Blood tests explained
MCV Blood TestUK: What High & Low Levels Mean
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed July 20266 min read
Every Helvy guide is written by our health editors, then checked by a qualified clinician before it goes live and re-checked as the science moves. We name clinical roles, not individuals, until each reviewer has agreed to be credited publicly. This is wellness guidance to help you understand your own data, not a diagnosis.
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MCV, or mean corpuscular volume, measures the average size of your red blood cells. A normal result is about 80 to 100 femtolitres (fL). A low MCV usually points to iron deficiency. A high MCV points to a vitamin B12 or folate deficiency, heavy alcohol use, or an underactive thyroid.
See which test checks the cause →As of July 2026. An MCV flag on a blood report sends a lot of people searching. This guide gives the plain read: what MCV measures, what a high or low result points to, and the one step that turns a number into an answer.
Your MCV is one small line on a routine blood test. On its own it rarely diagnoses anything. But it is a useful signpost, and it often points straight at a cause you can fix.
1. What is an MCV blood test?
MCV stands for mean corpuscular volume. It measures the average size of your red blood cells, the cells that carry oxygen around your body.
You do not order an MCV on its own. It comes as one line on a full blood count, the routine test that counts and sizes your blood cells. Most general health checks include it.
Cell size is a clue. Red cells that come out too small, or too large, often point to a specific and treatable cause. That is why the MCV is worth understanding, even when the rest of your count looks normal.
2. What is a normal MCV level?
UK labs report MCV in femtolitres (fL), a very small unit of volume. A normal adult range is roughly 80 to 100 fL. Your lab's exact range may differ a little, so read the numbers on your own report.
| MCV RESULT | WHAT IT IS CALLED |
|---|---|
| Below 80 fL | Low (microcytic): cells smaller than normal, most often from iron deficiency |
| 80 to 100 fL | Normal (normocytic): average cell size within the usual band |
| Above 100 fL | High (macrocytic): cells larger than normal, often from B12 or folate |
The number on its own means little. What matters is whether it sits inside that band, below it, or above it. The two sections that follow take each case in turn.
3. What does a low MCV mean?
A low MCV means your red cells are smaller than normal. Doctors call this microcytic. The most common cause by far is iron deficiency. Without enough iron, the body builds smaller, paler cells.
Heavy periods, a low-iron diet, pregnancy and gut problems are the usual reasons iron runs low. A low MCV is often an early sign, sometimes before you feel tired. A ferritin test confirms it, because ferritin is your iron store.
One other cause is worth knowing. Thalassaemia trait, an inherited condition, also lowers MCV. It is usually harmless, but it can look like iron deficiency on paper. Further tests tell the two apart.
4. What does a high MCV mean?
A high MCV means your red cells are larger than normal. Doctors call this macrocytic. The classic cause is a shortage of vitamin B12 or folate. Both are needed to build healthy cells. The NHS puts it plainly.
“A lack of vitamin B12 or folate causes the body to produce abnormally large red blood cells that cannot function properly.”
— NHS, Vitamin B12 or folate deficiency anaemia
Other causes raise MCV too. Regular heavy drinking is a common one. An underactive thyroid can do it, as can some medicines and pregnancy. Because the causes differ so much, a high MCV is a prompt to test further, not a diagnosis in itself.
5. What should I do about an abnormal MCV?
An abnormal MCV is a starting point, not an answer. The next step is to find the cause. That usually means testing the nutrients behind red cell size: iron (through ferritin), vitamin B12 and folate. A thyroid check is often added, since an underactive thyroid raises MCV.
Helvy's General Energy & Wellness panel (£149) measures vitamin B12 and thyroid (TSH and free T4), two of the common reasons an MCV runs high. If iron deficiency looks more likely, you can build a test that adds ferritin and iron studies. Both are home finger-prick kits from a UKAS-accredited UK laboratory.
See a GP if your MCV is well outside the range, if you feel very tired or breathless, or if a result worries you. A blood test measures the markers. A qualified clinician reads them alongside your history. Our energy and fatigue hub covers the wider set of markers behind low energy.
READY TO TEST?
Check the cause behind your MCV
A high or low MCV points to nutrients or thyroid, not to a diagnosis. Helvy's home finger-prick tests measure vitamin B12, thyroid and, when you add them, iron studies, so you can act on numbers rather than a hunch.
Frequently asked questions
What is a normal MCV level?
A normal adult MCV is roughly 80 to 100 femtolitres (fL). Labs vary slightly, so the range on your own report takes precedence. Below 80 is called low, or microcytic; above 100 is high, or macrocytic.
What does a low MCV mean?
A low MCV means your red cells are smaller than normal. The most common cause is iron deficiency, often from heavy periods, a low-iron diet or blood loss. A ferritin test confirms whether iron stores are low. Thalassaemia trait can also lower MCV.
What does a high MCV mean?
A high MCV means your red cells are larger than normal. The usual causes are a vitamin B12 or folate deficiency, regular heavy drinking, or an underactive thyroid. Because these differ so much, a high MCV is a reason to test further, not a diagnosis on its own.
Can my MCV be normal but I still have anaemia?
Yes. This is called normocytic anaemia, where cell size is normal but the count or haemoglobin is low. Early iron deficiency, mixed deficiencies and long-term illness can all cause it. That is why doctors read MCV alongside the rest of the full blood count.
Do I need to fast for an MCV test?
No. MCV is part of a full blood count, which does not need fasting. You can give the sample at any time of day. If your test bundles other markers that do need fasting, the instructions will say so.
Related guides
Full Blood Count Explained
What every line on your FBC means, from haemoglobin to platelets and MCV.
Iron Deficiency Blood Test
Why a low MCV often points to iron, and which markers confirm it.
Vitamin B12 Blood Test UK
The deficiency behind many high-MCV results, and how to read your level.
Anaemia Blood Test UK
How cell size sorts the different types of anaemia and points to the cause.