The Complete Blood Count (CBC) is one of the most common and informative blood tests ordered by healthcare providers. The CBC provides a detailed snapshot of the cells circulating in the bloodstream, including red blood cells, which carry oxygen. The Mean Corpuscular Volume (MCV) is a calculation that determines the average size of these red blood cells. A reading outside the typical range signals a potential underlying issue affecting cell production.
Defining Mean Corpuscular Volume
Mean Corpuscular Volume (MCV) is a direct measure of the average size and volume of red blood cells in a blood sample. This value is expressed in femtoliters (fL). For most adults, the normal range for MCV falls between 80 and 100 fL. If the measured MCV is below 80 fL, the cells are smaller than normal (microcytosis). Conversely, an MCV reading above 100 fL means the cells are larger than normal (macrocytosis). The MCV helps a provider quickly narrow down the potential causes of anemia.
Interpreting Low MCV Readings
A low MCV result indicates that the red blood cells are abnormally small, leading to microcytic anemia. This reduction in cell size is often linked to an impairment in the body’s ability to produce sufficient hemoglobin, the protein that transports oxygen. The most frequent cause of microcytosis globally is Iron Deficiency Anemia (IDA), where a lack of iron prevents the proper construction of hemoglobin. Without enough iron, red blood cell precursors in the bone marrow shrink in size.
Another cause of a low MCV is Thalassemia, an inherited blood disorder affecting the synthesis of the globin chains that form hemoglobin. This genetic defect results in fewer globin chains and smaller, paler red blood cells. Thalassemia is distinct from iron deficiency because it is a problem of production, not a lack of raw material.
Anemia of Chronic Disease (ACD) can also present with a low MCV, particularly in long-term inflammatory conditions like autoimmune diseases or cancer. In ACD, inflammation disrupts the body’s iron utilization, preventing its efficient use for hemoglobin production.
Interpreting High MCV Readings
An elevated MCV, measuring above 100 fL, suggests the red blood cells are enlarged (macrocytosis). This increase in cell volume is frequently tied to problems with cell development and DNA synthesis within the bone marrow. The most common cause of significant macrocytosis is megaloblastic anemia, which arises from a deficiency in Vitamin B12 or Folate. A shortage of these necessary cofactors for DNA synthesis leads to delayed cell division, causing the red blood cell precursors to grow larger than normal before they divide.
Other health issues can lead to non-megaloblastic macrocytosis. Chronic, excessive alcohol use is a common factor, as alcohol can directly interfere with bone marrow function or be associated with poor nutrition. Liver disease can cause a buildup of cholesterol and phospholipids on the red blood cell membranes, physically increasing the cell’s volume. Hypothyroidism (an underactive thyroid) is another condition that can cause red blood cells to enlarge.