Mean Corpuscular Volume or MCV is a blood test that measures the average size of your red blood cells. Red blood cells or erythrocytes are small, round, biconcave cells that contain hemoglobin which carries the oxygen from the lungs to tissues and carbon dioxide back from tissues to the lungs. The amount of oxygen in your body is tightly connected to the amount of red blood cells in your body and their function. When the red blood cells in your body are either too big or too small, it can be a sign of certain blood disorders such as anemia, lack of iron, or lack of certain vitamins.  

MCV is usually a part of a complete blood count or CBC. The MCV test alone is not used to confirm any diagnosis. However, it helps doctors to make a diagnosis when its value is taken into account with other test results and symptoms.  

When MCV levels are elevated, your red blood cells are bigger than normal. That can be a sign that your body is lacking vitamin B12 and folate. It can also be connected to certain autoimmune diseases such as Crohn's disease or celiac disease.  

Low levels of MCV mean that your red blood cells are smaller than normal. That is usually connected to iron-deficiency anemia, which is the most common type of anemias. Small red blood cells are known to be present also in people with thalassemia, an inherited blood disorder where the body produces less healthy red blood cells and hemoglobin. 

There are cases when the MCV level is normal, but people still experience anemia. Anemia can be caused by a sudden loss of blood, kidney failure, or very rarely aplastic anemia. On the other hand, however, MCV might be out of the reference range, but that doesn't necessarily mean you have any medical problems. Diet, menstruation, and your activity level all have an impact on your MCV levels.