Full blood count explained: haemoglobin, white cells, platelets and MCV
Short answer: a full blood count (FBC) counts the red cells, white cells and platelets in your blood. It checks for anaemia, signs of infection or inflammation and problems with the cells that help blood clot. It is one of the most common blood tests because it gives a broad picture of your health from a single sample.
What a full blood count measures
Haemoglobin: the oxygen-carrying protein in red cells. A low level is called anaemia.
MCV (mean cell volume): the average size of your red cells. Small cells can point towards low iron, and large cells towards low B12 or folate, alcohol or an underactive thyroid.
White cells: your infection-fighting cells, often split into neutrophils, lymphocytes, monocytes, eosinophils and basophils.
Platelets: small cell fragments that help your blood clot.
You can book a full blood count on its own. If you are looking into anaemia, the anaemia profile adds checks for the likely causes.
What an abnormal result can mean
Low haemoglobin: iron deficiency, low B12 or folate, blood loss, long-term illness or inherited blood conditions.
High white cells: infection, inflammation, smoking or some medicines, such as steroids.
Low white cells: often a recent viral illness or a medicine effect.
High platelets: can follow inflammation, infection or iron deficiency.
Low platelets: can follow a viral illness, some medicines or alcohol, and sometimes immune conditions.
One slightly abnormal value is common and is often repeated. The pattern across the whole report matters more than a single number.
Normal ranges vary
Ranges differ between laboratories, and between men and women, so use the range printed on your own report and ask a clinician to explain what it means for you.
When to seek advice promptly
See a doctor promptly if you have unexplained bruising or bleeding, repeated infections, persistent fevers, drenching night sweats or marked breathlessness.
Frequently asked questions
Do I need to fast for a full blood count?
No. A normal meal does not change blood cell counts.
How long do results take?
A single test like this typically takes 1 to 3 working days, though this can vary.
Is a full blood count the same as a full body health check?
No. A full blood count only looks at blood cells. A full health screening adds tests such as kidney, liver, cholesterol and blood sugar.
Related reading
Clinically reviewed by Zak Toon, ACP, on 15 October 2026.
This article is general information based on UK guidance from NHS and NICE. It is not personal medical advice. If you are worried about your health or have symptoms, speak to your GP or call 111.
