MCV

MCV is calculated as (Hematocrit/RBC count) x 10 and is reported automatically by automated hematology analyzers as part of the complete blood count (CBC). Classifying anemia by MCV narrows the differential diagnosis significantly:

Microcytic anemia (MCV <80 fL): Caused by defects in hemoglobin synthesis. Mnemonic 'TAILS': Thalassemia, Anemia of chronic disease (can also be normocytic), Iron deficiency anemia, Lead poisoning, Sideroblastic anemia. Iron deficiency is the most common cause worldwide and is distinguished from thalassemia by iron studies (low ferritin, low iron, high TIBC in iron deficiency vs. normal/increased iron studies in thalassemia) and elevated RDW in iron deficiency.

Normocytic anemia (MCV 80-100 fL): Divided into hemolytic and non-hemolytic causes based on reticulocyte count. Causes include acute blood loss, anemia of chronic disease, aplastic anemia, chronic kidney disease (decreased erythropoietin), and early iron deficiency. Hemolytic causes include hereditary spherocytosis, G6PD deficiency, sickle cell disease, and autoimmune hemolytic anemia.

Macrocytic anemia (MCV >100 fL): Subdivided into megaloblastic and non-megaloblastic. Megaloblastic causes involve impaired DNA synthesis: vitamin B12 deficiency, folate deficiency, and drugs like hydroxyurea or methotrexate; hypersegmented neutrophils are a classic finding. Non-megaloblastic causes include alcoholism, liver disease, hypothyroidism, and reticulocytosis (since reticulocytes are larger than mature RBCs).

Clinical use: MCV should always be interpreted alongside RDW (red cell distribution width), reticulocyte count, peripheral smear findings, and iron/B12/folate studies to reach a specific diagnosis. An elevated RDW suggests a mixed population of cell sizes (e.g., iron deficiency, combined deficiencies) whereas a normal RDW suggests a more uniform population (e.g., thalassemia trait, anemia of chronic disease).