between different practices; typically, you should not have to wait very long for results but your doctor will explain when and how to get your results after the test has been completed. If you are worried about the results, you should discuss this with your GP.

What do my test results mean?

The full blood count examines several different components in the blood; it can determine the levels of different component cells and test their function. Variations in the number or size of the different cells in the blood can contribute to medical problems, including:

  • Red blood cells: low numbers of red blood cells usually indicate anaemia, while high numbers of red blood cells can cause the cells to stick together and block small blood vessels. A high number of red blood cells may also indicate an underlying kidney or lung condition. Average red blood cell counts are 5-6 million cells/mcL (cells per microlitre) for males and 4-5 million cells/mcL for females.
  • White blood cells: a low number of white blood cells may indicate a problem with the bone marrow; this is usually either an infection or a disease, such as cancer (this is known as leukaemia) or a problem with the liver (this may be caused by alcoholism). A high number of white blood cells usually indicates that an individual has an infection, which the body is currently trying to fight. Average numbers of white blood cells are 4,500-10,000 cells/mcL.
  • Platelets: a high number of platelets usually indicates an inflammatory condition, such as rheumatoid arthritis or a problem with the spleen. A low platelet count may indicate a viral infection, such as rubella or a problem with the immune system; autoimmune conditions cause the body to mistake healthy tissue for harmful tissue and this causes the body to effectively attack itself; an example of this is lupus). Average numbers of platelets range from 140,000 to 450,000 cells/mcL.
  • Mean corpuscular volume: an increased MCV may indicate conditions including an underactive thyroid gland, vitamin B12 and folate deficiency (this may be more common during pregnancy), liver disease and alcoholism. A decreased MCV may indicate iron deficiency and inflammatory conditions. The mean corpuscular volume should be between 80 and 95 femtolitre.

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