What Is Macrocytosis?

Medically Reviewed by Dan Brennan, MD on June 01, 2021

Macrocytosis is a condition in which your red blood cells are larger than they should be. While it isn’t a condition of its own, macrocytosis is a sign that you have an underlying health condition and may lead to a severe form of anemia called macrocytic normochromic anemia.

Understanding Macrocytosis

‌Macrocytosis is also called megalocytosis or macrocythemia. When you complete blood tests, the size of red blood cells is reported in your complete blood count. Because macrocytosis often develops into severe anemia, called macrocytic anemia, it is important to pay attention to these blood test results.‌

The two specific indicators that your doctor looks at on blood test results include the mean corpuscular volume (MCV) and the mean corpuscular hemoglobin (MCH). If either level is elevated, it’s a good indication that macrocytosis is present.

Symptoms of Macrocytosis

Macrocytic normochromic anemia develops slowly over time and may not show any symptoms until it is already severe. Common symptoms include: 

Causes of Macrocytosis

Macrocytosis isn't a disease itself but is instead a condition that results from other underlying health problems that include:

Macrocytosis may also be the side effect of some medications prescribed to treat cancer, seizures, or autoimmune disorders. If you lose a lot of blood because of an accident or injury, your bone marrow may produce more red blood cells to address the problem. Bone marrow cancer, also called myelodysplastic syndrome, may also lead to macrocytosis.

Diagnosing Macrocytosis

Blood work to test for macrocytosis should include:

Following a blood test, your doctor may want to assess the severity of your anemia. Your doctor will assess your health history to determine the likelihood of macrocytic anemia. A diagnosis is usually made based on the peripheral smear, with other results also contributing.‌

The smear provides imagery of your red blood cells’ shape and size for examination. Macrocytosis doesn’t necessarily mean you have an iron deficiency. When macrocytosis is fully developed, your MCV levels are 100 fluid liters per cell if you don’t also have an iron deficiency, thalassemia trait, or kidney disease. Other indicators include:

  • Howell-Jolly bodies 
  • Reticulocytopenia
  • Hypersegmentation of the granulocytes early on
  • Neutropenia in later stages
  • Thrombocytopenia in severe cases‌
  • Platelets of an unusual size and shape 

Keep in mind that if you do have an iron deficiency, macrocytosis may be overlooked. However, the presence of Howell-Jolly bodies and granulocyte hypersegmentation are red flags of macrocytic normochromic anemia.‌

If you have a B12 or folate deficiency, your doctor completes additional testing to determine the cause of your deficiencies. By addressing the underlying cause, you can increase your vitamin levels and improve your macrocytosis.

Treating Macrocytosis

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.  

Addressing a vitamin B12 deficiency. If you’re not getting enough vitamin B12 or folate in your diet, eat foods rich in these nutrients. If you’re still not getting enough, you may need to take supplements. 

If you are 14 or older, you need to get 2.4 micrograms of vitamin B12 every day. If you are pregnant or breastfeeding, you need slightly more. In this case, aim for between 2.6 and 2.8 micrograms each day.‌

There is no upper limit set for vitamin B12, meaning that too much isn’t toxic for you. However, some studies show that taking in 25 or more micrograms per day increases your risk for bone fracture.

Foods rich in vitamin B12 include:

  • Fish and shellfish
  • Liver
  • Red meat
  • Eggs
  • Poultry
  • Dairy products like milk, cheese, and yogurt
  • Fortified nutritional yeast
  • Fortified breakfast cereals‌
  • Enriched plant-based milks‌

Addressing a vitamin folate deficiency. If you are 19 years or older, you need 400 micrograms of dietary folate equivalents (DFE) each day. If you are pregnant, you need 600 micrograms each day, and if you are breastfeeding, you need 500 micrograms each day.‌

If you drink alcohol regularly, you should also try to get 600 micrograms of folate. High levels of alcohol consumption may impair your body’s ability to absorb folate. Try not to take more than 1,000 micrograms per day of folate in the form of a supplement. When you get folate from food, there is no limit placed on how much you can eat. Foods rich in folate or dietary folate equivalents include:

  • Dark green leafy vegetables like turnip greens, spinach, romaine lettuce, asparagus, brussels sprouts, and broccoli
  • Beans
  • Peanuts
  • Sunflower seeds
  • Fresh fruit
  • Whole grains
  • Liver
  • Seafood
  • Eggs‌
  • Fortified foods 
WebMD Medical Reference



Harvard TH Chan: “Folate (Folic Acid) – Vitamin B9,” “Vitamin B12.”

Kauffmann, T. Macrocytosis, StatPearls, 2021.

Mayo Clinic: “Macrocytosis: What causes it?”

Merck Manual: “Megaloblastic Macrocytic Anemias.”

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