What to Know About Anisocytosis

Reviewed by Dan Brennan, MD on April 24, 2021

‌A simple blood smear can tell your doctor a lot about your health. It allows them to observe the number, size, and shape of your blood cells. Sometimes it can lead to a diagnosis of anisocytosis. This means that your red blood cells (RBCs) are of mixed sizes. Normally, red blood cells should be about the same size. 

Any abnormality of your red blood cells is significant because of the important role they play in your body. Scientists are still investigating anisocytosis and its relationship with disease.

Red Blood Cells and What They Do

Red blood cells are the most abundant blood cells in your body. They make up almost half of your blood's volume. They grow in your bone marrow and take about 7 days to mature. A hormone called erythropoietin controls the production of your RBCs, which are also known as erythrocytes.

Red blood cells are uniquely suited for their twin jobs. These include carrying oxygen from your lungs to the rest of your body and returning with the carbon dioxide that you exhale. A protein called hemoglobin inside RBCs transports the oxygen and carbon dioxide. 


Red blood cells are shaped like disks that are thinner in the middle. They resemble doughnuts without the hole. They don't need a thick middle because they have no nuclei. This shape allows them to fold over so they can fit through your narrowest blood vessels.

The lack of a nucleus means that red blood cells don't live long. Their life span is just 3 months. They perform such important jobs in your body that abnormalities in size, shape, and number will seriously affect your health. 

Measuring Anisocytosis

Anisocytosis is diagnosed using a measurement called red blood cell distribution width (RDW). A high RDW value means that the variation in the size of your red blood cells is higher than normal. A high RDW value can be caused by cells that are larger than normal, smaller than normal, or both.

Your healthcare provider will combine the results of your RDW test with other tests to make decisions about your health. A high RDW value doesn't always mean that you are ill. RDW values increase with age and can be higher in certain ethnic groups. They may also increase during the later stages of pregnancy and after physical exercise.

Anemias and Anisocytosis

Blood disorders known as anemias are the most common cause of anisocytosis. It's important to remember that anemia is not a diagnosis. Instead, it is a symptom of an underlying problem.

Common signs and symptoms of anemia include:

Anemia is treated by addressing its cause. Sometimes that means taking supplements or changing your diet. You may need blood transfusions if your level of red blood cells gets too low. Some types of anemia can require a bone marrow transplant.

Kinds of Anemia

Anisocytosis may be present in several types of anemia, including the following.

Iron deficiency anemia. Red blood cells may be abnormally small and pale in this type of anemia. Iron deficiency is most often caused by blood loss. Insufficient iron in your diet or a problem absorbing the iron you consume can also cause it.

Vitamin deficiency anemia. A shortage of vitamin B12 or folate or both can cause this anemia. It is also known as megaloblastic anemia because some of the red blood cells are large and abnormally shaped.

Some kinds of anemia do not have anisocytosis as a symptom. These are called normocytic because the red blood cells are of normal size. An RDW test is still helpful here. It can rule out some types of anemia.

Other Conditions Associated With Anisocytosis

Researchers have discovered that many conditions besides anemia are associated with anisocytosis as measured by an RDW test. In fact, a high RDW value has been associated with a higher overall death rate.

Doctors aren’t sure whether anisocytosis plays a role in the deterioration of health or if it is simply a marker of illness. Is it a cause or an effect? Could it be used more widely in the diagnosis of illness? Can it even predict outcomes? Scientists are interested in these questions.  

Other than anemia, some conditions that are associated with anisocytosis include:

Cardiovascular disease. Many people with atrial fibrillation, coronary artery disease, and other heart problems have higher than normal RDW scores. This indicates anisocytosis.

Liver disease. One study suggested that RDW could be used to predict 3-month mortality in people with liver disease. Another study found that high RDW values were related to poor outcomes from hospitalization.


Kidney disease. In one study of people with acute kidney failure, high RDW scores were identified in people who died within 28 days. A study of patients with kidney disease showed that RDW scores increased as people experienced more serious stages of the disease.

Colorectal cancer. Persons with colorectal cancer had lower overall survival rates when they had high RDW scores, showing anisocytosis. This was true whether the test was done before surgery or after.

WebMD Medical Reference



American Society of Hematology: "Blood Basics."

Critical Reviews in Clinical Laboratory Studies: "Red blood cell distribution width: A simple parameter with multiple clinical applications."

JOHNS HOPKINS: "Facts About Blood."

MAYO CLINIC: "Anemia."

MERCK MANUAL: "Evaluation of Anemia," “Iron Deficiency Anemia," "Vitamin Deficiency Anemia."

QJM: An International Journal of Medicine: "Clinical and prognostic significance of anisocytosis measured as a red cell distribution width in patients with colorectal cancer."

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