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For people with Down syndrome, family members, caregivers and professionals.

Elevated Red Blood Cell Count (Polycythemia)

January 2024 | Brian Chicoine, MD - Medical Director, Adult Down Syndrome Center

Key Points

  • Red blood cells carry oxygen and carbon dioxide in the bloodstream. 

  • An elevated red blood cell count (polycythemia) occurs when there is an excess of red blood cells in the bloodstream.

  • Dehydration is a common cause of an elevated red blood cell count in people with Down syndrome. 

  • A low oxygen level (hypoxemia) can cause polycythemia.

  • Consider an evaluation for causes of low oxygen when elevated red blood cells are found.

What is polycythemia?

Having an abnormally high red blood cell count is called polycythemia.

Blood is made up of red blood cells, white blood cells, platelets, and fluid. Red blood cells opens in new window carry oxygen from the lungs to the tissues of the body and then carry carbon dioxide back to the lungs from the tissues. Red blood cells are made in the bone marrow (the inside of bones) and released into the blood stream (network of blood vessels).

Red blood cells are measured in a blood test called a complete blood count (CBC). Some of the measures in a CBC include: 

  • The red blood cell count

  • Hematocrit - the percentage of the blood that is red blood cells

  • Hemoglobin - a protein in red blood cells. On a CBC, it is an indirect measure of red blood cells.

The condition of too few red blood cells is called anemia. Iron deficiency and vitamin B12 deficiency are two possible causes of anemia.

What are causes of polycythemia?


When a person is dehydrated, there is less fluid in the bloodstream. Dehydration does not increase the actual number of red blood cells. However, the percentage of the blood that is red blood cells increases (because there is less fluid). This causes the values for hemoglobin and hematocrit to be elevated. We have found that many people with Down syndrome have mild to moderate dehydration and elevated hematocrit and hemoglobin because they do not drink enough fluids. See our Tips for Staying Hydrated article for resources on hydration for people with Down syndrome.

Low oxygen levels

If oxygen levels are chronically low, the tissues in the body do not get enough oxygen. The body responds by making more red blood cells to try to get more oxygen. Chronic decreased oxygen can be seen in some forms of heart disease that children with Down syndrome are born with (congenital heart disease)

Sleep apnea can cause chronic oxygen deprivation at night and can cause polycythemia.

Blood cancer

A type of blood cancer, polycythemia vera opens in new window, can also cause polycythemia. It is rare in all people and also appears to be rare in people with Down syndrome.

What are symptoms of polycythemia?

Symptoms of polycythemia include fatigue, headaches, dizziness, and others. For most people with Down syndrome, there are no symptoms; it is just found on a blood test.

Abnormal clotting can occur if the red blood cell count becomes significantly elevated, particularly if the platelet count is elevated (which can occur with polycythemia vera).

In people with Down syndrome, abnormal clotting is not common unless the red blood cell count is very high. 


Treatment usually consists of treating the underlying condition that is causing the elevated red blood cell count.  This may include:

  • Drinking more fluids

  • Assessing for and treating sleep apnea

  • Assessing for and treating heart disease

  • Treatment for polycythemia vera

What is important to know about polycythemia in people with Down syndrome?

Polycythemia is commonly seen in infants with Down syndrome, even in those without heart disease and/or low oxygen.  It is typically transient (temporary).

For an adult with Down syndrome with polycythemia, considerations include:

  • Evaluation for dehydration and increasing fluid intake

  • Evaluation for sleep apnea

  • Cardiac evaluation for congenital heart disease

  • Evaluation for other causes of low oxygen

Find More Resources

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Please note: The information on this site is for educational purposes only and is not intended to serve as a substitute for a medical, psychiatric, mental health, or behavioral evaluation, diagnosis, or treatment plan by a qualified professional. We recommend you review the educational material with your health providers regarding the specifics of your health care needs.