Mean Corpuscular Volume (MCV) is a standard part of a complete blood count that measures the average size of red blood cells. A result higher than the established reference range indicates that red blood cells are larger than normal, a condition known as macrocytosis. While discovering an abnormal lab result can be concerning, an elevated MCV is common and often points to manageable causes. The MCV value is a measurement, not a diagnosis, and serves as an important clue for your healthcare provider to investigate the underlying reason for the cell size change.
Understanding Macrocytosis
The MCV is reported in femtoliters (fL) and represents the average volume of red blood cells circulating in the bloodstream. A typical normal range for the average adult falls between 80 fL and 100 fL; macrocytosis is defined as an MCV measurement exceeding 100 fL.
Red blood cells transport oxygen throughout the body. When they are too large, they can be structurally impaired and may not pass through tiny blood vessels efficiently. This inefficiency can lead to a reduced oxygen-carrying capacity, resulting in macrocytic anemia. Macrocytosis can exist with or without anemia, but the elevated MCV value always signals a disruption in the bone marrow’s red blood cell production process.
Nutritional Deficiencies as Primary Contributors
The most frequent causes of an elevated MCV are deficiencies in Vitamin B12 (cobalamin) and Folate (Vitamin B9). Both micronutrients are required for proper DNA synthesis, a necessary step for the bone marrow to produce new red blood cells. When B12 or folate is lacking, developing red blood cell precursors cannot complete cell division correctly.
This failure causes the cells to continue growing into oversized, immature cells known as megaloblasts, which are released as large red blood cells. Deficiencies arise from inadequate dietary intake (more common with folate) or from malabsorption issues (frequently affecting B12). Pernicious anemia, for example, is an autoimmune condition preventing B12 absorption due to a lack of intrinsic factor. When macrocytosis is caused by these deficiencies, the MCV reading is often severely elevated, sometimes ranging between 116 fL and 130 fL.
High MCV Related to Systemic Health and Lifestyle
Causes of an elevated MCV are not limited to nutritional issues; they frequently involve systemic health conditions or lifestyle factors. Chronic, heavy alcohol consumption is a common non-nutritional cause of macrocytosis. Alcohol is directly toxic to the bone marrow, disrupting red blood cell formation, and it also interferes with the absorption and metabolism of folate.
Liver disease, regardless of whether it is alcohol-related, can also cause red blood cells to enlarge. Liver dysfunction can lead to changes in the fats and lipids within the red blood cell membrane, causing the cells to swell. Macrocytosis is also a finding in individuals with hypothyroidism (an underactive thyroid gland), as thyroid hormones regulate blood cell production in the bone marrow.
Furthermore, certain medications can interfere with the bone marrow’s ability to produce blood cells at a normal rate and size. Examples include some drugs used in chemotherapy, antiretroviral medications, and certain anticonvulsants. These medications can impair DNA synthesis, mimicking the effects of vitamin deficiencies and leading to the release of larger cells.
Addressing Your Concerns and Next Steps
An isolated high MCV reading is rarely an immediate medical crisis, but it should not be ignored, as it indicates an underlying process requiring identification. The level of concern depends on the degree of elevation and the presence of associated symptoms. If a high MCV is accompanied by persistent fatigue, weakness, shortness of breath, or a pale or jaundiced appearance, a prompt consultation with a healthcare provider is warranted.
Neurological symptoms, such as tingling or numbness in the hands and feet, are particularly suggestive of a Vitamin B12 deficiency and require immediate attention to prevent long-term damage. Follow-up testing is necessary to pinpoint the exact cause of the macrocytosis. This typically includes measuring blood levels of Vitamin B12 and folate, performing liver function tests, and a thyroid panel. Once the cause is confirmed, treatment can be targeted, involving dietary changes, supplementation, or management of the underlying systemic condition.