A complete blood count (CBC) is a common blood test that provides a snapshot of the cells circulating in your bloodstream. The Mean Corpuscular Volume (MCV) measures the average size of your red blood cells. A high MCV indicates that the red blood cells are larger than normal, a condition called macrocytosis. This article clarifies the meaning of a high MCV reading and explores the underlying causes that necessitate further medical investigation.
What is Mean Corpuscular Volume (MCV)?
The MCV is a calculated value representing the average volume of a single red blood cell, measured in femtoliters (fL). Red blood cells transport oxygen throughout the body, and their size provides an important clue about their production and function. A typical adult reference range for MCV falls between 80 and 100 fL.
A reading above this range, known as macrocytosis, means the red blood cells being produced are abnormally large. This finding is not a diagnosis itself but a laboratory sign that something is interfering with normal red blood cell formation within the bone marrow. The elevated MCV signals the need for a targeted investigation to uncover the specific root cause.
Common Nutritional Deficiencies Linked to High MCV
The most common causes of macrocytosis are deficiencies in Vitamin B12 (cobalamin) and Folate (Vitamin B9). Both B vitamins are necessary co-factors in DNA synthesis, a process active in rapidly dividing cells like those in the bone marrow. When these nutrients are lacking, DNA synthesis becomes impaired, slowing the cell division process.
Red blood cell precursors attempt to divide but cannot complete the process properly, leading to fewer but physically larger cells released into circulation. This results in macrocytosis, which is termed megaloblastic anemia when accompanied by a low red blood cell count. Folate deficiency often arises from poor dietary intake or conditions that increase the body’s demand, such as pregnancy.
Vitamin B12 deficiency is more commonly caused by malabsorption than by diet alone. A frequent cause of malabsorption is pernicious anemia, an autoimmune condition where the body attacks the intrinsic factor protein required for B12 absorption in the small intestine. Certain medications, such as those used to reduce stomach acid, can also interfere with B12 absorption.
Other Underlying Conditions That Raise MCV
While nutritional issues are primary culprits, many other systemic conditions and lifestyle factors can cause macrocytosis. Chronic and excessive alcohol consumption is a frequent cause of a high MCV, often without an accompanying vitamin deficiency. Alcohol is directly toxic to the bone marrow, disrupting red blood cell production, and it can also interfere with folate absorption and metabolism.
Chronic liver disease can also lead to an elevated MCV. This occurs because altered fat metabolism causes abnormal lipids to deposit onto the red blood cell membranes, increasing the cell’s surface area and size. Macrocytosis may also be observed in individuals with an underactive thyroid gland, known as hypothyroidism.
Medications and Bone Marrow Disorders
Certain medications are known to increase MCV by interfering with DNA synthesis or vitamin metabolism. Examples include chemotherapy agents, some antiretroviral drugs used for HIV treatment, and anticonvulsants like phenytoin. In rare cases, a high MCV can be a sign of a primary bone marrow disorder, such as Myelodysplastic Syndromes (MDS), where the bone marrow fails to produce healthy blood cells.
Next Steps After Receiving a High MCV Result
The discovery of a high MCV on a routine CBC is the starting point for a directed investigation. The healthcare provider will first review the entire CBC, paying close attention to hemoglobin levels and the counts of other blood cells to determine if anemia or other blood abnormalities are present. A comprehensive medical history is collected, focusing on diet, alcohol use, and all current prescription and over-the-counter medications.
The next step is typically to order specific blood tests to measure the levels of Vitamin B12 and Folate. If these are normal, the investigation will broaden to include tests for liver function and thyroid hormone levels to screen for systemic causes. A reticulocyte count, which measures immature red blood cells, may be ordered to check if the body is actively trying to replace lost blood, as these younger cells are naturally larger.
The ultimate goal of this workup is to identify the precise underlying cause of the macrocytosis. Treatment is directed at correcting that specific issue, whether it involves nutritional supplementation, managing a chronic condition like liver or thyroid disease, or adjusting a medication regimen. Addressing the cause is the pathway to normalizing the MCV and restoring healthy red blood cell production.