Macrocytosis is a term used to describe red blood cells that are larger than normal. Also known as megalocytosis or macrocythemia, this condition typically causes no signs or symptoms and is usually detected incidentally on routine blood tests.

Macrocytosis isn't a specific disease, but it may indicate an underlying problem that requires medical evaluation. Common causes of macrocytosis include:

• Vitamin B-12 deficiency
• Folate deficiency
• Liver disease
• Alcoholism
• Hypothyroidism
• A side effect of certain medications, such as those used to treat cancer, seizures and autoimmune disorders
• Increased red blood cell production by the bone marrow to correct anemia, for example, after blood loss
• An underlying bone marrow cancer called myelodysplastic syndrome

If you have macrocytosis, blood tests can help determine its cause. In some cases, it may be necessary to remove a sample of your bone marrow — the spongy tissue inside your bones — for testing.

Management of macrocytosis consists of finding and treating the underlying cause. In the case of vitamin B-12 or folate deficiency, treatment may include diet modification and dietary supplements or injections. If the underlying cause is resulting in severe anemia, you might need a blood transfusion.