The Mean Corpuscular Volume (MCV) and Mean Corpuscular Hemoglobin (MCH) are two standard measurements included in a routine Complete Blood Count (CBC) test. MCV reflects the average size of your red blood cells (RBCs), while MCH indicates the average amount of oxygen-carrying hemoglobin contained within each of those cells. Elevated results for either of these values suggest a change in red blood cell production or structure, and a medical professional must follow up on such findings.
What High MCV and MCH Indicate
An elevated MCV, typically above 100 femtoliters (fL), signifies the presence of abnormally large red blood cells, a condition referred to as macrocytosis. High MCH levels usually accompany a high MCV because a larger cell has more physical space to hold a greater quantity of hemoglobin. This combination of elevated indices is not a diagnosis itself but rather a sign that an underlying health issue is affecting the process of blood cell formation.
The production of these oversized cells, which can sometimes lead to macrocytic anemia, often results from a defect in the maturation process of red blood cell precursors in the bone marrow. Because the cells are large and often immature, they may be less efficient at transporting oxygen. These cells can also have a shorter lifespan than healthy red blood cells. Macrocytosis is categorized into two main types: megaloblastic (involving a specific defect in DNA synthesis) and non-megaloblastic (caused by other mechanisms).
Causes Related to Vitamin Deficiencies
Deficiencies in Vitamin B12 (cobalamin) and Folate (Vitamin B9) represent the most frequent cause of macrocytosis, leading to a specific type known as megaloblastic anemia. Both vitamins are necessary cofactors for DNA synthesis, which is required for the rapid division and maturation of red blood cell precursors. When one of these vitamins is lacking, DNA synthesis slows down significantly, while the production of cellular components like proteins and RNA continues normally.
This imbalance creates an asynchrony where the cell’s nucleus matures slowly, but the cytoplasm grows rapidly, causing the cell to enlarge without dividing properly. The result is the release of large, immature, and fragile red blood cells, known as macro-ovalocytes, into the bloodstream. A B12 deficiency can also create a functional folate deficiency, known as the “methyl trap,” by preventing the proper utilization of folate in the metabolic pathway.
Deficiencies can stem from inadequate dietary intake, particularly in strict vegans, or from malabsorption issues within the digestive tract. A common cause of B12 deficiency is Pernicious Anemia, an autoimmune disorder that prevents the stomach from producing intrinsic factor, a protein required for B12 absorption in the small intestine. Certain medications, such as those used to treat seizures or ulcers, can also interfere with the absorption or metabolism of these crucial vitamins.
Macrocytosis Due to Other Medical Conditions
Macrocytosis can also arise from conditions that do not involve a primary deficiency of B12 or folate, falling into the non-megaloblastic category. Chronic, excessive alcohol consumption is a very common cause, often exerting a direct toxic effect on the bone marrow, which disrupts the normal development of red blood cells. Alcohol abuse may also contribute to macrocytosis by impairing nutrient absorption or causing liver damage.
Liver disease, such as cirrhosis, is another frequent cause of enlarged red blood cells. This effect is often attributed to changes in lipid metabolism, which alters the composition of the red blood cell membrane, causing the cells to swell and appear as round macrocytes. Certain medications, especially chemotherapy agents or some antiviral drugs used to treat HIV, can interfere with DNA synthesis in the bone marrow, mimicking the effects of a vitamin deficiency.
Other systemic conditions can also impact red blood cell size. Hypothyroidism, a state of low thyroid hormone, is associated with macrocytosis, sometimes even without anemia. Conditions affecting the bone marrow directly, such as Myelodysplastic Syndromes (MDS), are characterized by the production of defective, large red blood cells. A high volume of Reticulocytes (young, larger-than-average red blood cells) released prematurely due to rapid blood loss or destruction can also cause a transient elevation in the MCV reading.
Next Steps for Diagnosis and Treatment
Identifying the cause of high MCV and MCH requires a systematic diagnostic approach initiated by a healthcare provider. The initial step typically involves a review of the complete blood count, including the Reticulocyte count, which helps determine if the bone marrow is releasing immature cells. A peripheral blood smear is also performed to visually examine the shape and appearance of the red blood cells, distinguishing between the oval macrocytes (vitamin deficiencies) and the round macrocytes (liver disease).
Further laboratory tests are used to narrow the possibilities, including measuring serum levels of Vitamin B12 and Folate. To differentiate between these two deficiencies, the levels of Methylmalonic Acid (MMA) and Homocysteine are often checked, as their elevation patterns are distinct. Liver function tests and a Thyroid Stimulating Hormone (TSH) test may also be ordered to rule out hepatic or endocrine causes.
Treatment is entirely dependent on the underlying cause identified through these investigations. If the macrocytosis is due to a simple dietary deficiency, supplements and dietary changes are often sufficient. For conditions like Pernicious Anemia, Vitamin B12 injections may be necessary to bypass the malabsorption issue. If alcohol use is the factor, cessation or reduction of alcohol intake is required for the MCV to normalize, which can take several months.