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Erythrocytosis

Overview

Erythrocytosis means your body has too many red blood cells (erythrocytes). Red blood cells carry oxygen throughout the body, so having too many can make your blood thicker than normal. This may cause symptoms like headaches, fatigue, or—in more serious cases—increase the risk of blood clots. Your care team will determine the specific type of erythrocytosis you have, since different types have different causes and treatments.

The most common form of erythrocytosis is polycythemia, a condition your doctor can help diagnose and manage.

There are two main categories:

Absolute erythrocytosis makes more red blood cells than normal.

Relative erythrocytosis — normal red blood cell levels appear high because the plasma (liquid part of blood) is low, often from dehydration.

What Causes Erythrocytosis?

Primary Erythrocytosis

This type results from changes inside the bone marrow that cause it to produce too many red blood cells. It may be:

  • Congenital (inherited)
  • Acquired, including conditions such as polycythemia vera, a rare blood disorder that increases red blood cell production and can lead to blood thickening and clotting if untreated.

Secondary Erythrocytosis

This type occurs when another condition causes the body to make extra red blood cells—usually in response to low oxygen levels. Causes include:

  • Chronic lung disease or sleep apnea
  • Heart conditions
  • Living at high altitude
  • Kidney disease, which can increase erythropoietin (EPO), a hormone that triggers red blood cell production
  • Smoking
  • Certain inherited hemoglobin conditions

Relative Erythrocytosis

This happens when blood plasma levels drop, for example, from dehydration—making red blood cell levels appear high.

Common Symptoms

Symptoms vary depending on how high your red blood cell levels are and what is causing the condition.

You may experience:

  • Headaches
  • Dizziness or feeling light-headed
  • Fatigue
  • A feeling of “fullness” in the face
  • Tendency toward clotting or bleeding
  • Vision changes

Some people have no symptoms and learn they have erythrocytosis through routine bloodwork.

How We Diagnose Erythrocytosis

A diagnosis usually begins with blood tests that check:

  • Hemoglobin and hematocrit levels (often elevated in erythrocytosis)
  • The ratio of red blood cells to plasma

Your provider may also:

  • Check oxygen levels
  • Evaluate kidney function
  • Discuss potential causes like smoking, sleep apnea, or dehydration
  • Look for signs of conditions such as polycythemia vera

Treatment Options

Treatment depends on the underlying cause and may include:

Treating lung disease, heart problems, sleep apnea, or kidney issues can help correct secondary erythrocytosis.

For certain types—especially polycythemia—periodic removal of blood may help lower red blood cell levels and reduce symptoms or clot risk.

Some patients may need medicines to reduce red blood cell production or treat complications. Your provider will discuss these options if needed.

  • Stay well‑hydrated
  • Avoid smoking
  • Manage sleep apnea if present
  • Report new symptoms promptly

When To See Your Provider

Seek medical care right away if you notice:

Sudden severe headaches

Chest pain or shortness of breath

Vision changes

Signs of blood clots, such as swelling or pain in the arms or legs

Outlook

The outlook for erythrocytosis depends on its cause. Some cases—such as dehydration‑related erythrocytosis—resolve quickly, while others, like polycythemia vera, require lifelong monitoring. Early diagnosis and proper management help prevent serious complications.

Why Choose Illinois CancerCare

A blood disorder diagnosis can feel overwhelming. At Illinois CancerCare, we are committed to ensuring you never have to face it alone. Providing advanced, comprehensive hematology and oncology care for our patients is at the heart of everything we do. Since 1977, our specialists have focused on individualized, evidence-based treatment plans, access to world-class clinical trials, and thorough follow-up. With Illinois CancerCare, you can move forward with confidence knowing trusted expertise and compassionate support are always close to home.‑based treatment plans, access to world‑class clinical trials, and thorough follow‑up.

Sources & Patient Friendly References

Information sourced from Cleveland Clinic and ASH (American Society of Hematology).