Macrocytosis is the medical term for an elevated red blood cell size, a finding frequently noted during a routine complete blood count. This result indicates that the body is producing red blood cells that are larger than the typical size range. While often not an immediate cause for alarm, macrocytosis is an important sign that an underlying health issue may require investigation. It points toward various potential causes, ranging from nutritional deficiencies to chronic conditions.
Macrocytosis Explained: Measuring Red Blood Cell Size
The size of red blood cells is determined by a specific calculation known as the Mean Corpuscular Volume (MCV). This measurement is a part of the standard complete blood count and reports the average volume of the red cells in femtoliters (fL). A normal MCV value for adults typically falls within the range of 80 to 100 fL.
Macrocytosis is confirmed when the MCV rises above this 100 fL threshold, indicating the presence of abnormally large cells in the bloodstream. Macrocytosis itself is a laboratory finding, not a definitive disease diagnosis. The elevated MCV signals that a process is interfering with the normal development of red blood cells, necessitating further medical evaluation to pinpoint the exact cause.
The Role of Vitamin Deficiencies
The most common causes of macrocytosis are deficiencies in Vitamin B12 (cobalamin) and Folate (Vitamin B9). These two B vitamins are fundamental cofactors required for the proper synthesis of DNA within the bone marrow. When either vitamin is insufficient, DNA production is impaired, which disrupts the normal division and maturation of red blood cell precursors.
This failure in cellular division results in the production of fewer, yet larger, immature red blood cells, a condition known as megaloblastic anemia. B12 deficiency can be serious because this vitamin is also necessary for maintaining the health of the nervous system. A lack of cobalamin can lead to neurological symptoms, such as difficulty with balance, memory problems, and numbness or tingling in the extremities.
Folate deficiency is often linked to poor diet or malabsorption, while B12 deficiency is more commonly caused by an inability to absorb the vitamin, often due to a lack of intrinsic factor in the stomach. Identifying which specific deficiency is present is important because treating one without addressing the other can sometimes mask the progression of neurological damage associated with a lack of B12.
Other Non-Nutritional Causes of Enlarged Cells
While vitamin deficiencies are frequent culprits, macrocytosis can also stem from several non-nutritional issues. Chronic alcohol consumption and liver dysfunction are significant causes. Alcohol can be directly toxic to the bone marrow, interfering with red blood cell production. Liver disease also alters the composition of fats surrounding the red cell membrane, which can cause the cells to swell and appear enlarged on the MCV test.
Another potential cause involves an underactive thyroid gland, or hypothyroidism, which can slow the rate of red blood cell production. Certain prescription medications are also known to interfere with cell division, leading to macrocytosis as a side effect. These drug classes include chemotherapy agents, some antiretrovirals used for HIV, and anticonvulsant medications.
Macrocytosis may also indicate a primary bone marrow disorder. Conditions like Myelodysplastic Syndromes (MDS) involve the bone marrow producing defective blood cells, including abnormally large red cells. This finding, especially when accompanied by low counts of other blood cell types, prompts a thorough hematological investigation.
Next Steps: Diagnosis and Treatment
Once an elevated MCV is detected on a complete blood count, follow-up blood tests are necessary to determine the underlying cause. These initial tests typically include checking serum levels of Vitamin B12 and Folate. Comprehensive liver function tests and a thyroid panel are also routinely ordered to screen for organ-related causes.
A reticulocyte count, which measures the number of new, immature red cells, helps determine if the bone marrow is responding appropriately to a problem. A healthcare professional will also examine a peripheral blood smear under a microscope. This smear can distinguish between the oval-shaped, megaloblastic cells typical of vitamin deficiencies and the rounder cells often seen in liver disease.
Treatment is determined by the identified cause; for example, a deficiency is managed with vitamin supplementation, while alcohol-related macrocytosis requires abstinence for the MCV to normalize. Consulting with a physician ensures a personalized diagnostic plan and appropriate management strategy.