The Mean Corpuscular Volume (MCV) measures the average size of red blood cells in a standard blood test. A high MCV reading, known as macrocytosis, indicates that red blood cells are larger than the typical range of 80 to 100 femtoliters (fL). Macrocytosis is a finding, not a diagnosis. Effective treatment depends entirely on accurately identifying the underlying medical condition causing the cell size increase.
Identifying the Underlying Cause
Determining the reason for macrocytosis requires a detailed review of the patient’s medical history. A healthcare provider will inquire about dietary habits (including vegetarian or vegan diets that may lack Vitamin B12) and quantify alcohol consumption. The medication list is also examined, as certain drugs (such as chemotherapy agents, anticonvulsants, and antiretrovirals) can interfere with DNA synthesis and lead to cell enlargement.
To guide the treatment plan, additional blood work is ordered to pinpoint the specific cause. These tests often include measuring Vitamin B12 and folate levels, as deficiencies in these nutrients are the most common cause. A reticulocyte count measures the number of immature, larger red blood cells, which can be elevated due to conditions like acute blood loss or hemolysis.
Liver function tests and thyroid panels are conducted to rule out other non-nutritional causes. If the initial workup is inconclusive, or if other cell lines are low, a bone marrow evaluation may be necessary to check for hematological disorders like myelodysplastic syndrome. Treatment is effective only once the specific root cause has been confirmed.
Treatment for Nutritional Deficiencies
Deficiencies in Vitamin B12 (cobalamin) and folate (Vitamin B9) are frequent contributors to macrocytosis and are treated through targeted supplementation. Both nutrients are necessary for proper DNA synthesis, and a lack of either causes red blood cells to grow larger than normal before division. It is recommended to address B12 deficiency before treating folate deficiency, as folate supplementation alone can worsen neurological symptoms if B12 deficiency is undiagnosed.
Treatment for Vitamin B12 deficiency depends on the severity and the reason for the deficiency. For individuals with dietary insufficiency or mild malabsorption, high-dose oral B12 supplements (often around 1000 micrograms (mcg) daily) may be sufficient. If the cause is pernicious anemia (an autoimmune attack preventing B12 absorption) or if neurological symptoms are present, intramuscular injections are typically required.
These B12 injections (often 1000 mcg) are administered frequently at first—such as daily for a week, then weekly for a month—before transitioning to a monthly lifelong maintenance schedule for non-correctable malabsorption issues. Folate deficiency is treated with oral folic acid supplementation, usually at a dose of 1 to 5 milligrams (mg) daily. Patients with malabsorption or increased requirements, such as during pregnancy, may require the higher end of this range.
The response to treatment is monitored, with full hematologic correction, including the normalization of the MCV, typically expected within about eight weeks of starting therapy. This confirms that the treatment has successfully corrected the underlying nutritional problem.
Addressing Non-Nutritional Causes
Macrocytosis that is not due to a B12 or folate deficiency necessitates treating the specific non-nutritional underlying condition. Chronic, excessive alcohol consumption is a common cause, as it is directly toxic to the bone marrow and disrupts red blood cell development. In this situation, the primary treatment is the reduction or complete cessation of alcohol intake.
The enlarged red blood cells associated with alcohol use typically take several months to shrink and normalize after abstinence, as the body replaces the existing larger cells with new, normal-sized ones. Liver disease, whether alcohol-related or from other causes, is another source of macrocytosis. Treatment focuses on managing the specific liver condition, such as addressing cirrhosis or hepatitis, and the macrocytosis often improves as the liver function stabilizes.
Certain medications can also cause an elevated MCV by interfering with cell production. If a medication is identified as the cause, the treatment involves a healthcare provider reviewing the necessity of the drug and potentially adjusting the dosage or switching to an alternative medication. This adjustment must always be done under medical supervision, especially for essential treatments like chemotherapy or anticonvulsants.
Hypothyroidism, an underactive thyroid gland, may also lead to macrocytosis, and the treatment involves hormone replacement therapy to restore normal thyroid function. By successfully managing the primary disorder, whether it is alcohol use, liver disease, medication effects, or a hormonal imbalance, the symptom of an elevated MCV is generally resolved as a secondary effect.