Mean corpuscular volume (MCV) is a blood test that measures the average red blood cell volume, in other words, its size. MCV is part of a Full Blood Count (FBC) test, and you will see it reported there along with other measures your red cells, white cells, and platelets.
You can have a high or low MCV without having an obvious disease process and without being anaemic; sometimes this is just a variation. But a high or low MCV is often seen with anaemia when your blood has a reduced red blood cell count and/or haemoglobin. In this case, the MCV is a useful value to determine what is causing the anaemia.
High MCV level is called macrocytic, which means that the red cells are larger than normal. A high MCV along with anaemia is called macrocytic anaemia. It can have a variety of causes including alcoholism, liver disease, and deficiencies of vitamin B12 and folic acid.
Low MCV level is called microcytic, which means that the red cells are smaller than normal. A low MCV along with anaemia is called microcytic anaemia. It is most commonly caused by iron deficiency, as the red cells are smaller than normal because of the lack of haemoglobin. A low MCV can be a clue that you have had on-going, silent blood loss that you may not have been aware of, which is an important clue for your doctor to explore. It can also be seen in lead poisoning, inflammation, and thalassemia.