Is your MCV value normal? What your number means

Your blood is full of red blood cells. MCV tells you how big one of those cells is on average. The lab does not measure that with a ruler: it counts how many cells are in your sample, measures how much space they take up together, and divides one by the other. What comes out is an average, expressed in femtolitres (fl). A femtolitre is unimaginably small, so do not let the number put you off. It is simply a measure of small, normal or large.

MCV is a standard part of the complete blood count, alongside your haemoglobin and your haematocrit. You rarely request it on its own; it comes out automatically whenever you have blood drawn.

The reference range shown on this page is 80 to 100 fl. Almost every result therefore falls into one of three boxes, and each box has its own follow-up question.

One thing matters more than the rest, and it is the reason MCV is never read on its own: MCV is an average and says nothing about the spread. If you have both small and large cells, they average each other out and a perfectly normal number comes back. That is why a lab always reads the RDW next to it. RDW shows how much your cells differ from one another in size, and sometimes that difference is the only trace there is.

Your MCV on its own is not a disease and not a diagnosis. It is a signpost. The moment your haemoglobin turns out to be on the low side, MCV tells you which direction to look in. Without MCV you only know that something is off; with MCV you know where to start. That saves months of searching in the wrong corner.

If your cells are small, below 80 fl, the first follow-up question is almost always how your iron is doing. You do not answer that with the iron in your blood alone, because that swings from day to day and even from meal to meal. Ferritin shows your stores, and that is the value the conversation usually turns on. Small cells alongside a normal haemoglobin and a normal ferritin can also belong to an inherited trait, thalassaemia trait. It is more common in people with a Mediterranean, Asian, African or Middle Eastern background. Doctors sometimes use the Mentzer index here: your MCV divided by your red cell count. Above 13 points more towards iron deficiency, below 13 more towards thalassaemia trait. That is an indication, not proof.

If your cells are large, above 100 fl, vitamin B12 and folate are the first things added to the next tube. They are not the only explanation, though. Alcohol makes red blood cells larger even when there is nothing wrong with your B12, and it fades only slowly: allow roughly three months. An underactive thyroid, a liver problem and some medicines do the same. And if you have just started iron or B12, or you recently lost blood, your bone marrow makes extra young cells. Those young cells, reticulocytes, are bigger than mature ones. Your MCV goes up while something good is actually happening.

Then there is the part most people miss: a normal MCV does not rule out a deficiency. If you are short of iron and short of vitamin B12 or folate at the same time, you make both small and large red blood cells. In the average they cancel each other out. Your result can read a tidy 90 fl while two deficiencies sit side by side. What usually does stand out in that situation is the RDW: it runs high, because your cells differ so much from one another in size. A normal MCV with a raised RDW is not a reason to stop looking.

In short: one number decides nothing. MCV points the direction, the follow-up value fills it in, and your GP puts your own story next to it.

You rarely measure MCV on its own. It comes out of every complete blood count, so the real question is when to have blood drawn at all. That makes sense if you are tired without a clear reason, if you are short of breath doing things you normally find easy, if you look pale, if you get dizzy standing up, or if you have tingling in your hands or feet. If you have had an abnormal haemoglobin before, a check is sensible too, even when you feel perfectly fine.

Did your GP hand you a result where MCV sits just outside the reference range while everything else is normal? That is often not an alarm. You can simply raise it at your next appointment instead of reacting to it straight away.

Do go back to your GP sooner if your MCV is off and your haemoglobin is low, if your MCV is clearly above 100 fl, if you have symptoms that will not go away, or if your result has shifted a lot in a short time. That is not a panic button, but it is the moment to have someone with your full file look at it.

If you want to repeat the test yourself, leave at least eight to twelve weeks between two measurements. Red blood cells live around three months, so testing sooner mostly shows you noise rather than change.