Mean corpuscular volume (MCV) is a biomarker in the complete blood count that measures the average size of red blood cells. MCV provides valuable insights into various hematological conditions, particularly anemias. Understanding how to interpret MCV levels and use these results to guide medical decision-making is fundamental for accurate diagnosis and implementing effective treatment strategies in clinical practice.
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What Is MCV in a Blood Test?
MCV stands for mean corpuscular volume, and it's a measurement of the average size or volume of red blood cells (RBCs) in circulation. In blood tests, MCV is included in a complete blood count (CBC), which is a detailed evaluation of the different types of blood cells in circulation, including RBCs, white blood cells, and platelets.
MCV is calculated by dividing the percent hematocrit by the RBC count and multiplying the result by 10. MCV is expressed as femtoliters (fL). A normal MCV ranges between 80-100 fL. (27)
Having abnormally small or large RBCs can indicate the presence of various medical conditions. Therefore, MCV is an important laboratory parameter because it aids in diagnosing and managing health issues ranging from anemia to cancer. (27)
What Does High MCV Mean?
Based on their size (or MCV), RBCs can be categorized into three primary types:
- Normocytic RBCs have a normal size with an MCV ranging from 80-100 fL. These cells are typical in healthy individuals and reflect a balanced state of RBC production. (32)
- Microcytic RBCs are smaller than normal, with an MCV of less than 80 fL. Microcytosis is commonly associated with iron deficiency anemia. (30)
- Macrocytic RBCs are larger than normal, with an MCV exceeding 100 fL (26).
Usually, macrocytosis doesn't cause any symptoms. If symptoms do appear, they're related to what is causing your RBCs to enlarge. For example, if you have macrocytic anemia caused by vitamin B12 deficiency, you may have symptoms like fatigue, weight loss, memory loss, and sore tongue.
What Causes High MCV?
The most common causes of macrocytosis (high MCV) include:
Nutritional Deficiencies: The body needs vitamin B12 and folate for DNA synthesis and cell division, processes involved in the healthy production of RBCs by the bone marrow. Deficiencies in either vitamin hinder DNA synthesis, leading to the production of fewer but larger immature RBCs because the cells cannot divide properly, resulting in macrocytic anemia.
Hemolytic Anemia or Hemorrhage: Hemolytic anemia is a deficiency of RBCs in circulation caused by the destruction, rather than the underproduction, of RBCs. Hemorrhage is acute bleeding from a damaged blood vessel. In either scenario, the body compensates by releasing immature RBCs called reticulocytes, which are larger than mature RBCs, into the bloodstream. (26)
Excessive Alcohol Intake: Alcohol use disorder often coexists with malnutrition and can lead to multiple nutrient deficiencies, including folate, ultimately leading to macrocytic anemia.
Liver Disease: Liver disease of any cause (not just alcohol-related liver disease) can cause macrocytosis due to changes in lipid (fat) metabolism, leading to a buildup of lipids on RBC membranes and increased RBC surface area (25).
Bone Marrow Disorders: RBCs are produced in the bone marrow. Bone marrow dysfunction can interfere with DNA synthesis and cell division processes, resulting in abnormal, immature, and macrocytic RBCs. Bone marrow disorders are more commonly a cause of macrocytosis in older populations than in younger ones. (22)
Hypothyroidism: Macrocytosis is found to occur in 55% of patients with hypothyroidism (an underactive thyroid gland).
Medications: Macrocytosis can be a side effect of various medications, often through mechanisms such as inhibiting folate or vitamin B12 metabolism or directly affecting the bone marrow's ability to produce appropriately sized RBCs. The most common culprits include:
- Antiretroviral agents to treat HIV
- Anticonvulsant medications, such as valproic acid and phenytoin
- Folate antagonists, such as methotrexate
- Chemotherapy drugs to treat cancer
- An antibiotic called trimethoprim/sulfamethoxazole
- Metformin, used to lower blood sugar in people with diabetes (22, 27)
Lab Tests and Diagnosis
Various laboratory tests can help diagnose macrocytosis and identify its underlying cause. Here's an explanation of the role of each test:
- Complete Blood Count: A CBC includes RBC count, hemoglobin levels, hematocrit, and RBC indices (including MCV). An elevated MCV indicates the presence of macrocytosis. Macrocytosis often coexists with anemia, which is diagnosed by measuring a low RBC count, hemoglobin, or hematocrit.
- Reticulocyte Count: This test measures the number of young RBCs in the blood. A high reticulocyte count suggests active RBC synthesis (erythropoiesis), often observed in anemias caused by hemolysis or blood loss. In contrast, a low count indicates a production issue, as seen in nutrient deficiencies or bone marrow dysfunction.
- Peripheral Blood Smear: Examining a blood sample under a microscope allows for direct visualization of RBC morphology. Large RBCs and other abnormal RBC shapes can provide clues to specific causes.
- Vitamin B12 and Folate Levels: Measuring serum levels of vitamin B12 and folate helps identify deficiencies that can cause macrocytosis. Elevated homocysteine levels can indicate deficiencies in vitamin B12, folate, or both. To distinguish between the two deficiencies, homocysteine can be ordered with methylmalonic acid (MMA), which only elevates with vitamin B12 deficiency.
- Liver Function Tests: A comprehensive liver panel includes aspartate aminotransferase (AST), alanine aminotransferase (ALT), gamma-glutamyl transferase (GGT), alkaline phosphatase (ALP), bilirubin, and protein levels. Abnormalities in these can indicate liver inflammation and disease.
- Bone Marrow Biopsy: This invasive test provides detailed information about the bone marrow's cellular composition and function. It is required to diagnose bone marrow disorders like myelodysplastic syndromes.
Treatment Options for High MCV
Treatment strategies for elevated MCV levels focus on addressing the underlying cause. In cases where the diagnostic workup is otherwise normal and the patient doesn't present with any symptoms, additional treatment other than periodic monitoring may not be necessary.
Addressing Nutritional Deficiencies
Treating vitamin B12 and folate deficiencies can involve increasing dietary intake through diet and supplements.
Folate deficiency can usually be easily treated by administering 1 mg of folic acid orally once daily for about three weeks. The recommended daily allowance for folate to prevent nutritional deficiencies is 400 mcg DFE. Eating a varied diet that includes vegetables, fruits, legumes, grains, eggs, dairy, and meat is often sufficient to prevent folate deficiency once it has been corrected. (36)
Intramuscular injections of cyanocobalamin (a form of vitamin B12) are often recommended for treating vitamin B12 deficiency. A typical treatment regimen involves 1,000 mcg of intramuscular B12 for five days, followed by 1,000 mcg once weekly for four weeks. (24)
Vitamin B12 is primarily obtained in the diet through animal products, including fish, meat, poultry, eggs, and dairy. People who eat vegetarian or vegan diets are at higher risk of vitamin B12 deficiency and may require additional oral or intramuscular B12 supplementation to prevent deficiency. (35)
Managing Alcohol Intake
Limiting alcohol consumption can prevent and assist in treating high MCV levels. The Centers for Disease Control and Prevention (CDC) recommends that adults limit alcohol consumption to moderate levels, defined as up to one drink per day for women and up to two drinks per day for men.
The following strategies can help individuals successfully reduce alcohol consumption:
- Set Clear Goals: Define specific, achievable goals for reducing alcohol intake and monitor progress regularly. This could include setting limits on the number of drinks consumed per week.
- Identify Triggers: Recognize situations or emotional states that lead to excessive drinking and develop strategies to manage these triggers effectively.
- Seek Support: Engage in support groups, counseling, or therapy to address underlying issues related to alcohol use and learn strategies for moderation.
- Find Alternatives: Replace alcoholic beverages with non-alcoholic alternatives or engage in activities that do not revolve around drinking, such as hobbies or exercise.
- Practice Mindful Drinking: Be aware of the reasons for drinking and savor each drink slowly, paying attention to the effects of alcohol on the body and mind.
Treat Underlying Medical Conditions
If macrocytosis is a side effect of a medication, your doctor may modify the dose or prescribe an alternative agent.
Treatments for bone marrow disorders may include blood transfusions, chemotherapy, and stem cell transplants (31).
The goal of treating hypothyroidism is to normalize thyroid levels in circulation and prevent irreversible damage to the thyroid gland. The following Rupa Health article discusses this approach in detail:
The primary goals of treating liver disease are to manage symptoms, halt or reverse disease progression, and restore liver function. Read about the therapeutic interventions doctors often recommend to meet these goals in this Rupa Health Article:
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Key Takeaways
- Mean corpuscular volume (MCV) is one parameter included in a complete blood count.
- High MCV is often discovered incidentally during routine blood tests, as it typically does not present with specific symptoms. However, abnormal MCV levels can indicate a range of medical conditions, each of which may lead to symptoms and complications if not properly managed.
- By providing early insights into conditions ranging from macrocytic anemia to bone marrow disorders, MCV measurement facilitates prompt diagnosis and intervention.
- Early identification and treatment based on MCV results can help prevent the progression of underlying conditions into more serious health issues, thereby improving overall patient outcomes.
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