Erythrocytosis is having a high concentration of red blood cells. Your levels may be high for many reasons. Some causes, like dehydration, are less concerning than others, like polycythemia vera, a serious blood disorder. Getting diagnosed and receiving treatment can prevent complications associated with erythrocytosis, like life-threatening blood clots.


What is Erythrocytosis (polycythemia)?

Erythrocytosis involves having a higher-than-normal concentration of red blood cells (erythrocytes) in your blood. Your blood consists of solid parts, including red blood cells, white blood cells and platelets. It also includes a liquid portion called plasma. Erythrocytosis describes having too many red blood cells (a solid part) in relation to plasma (the liquid part).

Erythrocytosis causes you to have high levels of hematocrit and/or hemoglobin.

  • Hematocrit levels: Hematocrit is the number of red blood cells you have.
  • Hemoglobin levels: Hemoglobin is an essential protein found in red blood cells.

Erythrocytosis is also commonly called polycythemia.


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How does erythrocytosis affect my body?

Red blood cells are important because they transport oxygen throughout your body. They keep your tissues oxygenated, nourished and healthy.

Too many red blood cells can impact your body in a variety of ways depending on what’s causing your erythrocytosis. Some effects are mild while others are more serious. Many people only experience unpleasant symptoms, like headache or fatigue. More serious causes of erythrocytosis can cause your blood to become too thick, putting you at risk of blood clots, heart attacks or strokes.

It’s important to work with your provider to determine what’s causing your erythrocytosis so you receive the right treatment to provide symptom relief or prevent complications, as needed.

What are the types of erythrocytosis?

Erythrocytosis is classified based on the blood composition that causes the high concentration of red blood cells.

  • Absolute erythrocytosis causes a high concentration of red blood cells because you have more red blood cells than normal.
  • Relative erythrocytosis causes a high concentration of red blood cells because you don’t have enough plasma in relation to the number of red blood cells. For example, dehydration may lead to you not having enough liquid or plasma in your blood.

Absolute erythrocytosis involves a greater variety of causes than relative erythrocytosis. It’s further broken down based on whether its causes are considered primary or secondary.

Primary erythrocytosis

Primary erythrocytosis results from problems in the spongy tissue inside of your bones called bone marrow. Bone marrow is the “primary” site in your body where red blood cells get made.

With primary erythrocytosis, red blood cell production increases in your bone marrow because of a genetic defect in the cells that eventually mature into red blood cells. These cells are called stem cells or progenitor cells. A genetic defect is an error in a cell’s code that tells it how to behave. A genetic defect can cause a cell to multiply out of control, creating too many red blood cells.

You can develop a genetic mutation over time (acquired), or you may be born with a genetic disorder (inherited).

  • Acquired primary erythrocytosis: The most common type of acquired erythrocytosis is polycythemia vera. Polycythemia vera is sometimes confused with erythrocytosis (polycythemia), but the conditions aren’t the same. Instead, polycythemia vera is a specific type of erythrocytosis. It’s one of the most serious conditions associated with erythrocytosis.
  • Inherited primary erythrocytosis: You can inherit gene mutations from your parents that cause your bone marrow to produce too many red blood cells.

Secondary erythrocytosis

Secondary erythrocytosis results from problems outside of your bone marrow. Most secondary erythrocytosis involves your body producing too much of a hormone called erythropoietin (EPO). Like all hormones, EPO is a chemical messenger. EPO tells your bone marrow to make more red blood cells.

For example, conditions that deprive your body of oxygen, like lung disease, trigger your body to produce more EPO. EPO tells your bone marrow to produce more red blood cells. Red blood cells, in turn, transport more oxygen to your body’s tissues. With secondary erythrocytosis, the red blood cell production is too much.

Some genetic conditions can also cause secondary erythrocytosis. Unlike primary erythrocytosis, these mutations don’t involve genetic mutations in your bone marrow.


What levels are associated with erythrocytosis?

Many factors can influence what’s considered a normal concentration for red blood cells. Your age, sex and even the altitude where you live all play a role.

Erythrocytosis is associated with elevated hematocrit and/or hemoglobin levels in adults. Your provider can measure these levels during a blood test.

Men and people assigned male at birth

  • Hematocrit (HCT): >50%
  • Hemoglobin (HB): >17.5 g/dL

Women and people assigned female at birth (non-pregnant)

  • Hematocrit (HCT): >45%
  • Hemoglobin (HB): >15.3 g/dL

Symptoms and Causes

What causes relative erythrocytosis?

Not having enough fluids can cause relative erythrocytosis. Vomiting, diarrhea and medicine called diuretics can cause you to have too little liquid in your blood. A diuretic helps your kidneys move water out of your body. You may need diuretics if you have high blood pressure or if excess fluid has collected in your body.


What causes absolute erythrocytosis?

Primary and secondary erythrocytosis have different causes.

Primary erythrocytosis

Primary erythrocytosis often results from a genetic mutation that affects your bone marrow. Causes include:

  • Polycythemia vera and other myeloproliferative neoplasms: Myeloproliferative neoplasms (myeloproliferative disorders) are conditions that can cause too many blood cells — including red blood cells — to get made in your bone marrow. Polycythemia vera is the most common type of myeloproliferative disorder associated with erythrocytosis.
  • Conditions involving inherited genetic mutations: You can be born with genetic mutations that cause your marrow to make too many red blood cells.

Secondary erythrocytosis

Secondary erythrocytosis usually involves high levels of EPO. Causes include:

  • Conditions that deprive your tissues of oxygen: Multiple conditions can cause your body to have too little oxygen. In response, EPO levels rise, so more red blood cells are made. Conditions include lung disease, heart disease and sleep apnea. A group of conditions called hemoglobinopathy can also cause secondary erythrocytosis. These conditions involve problems with hemoglobin, an essential protein that makes up red blood cells.
  • Kidney conditions and transplants: Conditions that cause erythrocytosis include renal artery stenosis and hydronephrosis. Erythrocytosis can also occur after a kidney transplant.
  • Tumors: Some tumors secrete excess EPO. They include cancerous tumors like renal cell carcinoma, hemangioblastoma, hepatocellular carcinoma and parathyroid carcinoma. Benign tumors, like uterine fibroids, pheochromocytoma and meningioma, may also secrete excess EPO.
  • Carbon monoxide poisoning and smoking: Carbon monoxide poisoning can deprive your tissues of oxygen. Excessive smoking can cause carbon monoxide poisoning.
  • Drugs and medications: Anabolic steroids and other performance-enhancing drugs used in sports can cause secondary erythrocytosis. Taking testosterone can also lead to erythrocytosis.

What are the symptoms of erythrocytosis?

Your symptoms (including how severe they are) will depend on what’s causing your erythrocytosis. For example, secondary causes of erythrocytosis may cause symptoms that include:

  • Headaches.
  • Confusion.
  • Trouble sleeping.
  • Weakness and fatigue.

Primary causes of erythrocytosis may cause more severe symptoms, including:

  • High blood pressure.
  • Night sweats.
  • Unexplained weight loss.
  • Severe pain, swelling and tenderness in your joints (gout).
  • Bleeding problems, including frequent nosebleeds and bruising easily.
  • Itchy skin that may feel worse after you’ve been in warm water.
  • Tingling (pins and needles) feeling in your arms, legs, hands or feet.
  • Burning and redness, especially in your face, hands or feet.

Diagnosis and Tests

How is erythrocytosis diagnosed?

Your healthcare provider may perform various tests and procedures to determine what’s causing your high concentration of red blood cells. To start, they’ll rule out relative erythrocytosis (too little plasma in your blood) by asking questions about your medical history, medicines you’re taking, lifestyle and symptoms. They may also perform a physical exam.

The next step is to determine whether your erythrocytosis is primary or secondary. In addition to considering the information above, your provider may perform several tests.

What tests will be done to diagnose erythrocytosis?

Your provider may perform any of the following tests to diagnose erythrocytosis.

  • Complete blood count (CBC): A complete blood count shows how many red blood cells you have and your hemoglobin and hematocrit levels. Your provider will consider this information when making a diagnosis.
  • Peripheral blood smear (PBS): During a PBS, your provider examines your blood beneath a microscope to check for abnormal cells. Cells with unusual shapes or sizes may be a sign of a myeloproliferative neoplasm, like polycythemia vera.
  • Kidney and liver function tests: These tests can show signs of kidney disease or a tumor causing high levels of EPO or red blood cells.
  • Pulse oximetry: Pulse oximetry measures how much oxygen is in your blood (blood oxygen level). It involves placing a sensor on your finger that detects your oxygen levels. This test can show whether your erythrocytosis is likely related to conditions like heart or lung disease.
  • Erythropoietin (EPO) test: Your levels of EPO can help your provider determine whether you have primary or secondary erythrocytosis. EPO levels are low with primary erythrocytosis and high with secondary erythrocytosis.
  • Urinalysis: During a urinalysis, your provider examines a pee sample to check for abnormalities. Red blood cells in your pee may signal that kidney disease is causing your erythrocytosis.

Your provider may perform other tests, including imaging and a bone marrow biopsy if they suspect a specific condition like polycythemia vera.

Management and Treatment

How is erythrocytosis treated?

Most causes of erythrocytosis can’t be cured. Instead, treatment can help ease symptoms. With more serious causes of erythrocytosis, your provider may provide treatments to prevent potential complications, like blood clots.

Phlebotomy (venesection)

Phlebotomy is the most common treatment for polycythemia vera. Your healthcare provider will remove blood to eliminate excess red blood cells and reduce your overall blood volume. Secondary causes of erythrocytosis don’t often require this treatment.


Your provider may recommend you take a low dosage of aspirin regularly if you’re at high risk of developing blood clots. Your provider may also prescribe medicines that can lower your red blood cell count, including:

Your provider may recommend specific medications and procedures to treat the underlying condition causing erythrocytosis.

Outlook / Prognosis

What can I expect if I have this condition?

Your experience depends on what’s causing your erythrocytosis. Many inherited causes of primary erythrocytosis only cause mild symptoms and aren’t life-threatening. More serious conditions, like polycythemia vera, require lifelong treatment.

Living With

How can I take care of myself?

Follow your provider’s instructions based on your condition’s causes and severity. In the meantime, specific lifestyle changes that can help include:

  • Stopping smoking.
  • Maintaining a healthy body weight.
  • Managing high blood pressure.
  • Avoiding environments with high altitudes.

When should I go to the ER?

It’s a good idea to know the signs of a blood clot if you have a condition related to your erythrocytosis that puts you at risk. Get immediate medical attention if you notice signs of the following conditions:

Symptoms include:

  • Chest pain (heart attack/pulmonary embolism).
  • Shortness of breath (heart attack/pulmonary embolism).
  • Feeling lightheaded or dizzy (heart attack/pulmonary embolism).
  • Sudden loss of balance or coordination (stroke).
  • Muscle weakness or numbness in your body (stroke).
  • Swelling, pain or redness in one leg (DVT).

A note from Cleveland Clinic

Erythrocytosis involves having a higher-than-normal concentration of red blood cells (erythrocytes) in your blood. Your healthcare provider will work with you to determine what’s causing your high concentration of red blood cells. In some instances, simple lifestyle changes — like reducing or stopping a medication — may fix the issue. If a more serious condition is causing your erythrocytosis, your provider will monitor your condition closely to ensure you receive the treatment you need.

Medically Reviewed

Last reviewed on 07/05/2022.

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