what is tested in a full blood count
A full blood count (FBC), also called a complete blood count (CBC), is a blood test that measures the main types of cells in your blood and several related values that give a snapshot of your overall health.
Quick Scoop: What’s Tested in a Full Blood Count?
In an FBC, the lab looks at three main cell groups and some extra indices that help work out what might be going on if something is abnormal.
1. Red blood cells (RBCs)
These carry oxygen around your body.
An FBC will typically include:
- Red blood cell count (RBC) – how many red cells you have.
- Haemoglobin (Hb) – the amount of oxygen‑carrying protein in your red cells.
- Haematocrit (HCT or packed cell volume) – what percentage of your blood volume is made up of red cells.
- Mean cell volume (MCV) – average size of red cells, helpful in classifying types of anaemia.
- Mean cell haemoglobin (MCH) – average amount of haemoglobin in each red cell.
- Mean cell haemoglobin concentration (MCHC) – how concentrated the haemoglobin is inside the red cells.
- Red cell distribution width (RDW) – how much variation there is in red cell size.
These measurements help detect anaemia, some vitamin or iron deficiencies, bleeding, dehydration and certain bone marrow or chronic diseases.
2. White blood cells (WBCs)
White cells are part of your immune system and fight infection.
Most FBCs include:
- Total white blood cell count (WBC) – overall number of white cells.
- Differential white cell count – breakdown of types, often including neutrophils, lymphocytes, monocytes, eosinophils and basophils.
Abnormal WBC numbers or patterns can suggest infections, inflammation, allergies, some immune problems, or blood cancers like leukaemia.
3. Platelets
Platelets help your blood to clot and stop bleeding.
Typical platelet tests in an FBC:
- Platelet count – how many platelets are present.
- Mean platelet volume (MPV) – the average size of platelets.
Low platelets can cause easy bruising or bleeding, and high platelets can increase the risk of blood clots in some situations.
4. Extra details the machine may flag
Modern FBC analysers also assess the general size, shape and sometimes the colour characteristics of red and white cells and platelets.
If something looks unusual, a laboratory scientist may review a blood film under the microscope to confirm or clarify abnormalities.
Core FBC Components (at a glance)
| Parameter | What it relates to | What it tells you (in general) |
|---|---|---|
| RBC count | Red blood cells | Overall number of red cells; helps detect anaemia or high red cell states. | [3][5][9]
| Haemoglobin (Hb) | Red blood cells | Oxygen‑carrying capacity of the blood. | [1][5][3]
| Haematocrit (HCT) | Red blood cells | Percentage of blood volume taken up by red cells. | [9][1][3]
| MCV, MCH, MCHC, RDW | Red cell indices | Size and haemoglobin content/variation of red cells; helps classify types of anaemia. | [2][3][9]
| WBC count | White blood cells | Total number of immune cells; altered in infection, inflammation, some blood disorders. | [7][1][2][9]
| WBC differential | White blood cells | Breakdown into neutrophils, lymphocytes, etc., giving more specific clues. | [5][9]
| Platelet count | Platelets | Clotting cell numbers; low in bleeding disorders, high in some reactive or bone marrow conditions. | [5][7][9]
| Mean platelet volume (MPV) | Platelets | Average platelet size, sometimes helpful in working out why platelets are high or low. | [2][9]
Why people get an FBC
Doctors often order an FBC as a general health check, before operations, or when you have symptoms like tiredness, infections, bruising or weight loss.
It is also used to monitor known conditions such as anaemia, some chronic illnesses, and the effects of treatments like chemotherapy or certain medicines.
Important: An FBC can suggest that something is abnormal, but it does not usually give a final diagnosis on its own; interpretation always depends on your symptoms, history and other tests, so it should be discussed with a healthcare professional.
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