Understanding full blood count (FBC)


Red blood cells (RBC) -

  • Are bi-concave disk shaped cells which lack a nucleus, this allows them to increase the volume available and therefore, increases the amount of haemoglobin and conversely oxygen.

The main function of red blood cells is to contain haemoglobin which binds to oxygen and carbon dioxide and transfers them around the body. Oxygen is carried from the lungs to tissues and organs whereas carbon dioxide is collected at the tissue - blood interface and is taken to the lungs where it can be respired out the body.

Red blood cells can also bind to other molecules such as carbon monoxide. Carbon monoxide has a higher affinity (prefers to bind to) for haemoglobin when compared to oxygen and carbon dioxide, this saturates the cell rendering it useless.

Haemoglobin -

  • Is an iron containing oxygen transport protein, synthesised by mitochondria and is a key part of the respiration process.

This protein is found in red blood cells and binds oxygen to carbon dioxide. This process is explained in the red blood cell section. One molecule of haemoglobin can bind to 4 molecules of oxygen, within one red blood is roughly 300 million molecules of haemoglobin. Therefore, one red blood cell can carry 1,200,000,000 molecules of oxygen at any one time.

Haematocrit -

  • This is a ratio between red blood cells in blood and the total blood volume. This can be influenced by red blood size which is discussed in mean cell volume. This test has been used in conjunction with red blood cell levels and mean cell volume, to highlight this relationship please see below.

⬆ red blood cells + ⬆ cell volume = ⬆ haematocrit

⬆ red blood cells + Normal mean cell volume = ⬆ haematocrit

Normal red blood cells + ⬆ mean cell volume = ⬆ haematocrit

⬇ red blood cells + ⬇ mean cell volume = ⬇ haematocrit

⬇ red blood cells + normal mean cell volume = ⬇ haematocrit

Normal red blood cells + ⬇ mean cell volume = ⬇ haematocrit

Normal red blood cells + normal mean cell volume = normal haematocrit

White cell count -

  • This counts the number of immune cells in totality can be used as an indicator of infection or someone's ability to combat an infection.

A low reading would make it difficult for the patient to fight off an infection successfully, this can be supplemented via a transfusion and can be often controlled in a hospital setting. There are a number of different diseases which could reduce the number of white cells in the body, most commonly HIV. HIV positive patients present with low white cells in various stages of the disease process, this is because HIV hides inside white blood cells and uses them as vectors.

Lymphocytes -

  • Are a type of white blood cell which make up T cells, B cells and natural killer cells. High levels of lymphocytes indicates inflammation or infection. Conversely a decrease in lymphocytes could also indicate an infection.

Monocytes -

  • Are a type of white blood cells which can specialise into macrophages or dendritic cells. Monocytes circulate in blood and seek out pathogens, on recognition of a pathogen the monocyte has the ability to destroy or alert the surrounding cells of invasion which stimulates further release of white blood cells.

  • As monocytes mature they develop into macrophages which reside in the tissue rather than circulate in the blood.

Neutrophils -

  • Is a type of white blood cell and is part of the granulocyte family along with basophils and eosinophils. This is due to there being a high number of nuclei which can be considered granules and therefore, they are granular. Granularity and complexity are interchangeable when considering the immune system.

  • Neutrophils are soldiers of the immune system and travel to the site of infection to battle the pathogen on the front line. There are three different ways neutrophils destroy pathogens, click here to find out more.

  • Neutrophilia (high number of neutrophils) are indicative of a bacterial infection, however as seen before a low number of neutrophils can indicate infection, such as hepatitis.

Basophils -

  • Basophils are a white blood cell which defend the body from pathogens, allergies and parasites. They are part of the granulocyte family, in addition they increase vascular permeability and therefore aid in blood flow.

  • High levels of basophils in combination with eosinophils indicates presence of parasites

  • High levels of basophils and slightly elevated eosinophils indicates an allergic reaction (however, the action these cells have on allergic reactions is still unclear)

Eosinophils -

  • Are the last of the white blood cell family and are part of the granulocytes. These are essential in the attack on parasites. A high level of eosinophils indicates a parasitic infection, they are also shown in increased numbers during an allergic reaction. To see how eosinophils attack, click here.

Platelets -

  • Are also known as thrombocytes, their main functions are to clot. This can be used to prevent blood loss after injury however, these same clots cause strokes and can lead to irreversible brain damage and even death.

  • Thrombocytopenia is a term used to describe a low platelet count, this can slow clot formation and can even prevent them from forming at all. There is a condition known as haemophilia which is an inherited disease and is in the bloodline of the British royal family. The condition causes a reduction in clotting factor VIII (8) and IX (9), to see more on the clotting cascade check out. To reverse thrombocytopenia patients can be given platelets to ensure sufficient clotting.

Mean cell volume (MCV) -

  • Is the measure of the average volume of red blood cells throughout the body. Mean cell volume can be used to determine the presence of anaemia, in anaemic patients the red cells will be smaller in size due to the lack of iron present in the haemoglobin of the red cells.

  • Microcytosis = small red blood cells

  • Macrocytosis = large red blood cells

Red cell distribution width (RDW) -

  • Measuring the difference in size and volume of the red cells, a high RDW means there is a large difference between the smallest and largest. In a patient with no disease processes the red blood cells should be relatively uniform in shape, colour and volume.

  • Measuring the RDW is helpful in identifying an issue with red cell structure which most commonly is anaemia. This can also be assessed visually which can be seen in the blood film section. In diagnosis, a blood film must be used in conjunction with a full blood count.

Mean cell haemoglobin (MCH) -

  • Is the average number of haemoglobin molecules found in red blood cells, hypochromia is when the MCH is low this can be caused by iron deficiency. On the blood film, hypochromic patients present with red blood cells which are slightly less red when compared to ‘normal’ patients. In very rare cases, patients can present with hyperchromia. This is when the average red blood cells have more haemoglobin than typically found. This slightly increases the red hue caused from red blood cells.

  • Hypochromia normally comes hand in hand with microcytosis and is a tell tell sign of anaemia.

C-reactive protein (CRP) -

  • Is a highly sensitive marker of inflammation, an increase in CRP can be seen before any symptoms present. Testing CRP is common practice due to the sensitivity of the test and the reliability. Some medication (birth control pill) can increase CRP and therefore, the physician should consider medication alongside the result.

Blood film -

  • As you can see in the image below is a comparison between 3 states: normal, sickle cell and target cells.

  • Normal - the red cells are uniform in shape and size which some platelets present (small purple clumps). There are 2 white cells present, the one on the right is a neutrophil and the one on the left is a lymphocyte.

  • Sickle cell - some cells are uniform in size and look typical however, there are some sickle cells present. This is due to a mutation which affects how red cells develop, reducing the number of haemoglobin which is able to be carried by the red cells, leaving the patient feeling fatigued and tired.

  • Target cells - some typical red cells presence with some codocyte (target cells present). These are caused by a deficiency of lecithin cholesterol acyl transferase causing red cells to express excess cholesterol and phospholipid and decreased cellular haemoglobin giving it its target shape.