The Complete Blood Count (CBC) is a common laboratory test that details the cells circulating in the blood, including red blood cells, white blood cells, and platelets. Among these measurements is the Mean Corpuscular Volume (MCV), which offers specific insight into the physical characteristics of red blood cells.
Defining Mean Corpuscular Volume
The Mean Corpuscular Volume (MCV) is the average size or volume of the red blood cells in a blood sample. It is calculated as part of a standard CBC and provides information about the physical features of these oxygen-carrying cells. The MCV value is expressed in femtoliters (fL). For most adults, the reference range falls between 80 and 100 fL, indicating an average, or “normocytic,” size. If the MCV is outside this range, it signals that the cells are either smaller (microcytic) or larger (macrocytic) than normal, which helps classify the underlying cause of conditions like anemia.
Causes of Low MCV Readings
A low MCV reading (below 80 fL) indicates microcytosis, meaning the red blood cells are smaller than average. This finding often points to issues producing sufficient hemoglobin, the protein responsible for oxygen transport. The reduced hemoglobin content results in smaller cells because less material is available to fill the cell volume.
Iron deficiency anemia is the most common cause of a low MCV. Iron is a fundamental component of hemoglobin, and when stores are depleted, the bone marrow cannot properly synthesize the protein. This lack of proper synthesis leads to the production of small red blood cells.
Another primary cause is thalassemia, a group of inherited disorders that directly impair the body’s ability to produce normal hemoglobin chains. Although iron levels may be normal, the genetic defect results in small, microcytic red blood cells. Anemia of chronic disease, associated with long-term inflammatory conditions, can also present with a low MCV, as inflammatory signals interfere with the body’s iron utilization.
Causes of High MCV Readings
An MCV reading above 100 fL indicates macrocytosis, meaning the red blood cells are larger than normal. This condition is often categorized into two main groups: megaloblastic and non-megaloblastic macrocytosis.
Megaloblastic macrocytosis is characterized by a defect in DNA synthesis within the bone marrow, causing red cell precursors to grow large but fail to divide properly. The most frequent causes are deficiencies in Vitamin B12 and folate (Vitamin B9). These B vitamins are necessary cofactors for DNA synthesis, and their absence slows cell division, resulting in the release of large, immature red blood cells (macro-ovalocytes) into the bloodstream.
Macrocytosis can also occur without the characteristic DNA synthesis defect, known as non-megaloblastic macrocytosis. Chronic alcohol use is a common cause, as alcohol can be toxic to bone marrow cells and interfere with nutrient absorption. Liver disease is another contributor, where changes in blood lipid levels alter the red blood cell membrane, increasing the cell’s volume.