A high result for Mean Corpuscular Volume (MCV) on a routine blood test can be concerning, but it is rarely a cause for immediate alarm. This value measures the average size of your red blood cells, the oxygen carriers in your bloodstream. When the MCV is elevated, it indicates macrocytosis—meaning the red blood cells are larger than average. Macrocytosis is not a diagnosis itself, but a signpost pointing toward underlying causes, many of which are easily managed.
Understanding Mean Corpuscular Volume
The MCV is a standard component of a Complete Blood Count (CBC) and is reported in femtoliters (fL), which are units of volume. A typical normal range for the MCV in adults is approximately 80 to 100 fL, though this can vary slightly between laboratories and age groups. A result above this range, often over 100 fL, indicates the presence of macrocytosis. Macrocytosis often arises from issues during the production of red blood cells in the bone marrow, resulting in cells that are larger and frequently immature. The size of these cells provides insight into the health and function of your red blood cell production system.
Common and Treatable Causes of High MCV
The most frequent reasons for an elevated MCV are often not severe and involve easily corrected deficits. When red blood cell precursors in the bone marrow cannot synthesize DNA properly, they continue to grow in size before dividing, leading to the production of large cells. This mechanism is known as megaloblastic anemia.
Deficiencies in Vitamin B12 (cobalamin) and Folate (Vitamin B9) are the primary drivers of megaloblastic anemia. These vitamins are necessary cofactors in the synthesis of DNA, and when they are lacking, the developing red blood cells have impaired nuclear maturation. This delay in maturation contrasts with the normal development of the cell’s cytoplasm, resulting in a large, immature cell called a macro-ovalocyte. Correcting these deficiencies often involves simple dietary changes or supplementation with the missing vitamin.
Heavy alcohol consumption is another common cause of macrocytosis, ranking alongside vitamin deficiency and medication use as a top contributor. Alcohol has a direct toxic effect on the red blood cell precursors in the bone marrow, which interferes with their normal development. Chronic alcohol misuse can also impair the absorption and metabolism of folate, compounding the problem. The elevated MCV in these cases is often reversible, but the process may take several months of abstinence for the red blood cell size to return to the normal range.
When High MCV Signals Underlying Conditions
While many cases of high MCV are treatable, the result can occasionally point toward less common or more serious health issues. When the underlying cause is not a Vitamin B12 or folate deficiency, the condition is categorized as non-megaloblastic macrocytosis. This form is commonly linked to liver problems.
Impaired liver function, such as cirrhosis or chronic hepatitis, can lead to the deposition of excess lipids and cholesterol on the membranes of red blood cells. This change in membrane composition increases the surface area of the red blood cells, causing them to appear larger on the MCV measurement. Hypothyroidism, a condition where the thyroid gland does not produce enough hormones, is also a recognized, albeit less direct, cause of macrocytosis.
Certain medications are also known to affect red blood cell production and lead to an elevated MCV. Drugs that interfere with DNA synthesis, such as some chemotherapy agents or antiviral drugs used to treat HIV, can mimic the effects of B12 and folate deficiency. Other medications, including certain anticonvulsants, can also elevate the MCV. In rare instances, a high MCV can be an early indicator of a primary bone marrow disorder, such as myelodysplastic syndromes (MDS). MDS is a group of conditions where the bone marrow produces abnormal or immature blood cells.
Diagnostic Follow-Up and Management
When a high MCV is detected on a CBC, a physician will typically order a suite of additional laboratory tests to determine the precise cause. Testing specific levels of Vitamin B12 and folate is a necessary first step, given that deficiencies are so common. Further evaluation often includes liver function tests to assess the health of the liver and a thyroid-stimulating hormone (TSH) test to check for hypothyroidism.
A reticulocyte count, which measures the number of immature red blood cells, is also a valuable tool to differentiate between causes. A low reticulocyte count points toward a production problem, such as a nutritional deficiency, while a high count may suggest the body is rapidly replacing red blood cells lost to bleeding or destruction. Providing a detailed medical history, including information about diet, alcohol intake, and all current prescription and over-the-counter medications, is necessary for your doctor to narrow down the possibilities. Management of a high MCV is always focused on addressing the underlying cause. Treatment can range from simple vitamin supplements for nutritional deficits to adjusting medications or managing a chronic disease like liver dysfunction.