A routine blood test known as a Complete Blood Count (CBC) provides a snapshot of the various components circulating in the blood, including red blood cells, white blood cells, and platelets. One specific measurement within this panel is the Mean Corpuscular Volume, or MCV. An elevated MCV result signals that the red blood cells are larger than their typical size. Understanding this change is a key step in identifying potential underlying health issues.
Understanding the Mean Corpuscular Volume
The Mean Corpuscular Volume (MCV) measures the average size and volume of red blood cells in a blood sample. These cells, responsible for transporting oxygen throughout the body, are typically uniform in size. The MCV is measured in femtoliters (fL). For most adults, the normal range for MCV is between 80 and 100 fL.
A result above this upper limit, usually over 100 fL, is referred to as macrocytosis, which literally translates to “large cell condition.” Macrocytosis is an observation about cell size, not a specific diagnosis. It is a sign that the process of red blood cell formation in the bone marrow is altered, leading to the release of larger, sometimes less efficient, cells into the bloodstream. The degree of elevation can offer an initial clue, as an MCV significantly higher than 110 fL often suggests a specific nutritional deficiency.
Macrocytosis Caused by Vitamin Deficiencies
The most common causes of a high MCV are deficiencies in Vitamin B12 (cobalamin) and Vitamin B9 (folate). These two B vitamins are interdependent and play a necessary role as cofactors in the synthesis of DNA, a process required for cell division. When B12 or folate is lacking, bone marrow cells cannot properly synthesize DNA and divide quickly enough. This failure causes red blood cell precursors to grow larger than normal before dividing, resulting in megaloblastic anemia.
The resulting red blood cells (macro-ovalocytes) are oversized and often misshapen, which can lead to their premature destruction in the bone marrow. B12 deficiency also impairs DNA synthesis by preventing the proper recycling of folate, creating a functional folate deficiency.
Deficiencies arise from inadequate dietary intake (e.g., strict vegetarian or vegan diets for B12) or from various malabsorption issues. Pernicious anemia is a common cause of B12 deficiency, where an autoimmune attack prevents the body from absorbing B12 in the small intestine. Gastrointestinal issues, such as celiac disease, inflammatory bowel disease, or gastric surgery, can also impair B12 and folate absorption.
High MCV Related to Other Conditions
Macrocytosis not caused by B12 or folate deficiency is categorized as non-megaloblastic macrocytosis. Chronic, heavy alcohol consumption is a frequent non-nutritional cause of an elevated MCV. Alcohol directly interferes with bone marrow function and is often associated with concurrent folate deficiency.
Liver disease, including conditions like cirrhosis, also commonly leads to a high MCV. In this context, the red blood cell membrane accumulates excess cholesterol and lipids, increasing the cell’s surface area and size. Endocrine disorders, such as an underactive thyroid (hypothyroidism), can also result in a mild macrocytosis, though the exact mechanism is not fully understood.
Certain medications can induce macrocytosis by interfering with DNA synthesis or affecting vitamin metabolism. Examples include chemotherapy drugs, some antiretrovirals used for HIV, and specific anticonvulsants. In less common cases, a high MCV can be a sign of a primary bone marrow disorder, such as myelodysplastic syndromes, where the blood-forming cells are defective.
Next Steps After a High MCV Result
A single high MCV result is not a final diagnosis, but indicates that further investigation is necessary to find the root cause. A medical professional first reviews the complete blood count, looking at the hemoglobin level and the Red Cell Distribution Width (RDW). They also take a thorough patient history, focusing on diet, alcohol use, and current medications.
The next diagnostic step involves blood tests to measure serum levels of Vitamin B12 and folate. If those levels are normal, the physician may order liver function tests or a thyroid panel to rule out hepatic issues or hypothyroidism. A peripheral blood smear allows a pathologist to visually inspect the size and shape of red blood cells and look for characteristic features, such as hypersegmented neutrophils, a hallmark of megaloblastic anemia.
Treatment is directed specifically at the identified underlying cause. For nutritional deficiencies, this involves supplementation with B12 (often via injections for severe deficiency or malabsorption) or oral folate. For other causes, treatment may involve reducing alcohol consumption, adjusting medications, or managing the underlying liver or thyroid disease.