What Is Erythrocytosis?

Medically Reviewed by Carol DerSarkissian, MD on April 27, 2023
Written by WebMD Editorial Contributors
4 min read

Erythrocytosis is when you have more red blood cells than normal. Red blood cells are also called erythrocytes. 

Red blood cells carry oxygen throughout your body and remove carbon dioxide from your body. Your bone marrow (the tissue inside your bones) makes red blood cells and releases them into your bloodstream. Red blood cells live for about 120 days and then die.

There are two types of erythrocytosis, primary and secondary. While both cause an increase in red blood cells, each type happens for a different reason.

Primary erythrocytosis. This type of erythrocytosis occurs when there is a problem in your bone marrow that causes it to produce too many red blood cells. Primary erythrocytosis can be either congenital, which means you were born with it, or acquired, which means you developed it later.

Secondary erythrocytosis. This type of erythrocytosis also happens when your body produces too many blood cells, but it is not caused by a problem with your bone marrow. This type usually happens because something affects your erythropoietin (EPO), which is a hormone that your kidneys make that drives red blood cell production.  

Primary erythrocytosis can be a harmless disorder caused by a genetic trait that is passed down in families. If you only have too many red blood cells without having too many white blood cells or platelets, and you don't have secondary erythrocytosis, this may be why. 

Primary erythrocytosis can also be caused by a type of blood cancer called polycythemia vera. Polycythemia vera is rare and develops slowly. You can have it for years before it's diagnosed. If it isn't treated, polycythemia vera can be life-threatening because the extra red blood cells can cause your blood to thicken and slow down. This can cause blood clots to develop. 

Secondary erythrocytosis can have many causes. These can include: 

Conditions that cause low oxygen. Your red blood cells carry oxygen to your body. If your oxygen is low for any reason, your body may increase its production of red blood cells to give you more oxygen. Conditions that can cause you to have a low oxygen level include:

  • Smoking
  • Heart problems
  • Lung disease
  • Sleep apnea, which is a condition that causes you to stop breathing or breathe shallowly when you're sleeping
  • Hemoglobinopathy, which is a group of inherited blood disorders that affect the ability of your red blood cells to carry oxygen
  • High altitudes

Kidney disease. The hormone EPO that controls the production of red blood cells is made in your kidneys. Problems with your kidneys such as cysts, tumors, or narrowing of the arteries that go to the kidneys can cause your body to make more EPO, which makes more red blood cells. 

Some medicines. Anabolic steroids and other performance-enhancing drugs can cause your body to increase the number of red blood cells it produces.

Tumors. Some tumors secrete EPO. These can include tumors of the kidney, brain, liver, and uterus.

Dehydration. In some cases, you don't have too many red blood cells, but your red blood cells are too concentrated because you don't have enough fluid in your bloodstream. This is called relative erythrocytosis and can be caused by burns, vomiting, diarrhea, or anything else that can cause dehydration, such as not drinking enough fluids.

Relative erythrocytosis can also be caused by diuretics, medicines that cause your kidneys to eliminate salt and water more quickly than normal. 

Many of the symptoms of erythrocytosis are caused by your blood flowing more slowly than normal. These symptoms can include: 

Erythrocytosis is usually diagnosed first with a blood test. Your doctor will check the number of red blood cells you have as well as your EPO level. If your EPO level is high, your doctor may do more tests related to primary erythrocytosis. If your EPO level is normal or high, your doctor may do more tests to check for causes of secondary erythrocytosis.     

Treatment for erythrocytosis depends on the cause. Primary erythrocytosis may be treated with phlebotomy, which is when some of your blood is taken out to remove excess red blood cells and improve your blood flow. This is the primary treatment for polycythemia vera. 

Secondary erythrocytosis is treated by treating the underlying cause. This treatment may include:

  • Oxygen for people who have low oxygen levels
  • Advice and help to quit smoking
  • Treatment for any disorder that is causing low oxygen levels
  • Surgery to remove tumors
  • Stopping medicines that are causing the erythrocytosis
  • Treating a hormone disorder
  • Giving fluids