MCV: Normal Values and What a High or Low MCV Means

MCV stands for Mean Corpuscular Volume: the average volume of your red blood cells. The value is expressed in femtolitres (fl) and calculated from your haematocrit and red cell count.

MCV therefore does not tell you how many red cells you have, but how large they are on average. That size is surprisingly informative, because different causes of anaemia give red cells a characteristic size.

On the basis of MCV, anaemia is classified into three groups: microcytic (cells too small), normocytic (normal size) and macrocytic (cells too large). That classification steers the follow-up investigation.

A low MCV means your red blood cells are too small. That points mainly to iron deficiency, where there is too little iron to fill the cells properly with haemoglobin. Thalassaemia, an inherited condition of haemoglobin production, also produces small cells.

A high MCV means your red blood cells are too large. The classic causes are a deficiency of vitamin B12 or folate, in which cell division stalls while the cell keeps growing. Heavy alcohol use, a liver condition, an underactive thyroid and certain medicines can also raise the MCV.

It is exactly this directional value that makes MCV useful: with anaemia and a low MCV your doctor looks first at your iron status, while with a high MCV they look sooner at vitamin B12, folate and alcohol use.

MCV is measured as standard in the complete blood count and is particularly relevant when there is anaemia or unexplained fatigue.

The value is almost never assessed on its own. Your doctor reads MCV together with your haemoglobin, haematocrit and RDW, and uses the combination to decide which follow-up is useful: ferritin with a low MCV, vitamin B12 and folate with a high MCV.

An abnormal MCV without anaemia also occurs and is not always worrying, but it should be investigated if it persists, because it can be an early sign of a deficiency that has not yet lowered your haemoglobin.