Mean Corpuscular Volume (MCV) is a measurement included in a standard Complete Blood Count (CBC) that provides information about the size of your red blood cells. Red blood cells transport oxygen throughout the body, and their size is important for this function. A low MCV result, typically below 80 femtoliters (fL), means the average size of the red blood cells is smaller than normal. This finding suggests a problem with the production of hemoglobin, the protein inside red cells that binds to oxygen.
Understanding Low Mean Corpuscular Volume
A low MCV value indicates microcytosis, the defining characteristic of microcytic anemia. This anemia means the body has red blood cells that contain less hemoglobin, resulting in smaller cells. Since these small cells are less efficient at carrying oxygen, oxygen delivery to tissues may be compromised.
Common symptoms associated with reduced oxygen transport are often subtle. These can include persistent fatigue, overall weakness, paleness of the skin, and shortness of breath, especially during physical activity.
A low MCV is a laboratory finding, not a final diagnosis. It serves as an indication for a healthcare provider to conduct additional testing to pinpoint the specific underlying cause.
The Primary Causes of Low MCV
The two most frequent causes for a low MCV are insufficient iron levels and inherited genetic disorders affecting blood protein production. Iron deficiency is the most common nutritional deficiency worldwide and directly affects the ability of red cells to synthesize hemoglobin. Iron is a necessary component of hemoglobin; without enough of it, the body cannot produce hemoglobin properly. This results in red blood cells that are both small (microcytic) and pale (hypochromic).
Iron deficiency frequently results from chronic, low-level blood loss, such as heavy menstrual periods or slow bleeding in the gastrointestinal tract. Inadequate dietary intake or malabsorption can also be factors. When iron stores are depleted, the resulting red blood cells are characteristically small. A physician will investigate the source of the deficiency, especially in men and postmenopausal women, where gastrointestinal bleeding is a particular concern.
The second major cause is thalassemia, a group of inherited blood disorders affecting the synthesis of the globin chains of the hemoglobin molecule. Unlike acquired iron deficiency, thalassemia is genetic and ranges widely in severity. The defect in globin chain production leads to an imbalance in hemoglobin components, causing red blood cells to be fragile and poorly formed.
Thalassemia trait (thalassemia minor) is a mild form where the low MCV may be a lifelong finding causing few symptoms. This is an important consideration when a low MCV is discovered incidentally. The red cells are small due to the inherited production defect, even though the body’s iron stores are typically normal. Differentiating between thalassemia and iron deficiency is important because treating thalassemia with iron supplements is ineffective and can lead to a dangerous buildup of iron in the body.
Less Common Causes of Low MCV
While iron deficiency and thalassemia are the primary considerations, other conditions can also lead to a low MCV less frequently. Anemia of Chronic Disease (ACD) is one such cause, often occurring with long-term inflammatory conditions like autoimmune disorders, cancer, or chronic infections. ACD is typically classified as normocytic (normal-sized cells), but some cases present with a mildly low MCV.
In ACD, underlying inflammation interferes with how the body manages iron, trapping it within storage cells. The iron is present but cannot be efficiently released for use in hemoglobin synthesis, creating a functional iron deficiency. This impaired iron utilization can result in the production of smaller red blood cells.
Another rare cause of low MCV is sideroblastic anemia. In this condition, the body has sufficient iron but cannot effectively incorporate it into the heme structure to form functional hemoglobin. This defect leads to an iron buildup within red blood cell precursors in the bone marrow. Sideroblastic anemia can be inherited or acquired through toxins, such as chronic alcohol use or lead poisoning.
Diagnosis and Management of Low MCV Results
After a low MCV is identified on a CBC, specific blood tests are used to determine the exact cause. The most informative initial step is often an iron panel, which includes measurements for serum iron, total iron-binding capacity (TIBC), and ferritin. Ferritin reflects the body’s stored iron, and a low ferritin level is the most specific indicator of true iron deficiency.
The Red Cell Distribution Width (RDW) is another measurement that helps differentiate causes by measuring the variation in red cell size. Iron deficiency anemia often shows a high RDW, indicating a mix of small and normal-sized cells. Thalassemia trait typically has a normal RDW.
If iron studies are normal, or if a genetic condition is suspected, a Hemoglobin Electrophoresis test may be performed. This test analyzes the different types of hemoglobin present and is the definitive test for diagnosing thalassemia.
Management depends entirely on the underlying condition identified by diagnostic tests. For iron deficiency anemia, treatment involves identifying and stopping the source of blood loss, followed by iron supplementation to replenish stores. Patients with thalassemia trait usually require no treatment, only monitoring, and must avoid unnecessary iron supplements. For Anemia of Chronic Disease, the primary strategy is to treat the underlying inflammatory disease, which often leads to the resolution of the anemia.