MCV (mean corpuscular volume) measures the average size of red blood cells in femtolitres (fL). It is one of the most clinically useful red blood cell indices for classifying anaemia and guiding further investigation.

Low MCV (microcytic anaemia — small red cells) is most commonly caused by iron deficiency or thalassaemia. Normal MCV (normocytic anaemia) is associated with anaemia of chronic disease, haemolytic anaemia, or early nutritional deficiency. High MCV (macrocytic anaemia — large red cells) is typically caused by vitamin B12 or folate deficiency, but can also result from alcohol excess, hypothyroidism, liver disease, or certain medications including methotrexate and hydroxyurea.