Low MCV? How to read the size of your red blood cells

MCV, in full mean corpuscular volume, is the average volume of a single red blood cell. It is expressed in femtolitres (fl) and comes out of the cell counter automatically whenever a complete blood count is run. The reference range shown on this page is 80 to 100 fl and is deliberately not split by sex: the same interval applies to adult women and men.

Combined with haemoglobin, MCV gives anaemia a direction. If haemoglobin is low, MCV sorts that anaemia into three tracks: microcytic (MCV below 80 fl, small cells), normocytic (80 to 100 fl) and macrocytic (above 100 fl, large cells). Without MCV you know that haemoglobin is low, but not where to look next. MCV points; it never establishes a cause by itself.

One thing matters more than any other here: MCV is an average and says nothing about the spread. If the same sample contains two populations of cells, small ones and large ones, they cancel each other out in that average. This is exactly what happens in a combined deficiency, where a lack of iron coincides with a lack of vitamin B12 or folate. The MCV can then be entirely normal while two deficiencies exist side by side. A normal MCV therefore does not rule out a deficiency. The RDW, which describes the spread in cell size, is usually raised in that situation. That is why MCV is always assessed together with the RDW and the other cell indices, never on its own.

Finally, sample handling counts. Red cells slowly swell inside an EDTA tube. If a sample is only analysed after roughly 24 hours, the MCV comes out artificially high and the MCHC artificially low. A mildly raised MCV with nothing else out of range is therefore first a question about storage time, and only then a clinical question.

For women, the most common reason an MCV drifts downwards is monthly blood loss. Every period takes iron out of the body. If bleeding is heavy or prolonged — flooding, clots, more than seven days of bleeding — that loss can be structurally larger than what the diet replaces each month. The body then covers the difference from its own stores.

That sequence explains why MCV often reacts late. The iron store empties first, and you see that in a falling ferritin. Only once the store is genuinely empty does the bone marrow start producing smaller cells and the MCV fall. A normal MCV alongside a low ferritin is therefore a very ordinary and, above all, an early picture: it does not mean nothing is going on, it means you are catching it in time. If your MCV is low, the iron store has usually been under pressure for a while.

Not every low result comes from a deficiency. If the MCV is low while haemoglobin and ferritin are normal, inherited small red cells — a thalassaemia trait — can come into the picture. This is more common in people of Mediterranean, Asian, African or Middle Eastern background. The hallmark is a normal to high red cell count with cells that stay small. The Mentzer index, MCV divided by the red cell count, gives an indication here: above 13 points towards iron deficiency, below 13 towards an inherited cause. An indication is not proof, and certainly not a diagnosis. When you are planning a pregnancy this is worth discussing with a doctor, not least because testing your partner sometimes comes up. That decision belongs in the consulting room, not on this page.

The main causes of a low MCV and the value usually looked at next:

The other side of the spectrum is the macrocytic route. MCV can rise slightly in pregnancy. And on a vegetarian or vegan diet, vitamin B12 is the point of attention, because B12 occurs almost exclusively in animal and fortified products; a lack of B12 or folate makes red cells larger instead. Someone eating plant-based who also has heavy menstrual loss can have both at once — and then the MCV alone is not enough.

MCV is rarely requested on its own; it belongs to the complete blood count and is assessed there together with haemoglobin and haematocrit. Have it included whenever you have persistent fatigue, breathlessness on exertion, pallor or dizziness — and certainly when those complaints go together with heavy or prolonged periods.

There are a few moments when measuring is especially worthwhile. When you are planning a pregnancy and at the start of one, because iron requirements rise and a low value is better spotted early. After giving birth and during breastfeeding, when the store has often not recovered yet. And on a vegetarian or vegan diet, where vitamin B12 and folate are the values that complete the picture.

If you have started iron, retesting only makes sense after several weeks. Red cells live for roughly three months, so the average volume of the whole population changes slowly. In that case always measure ferritin alongside it, because ferritin moves sooner. Take an abnormal result to your GP or midwife, particularly if you have symptoms; they decide which follow-up investigation fits your situation.