MCV
The average size of your red blood cells, which points towards the cause of an anaemia.
What it means
The average size of your red blood cells — and one of the most useful pointers on a full blood count, because size suggests cause.
- Low MCV — small cells. Points towards iron deficiency, or an inherited condition such as thalassaemia trait
- High MCV — large cells. Points towards B12 or folate deficiency, an underactive thyroid, alcohol, or certain medicines
- Normal MCV with anaemia. Often anaemia of chronic disease, kidney disease, or recent blood loss
A raised MCV can appear before anaemia does, and is worth investigating on its own.
In practice
On every full blood count, alongside haemoglobin, white cells and platelets.
It is often the line that determines which tests get requested next, which is why it matters more than its unfamiliarity suggests.
Why it matters
If your MCV is high, ask whether B12, folate and thyroid have been checked, and be honest about alcohol.
If it is low, ask about ferritin — and if you are a man or over 50, ask what is being done to find the cause of any iron deficiency.
An abnormal MCV with a normal haemoglobin still deserves an explanation. It is frequently filed as unimportant when it is an early signal.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026