Anisocytosis describes red blood cells that are of different sizes. Normal red blood cells are generally the same size. Having red blood cells of unequal sizes may be a sign of anemia, a condition that can cause symptoms like fatigue and shortness of breath. Treatment depends on what’s causing anisocytosis.


What is anisocytosis?

Anisocytosis (pronounced “a-nuh-soe-sai-TOW-suhs”) describes red blood cells that vary in size. Sometimes, size variation in your red blood cells may signal an underlying medical issue. You may learn that your red blood cells are different sizes when you receive results from a blood test.

It may be helpful to remember what anisocytosis means by understanding its roots:

  • Aniso: unequal or uneven.
  • Cytosis: relating to cells (in this case, red blood cells in particular).

Normal red blood cells are all the same size and shape (rounded, like a disc). A red blood cell’s structure allows it to do its job, carrying oxygen to cells throughout your body. This oxygen provides the energy your organs need to function.

Size variation in your red blood cells (anisocytosis) or irregular shapes (poikilocytosis) may signal that your red blood cells aren’t fully capable of transporting the oxygen you need, resulting in anemia.

Types of red blood cell size variations

With anisocytosis, your red blood cells may be smaller or larger than normal red blood cells.

  • Macrocytosis means you have larger-than-normal red blood cells.
  • Microcytosismeans you have smaller-than-normal red blood cells.

You may also have a mix of both larger and smaller red blood cells.

It’s important to remember that these terms are just descriptive. Anisocytosis may or may not be a sign of an underlying health condition that requires treatment.


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Symptoms and Causes

What are the symptoms of anisocytosis?

The most common symptoms are the same as in anemia. You may experience:

What causes anisocytosis?

Anisocytosis is often a sign of anemia. Different forms of anemia have different causes. Anisocytosis may also signal other conditions that may or may not be associated with anemia.

Anemias associated with anisocytosis

Associated anemias include:

Anemia during pregnancy is common and may show up as anisocytosis on your bloodwork. Your body needs iron to make red blood cells. Getting the right amount of iron to support your body and the needs of a developing fetus can be difficult. Your pregnancy provider can recommend changes in your diet or supplements that can help.

Other conditions associated with anisocytosis

Conditions that may cause varied red blood cell sizes to show up in your blood work include:

Anisocytosis may show up in blood test results following a blood transfusion. Certain medications can cause anisocytosis, too.


What are the complications of anisocytosis?

Some underlying conditions that cause anisocytosis can cause complications without treatment. For example, some forms of anemia can lead to organ damage. In children, untreated anemia can cause developmental delays.

Mild anemia during pregnancy is common, but more severe anemia can lead to problems like preterm labor without treatment.

Your healthcare provider can prescribe treatments to prevent complications once they’ve identified what’s causing the size variation in your red blood cells.

Diagnosis and Tests

How is anisocytosis diagnosed?

Healthcare providers may use either of the following tests (or both) to identify anisocytosis.

  • Complete blood count (Red blood cell distribution width): A complete blood count (CBC) is a routine blood test that providers use to check on your blood cells. A particular value on this test called red blood cell distribution width (RDW) measures how varied your red blood cells are in size. Normal results are between 12% and 15%. Any number higher than that means there’s more size variation in your red blood cells than what’s considered normal.
  • Peripheral blood smear (PBS): During a PBS, a lab technologist smears a blood sample on a slide and views it with a microscope. It allows them to see red blood cell size variations and identify anisocytosis.

Additional tests to determine what’s causing anisocytosis

Your healthcare provider may perform additional tests to determine what’s causing size variation in your red blood cells. They may ask about your symptoms and medical history, including prescriptions and supplements you’re taking. They may ask about your diet.

They may check:

  • Vitamin B12 and folate levels: Your body needs these nutrients to make healthy red blood cells.
  • Iron levels: Your body needs iron to produce healthy red blood cells.
  • Ferritin levels. Ferritin is a protein that stores iron.
  • Liver function. Your liver absorbs the nutrients your body needs to make healthy red blood cells.

Your healthcare provider may perform other tests as needed.


Management and Treatment

How is anisocytosis treated?

Treatment depends on what’s causing the size variation in your red blood cells. Treatments may include:

  • Vitamin B12 and folate supplements, if a nutritional deficiency is the cause.
  • Blood transfusion, for certain inherited conditions, like sickle cell disease.
  • Bone marrow transplant, if you have problems with red blood cell production. Your body’s red blood cells are made in your bone marrow. You may need this treatment if myelodysplastic syndromes or some forms of cancer are causing anisocytosis.


Can anisocytosis be prevented?

Not all causes are preventable.

Still, you can reduce your risk of some conditions associated with anisocytosis by ensuring you get enough of the right nutrients in your diet, including:

  • Vitamin B12: Common in some dairy products and meats. Some cereals have added vitamin B12.
  • Folate: Common in some fruits, vegetables, nuts and seeds. Some cereals have added folate.
  • Iron: Common in some meats, beans and vegetables. Some breads and cereals have added iron.

Outlook / Prognosis

What is the outlook for anisocytosis?

Many causes of anisocytosis are reversible or manageable with treatment. Certain conditions associated with anisocytosis — like cancer, sickle cell disease and other chronic conditions — often require careful monitoring and ongoing treatment.

Some studies show that anisocytosis can predict a worse outlook for certain conditions. Conditions where anisocytosis may be associated with a worse prognosis include:

More research is needed to understand the relationship between abnormal red blood cell sizes and disease severity.

Living With

When should I see my healthcare provider?

See your healthcare provider if you’re experiencing symptoms of anemia, including fatigue, shortness of breath or pale skin. Don’t skip annual provider checkups, which include blood tests. The results can help your healthcare provider identify signs of potential health issues.

A note from Cleveland Clinic

“Anisocytosis” may sound like a serious diagnosis, but it’s just a word that describes size variation in your red blood cells. Don’t be concerned if you learn from your blood test results that you have anisocytosis. There are many reasons why red blood cell sizes may vary. Work with your healthcare provider to determine what’s causing the variation. If anisocytosis signals a medical condition, your provider can recommend treatments that can help.

Medically Reviewed

Last reviewed on 05/15/2023.

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