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What is a Full Blood Count?

by Doctorpal

The full blood count as the name suggests is a complete count of the main types of cells in your blood. It is therefore sometimes called a complete blood count. Let’s quickly look at what blood is made of. Blood consist of cells and fluid called plasma, there are 3 main types, red blood cells which carry oxygen, white blood cells which fight infections or any foreign thing that enters the body and platelets which help stop bleeding in the event that you get an injury. The red blood cells are responsible for the red color of blood due to a substance they contain called Hemoglobin (Hb) which carries the oxygen.

There are 5 main types of white blood cells, neutrophils, lymphocytes, eosinophils, basophils and monocytes. Each has its own role in protecting the body.

How is it done?

Usually, an elastic band called a tourniquet will be used to thigh anywhere along your upper limbs, commonly at your wrist or above your elbow. This causes blood going up to be pool at the that point much like a dam to let the veins become more prominent and visible so that a needle can easily be used to collect a small amount of blood.

How long does it take?

It doesn’t take long, just about 10 minutes or less because machines used are fully automated.

Let’s now move to the how the results look like and what your doctor can tell from it. We will categorize this into the three main cell types in the blood

Red Blood Cells (RBC)

From the full blood count, we can tell whether your hemoglobin level is low (anaemia) and further go on to tell what the possible cause might be, whether due to dietary factors, blood loss, a chronic illness etc. when you have too many red blood cells it called Polycythemia. If you are dehydrated your RBC count might seem to be high due to the relatively lower amount of plasma.

White Blood Cells (WBC)

These are the soldiers of the body and I mentioned earlier that there are 5 different types with each group having its own function. Generally, a high number of WBCs tells your doctor that you have an infection, very high levels could however suggest that there is a form of blood cancer. Very low levels of WBC below normal is suggestive of immunosuppressed state or the effect of drugs such as cancer drugs.

Neutrophils: these are the most abundant type of WBCs making about 60-70%, they are largely responsible for fighting bacteria and to a lesser extent fungi, when they kill bacteria and die in large numbers they end up forming what we call pus (pompo in local parlance).  Neutrophil levels therefore help us to decide whether you need antibiotics or not

Lymphocytes: lymphocyte levels are elevated when there is usually a viral infection (meaning you may not need antibiotics), however very very high levels could point to a more serious condition such as a lymphoma or leukemia. Very low levels of lymphocytes on your full blood count could mean immunosuppressed states such as seen in AIDS or some chemotherapy drugs.

Eosinophils: parasites and allergies is their job. So levels will be high on the FBC if you have a parasitic infection like worm infestation or an allergic reaction, it may also be high in asthmatics.

Monocytes: work like cleaners, they mop up dead cells and also move into the tissues to fight disease causing organisms. High levels generally suggest chronic diseases such as Tuberculosis and some other not very common diseases.

Basophils: when levels are too high could be due to some forms of cancer, whereas very low levels could be seen when you are recovering from a very severe allergic reaction.


Very low levels of platelets (thrombocytopenia) could mean you are at risk of bleeding more when you have an injury. On the other hand very high levels (thrombocytosis) mean your blood is more likely to clot. It also helps repair the body through it’s role in inflammation.

It is important to note that your doctor does not make his decisions solely on your laboratory results, his clinical judgment far supersedes the interpretation of your result. That is why the doctor first takes your history and examines you, this enables him to correlate his clinical findings with your result.

So what do you think? Do you want an FBC every time you visit the hospital?

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