The Mean Corpuscular Volume (MCV) is a measurement included in a standard Complete Blood Count (CBC). This value indicates the average size of your red blood cells, which carry oxygen. An MCV of 100 femtoliters (fL) is at the upper limit of the typical reference range, a finding known as macrocytosis. Macrocytosis describes the physical characteristic of large red cells, signaling that an underlying process is affecting blood cell production. This finding is informative for a healthcare provider, but it is not a diagnosis in itself.
Understanding Mean Corpuscular Volume
The normal reference range for MCV is generally between 80 and 100 fL for an adult. A result of 100 fL means the average size of your red cells is just at the threshold of being considered enlarged. Conversely, an MCV below 80 fL indicates microcytosis, where red blood cells are smaller than average.
The MCV is rarely interpreted in isolation and gains its meaning when considered alongside other components of the CBC, such as hemoglobin and hematocrit. If the enlarged cells are also low in number, this may indicate macrocytic anemia. Evaluating the red cell distribution width (RDW), which measures the variation in cell size, provides further context.
Primary Causes of Elevated MCV
An elevated MCV occurs when the bone marrow produces red blood cells that are too large, often due to impaired cell division or changes to the cell membrane. The common reasons for this finding fall into distinct categories, each with a different underlying mechanism.
Nutritional Deficiencies
Deficiencies in Vitamin B12 (cobalamin) and Folate (Vitamin B9) are frequent causes of macrocytosis. These vitamins are necessary cofactors for DNA synthesis, required for red blood cell maturation. When these nutrients are lacking, red cell precursors in the bone marrow cannot divide properly. This leads to the release of large, immature cells into the bloodstream, a condition known as megaloblastic anemia.
Vitamin B12 deficiency may stem from dietary insufficiency, particularly in strict vegans, or from poor absorption due to conditions like pernicious anemia or gastrointestinal surgeries. Folate deficiency is often related to inadequate dietary intake, though chronic alcohol use can also affect it. Macrocytosis may be the only sign of these deficiencies, even before anemia develops.
Lifestyle and Organ Function
Chronic, heavy alcohol consumption is a common cause of macrocytosis. Alcohol can directly inhibit bone marrow function, interfering with red cell production. It can also lead to poor nutrition, which exacerbates folate deficiency.
Liver disease, even independent of alcohol use, can also result in an elevated MCV. In this case, the red cell membrane develops an abnormal composition due to changes in circulating lipids and cholesterol. This alteration physically increases the cell’s surface area, making it appear larger. Hypothyroidism, or an underactive thyroid, is another condition associated with macrocytosis.
Medication Effects
Several classes of drugs can interfere with red blood cell metabolism and cause a rise in the MCV. Medications used in chemotherapy, such as antimetabolites, can disrupt DNA synthesis and mimic the effects of B12 or folate deficiency. Certain antiviral drugs used to treat HIV or medications used for seizure control have also been linked to macrocytosis.
Next Steps and Further Testing
An MCV of 100 fL warrants a discussion with a healthcare provider to determine the underlying cause. The initial diagnostic step involves reviewing your medical history, including diet, alcohol consumption, and current prescriptions. This information helps narrow the potential causes and guides the selection of further diagnostic tests.
Common follow-up blood tests include measuring serum levels of Vitamin B12 and folate to check for nutritional deficiencies. A liver function test is often ordered to assess for underlying liver issues. A thyroid panel may also be utilized to evaluate for hypothyroidism.
Another important test is the peripheral blood smear, where a laboratory professional visually examines the red blood cells under a microscope. This smear confirms the presence of large cells and identifies specific structural abnormalities, such as hypersegmented neutrophils, characteristic of megaloblastic conditions. Treatment for macrocytosis is always directed at resolving the underlying cause, whether through nutritional supplementation, medication adjustment, or managing a chronic disease.