What is the name of the condition that occurs when there is a high hematocrit or too many red blood cells?

September 29, 2025 •

3 min read

An elevated hematocrit, or having too many red blood cells, is a key indicator of the medical condition known as polycythemia or erythrocytosis. This state causes the blood to thicken, increasing its volume and potentially leading to serious complications like blood clots, heart attack, and stroke. Understanding this condition is the first step toward effective management and better health outcomes.

Quick Summary

Polycythemia, also known as erythrocytosis, is the medical term for having an increased red blood cell mass, which results in elevated hematocrit levels and thickened blood. It can be caused by either bone marrow disorders or underlying health issues affecting the body's oxygen levels.

Key Points

Condition Name: The condition involving too many red blood cells is called polycythemia, or erythrocytosis.
Types of Polycythemia: There are two main types: primary polycythemia (Polycythemia Vera), a bone marrow disorder; and secondary polycythemia, which is caused by an external factor like low oxygen levels.
Primary Cause (PV): Polycythemia Vera is often caused by a mutation in the JAK2 gene, leading to excessive, unregulated blood cell production.
Secondary Cause: Secondary polycythemia can result from conditions like COPD, sleep apnea, or living at high altitudes, which cause the body to produce extra red blood cells to compensate for low oxygen.
Main Risk: The thickening of blood due to high hematocrit increases the risk of dangerous complications, most notably blood clots that can lead to heart attack or stroke.
Diagnosis: Diagnosis typically involves a complete blood count (CBC), measuring erythropoietin levels, and potentially genetic testing for the JAK2 mutation.
Treatment: Management options include phlebotomy to remove excess blood, medications like Hydroxyurea, and treating the underlying cause.

Understanding Polycythemia and Erythrocytosis

Polycythemia and erythrocytosis are terms for an increased red blood cell count. Polycythemia is the broader term, while erythrocytosis specifically refers to a rise in red blood cells. This leads to high hematocrit, the percentage of blood volume that is red blood cells. The resulting thicker blood, or hyperviscosity, contributes to various symptoms and risks.

Primary vs. Secondary Polycythemia

Differentiating between primary and secondary polycythemia is vital for treatment. This distinction often relies on erythropoietin (EPO) levels.

Primary Polycythemia

Cause: Stems from a bone marrow issue, most notably Polycythemia Vera (PV).
Mechanism: Often linked to a JAK2 gene mutation, causing excessive red blood cell production.
EPO Levels: Typically low or normal.
Symptoms: Can involve increased white blood cells and platelets alongside red blood cells.

Secondary Polycythemia

Cause: Develops due to an underlying condition.
Mechanism: Usually a response to chronic low oxygen, prompting increased EPO production.
EPO Levels: Characterized by high levels.
Examples of Causes: Includes conditions like COPD, sleep apnea, smoking, heart disease, high altitude living, or EPO-secreting tumors.

How High Hematocrit Affects the Body

High hematocrit makes blood thicker, impairing flow and increasing risks.

Common Symptoms

Fatigue and weakness
Headaches and dizziness
Vision problems
Numbness, tingling, or burning in limbs
Itchy skin, especially after warming (aquagenic pruritus)
Shortness of breath
Enlarged spleen

Serious Complications

Blood Clots: Increased viscosity raises the risk of clots, potentially leading to heart attack, stroke, or pulmonary embolism.
Gout: Higher uric acid levels from cell turnover can cause gout.
Ulcers: Elevated histamine in PV can increase stomach acid, leading to ulcers.
Progression: Rarely, PV can advance to myelofibrosis or acute myeloid leukemia.

Diagnosing a High Hematocrit

Diagnosis involves multiple steps to find the cause.

Complete Blood Count (CBC): Measures hematocrit and is often the first sign.
Medical History and Physical Exam: Reviews symptoms, lifestyle, and existing health issues.
Erythropoietin (EPO) Level Test: Helps differentiate primary (low EPO) from secondary (high EPO) polycythemia.
Gene Testing: A JAK2 mutation test is common for suspected Polycythemia Vera.
Bone Marrow Biopsy: May be used to examine bone marrow and rule out other causes.

Treatment and Management Options

Treatment targets the underlying cause and aims to reduce red blood cells and complication risks.

Comparison of Treatment Options


Treatment Method	Best For	How it Works	Potential Side Effects
Phlebotomy	Primary and some secondary cases	Removes excess blood.	Iron deficiency, fatigue, dizziness
Medications (e.g., Hydroxyurea)	Primary PV	Suppresses bone marrow cell production.	Nausea, mouth sores, skin changes
JAK2 Inhibitors (e.g., Ruxolitinib)	Primary PV	Blocks the signaling pathway from the JAK2 mutation.	Anemia, bruising, low platelet counts
Treating Underlying Cause	Secondary polycythemia	Addresses the root issue (e.g., quitting smoking).	Varies
Low-Dose Aspirin	Both types (if prescribed)	Reduces blood clot risk.	Increased bleeding risk

Lifestyle Adjustments

Lifestyle changes can aid management and improve health.

Hydration: Dilutes blood viscosity.
Exercise: Improves circulation, helps prevent clots.
Smoking Cessation: Effective in lowering hematocrit, especially in secondary cases from hypoxia.
Diet: A balanced diet and monitoring iron intake can be helpful.
Avoiding Extreme Temperatures: Poor circulation increases sensitivity.
Managing Itching: Cool water baths and gentle soaps can relieve aquagenic pruritus.

Conclusion

High hematocrit indicates polycythemia or erythrocytosis, a condition of too many red blood cells. Accurate diagnosis determines if it's primary (like Polycythemia Vera) or secondary. Management, including phlebotomy, medication, and lifestyle changes, is crucial to reduce the risk of serious complications such as blood clots. Consulting a healthcare provider is key for managing this condition effectively. For more information, visit the National Heart, Lung, and Blood Institute website.

Frequently Asked Questions

High hematocrit, which is an elevated percentage of red blood cells in the blood, is a key characteristic of polycythemia. Polycythemia Vera is a specific type of primary polycythemia, caused by a genetic mutation in the bone marrow. High hematocrit can also be caused by other factors, known as secondary polycythemia.

The primary danger is that the blood becomes thicker, increasing the risk of blood clots. These clots can lead to serious cardiovascular events, including heart attacks, strokes, and deep vein thrombosis.

While lifestyle changes like hydration, exercise, and quitting smoking can help manage high hematocrit, especially in secondary cases, they are often not enough on their own. Many forms of polycythemia require medical intervention, such as phlebotomy or medication, to effectively lower red blood cell levels and reduce complications.

Common symptoms include fatigue, headaches, dizziness, blurred vision, itchy skin (especially after warm water exposure), and a burning sensation in the hands or feet.

High hematocrit is typically diagnosed through a routine blood test called a complete blood count (CBC). Further tests, such as measuring erythropoietin levels and genetic testing, may be conducted to determine the underlying cause.

Polycythemia vera is classified as a type of blood cancer (a myeloproliferative neoplasm) because it involves the uncontrolled production of blood cells in the bone marrow due to a genetic mutation. It is a slow-progressing cancer, and many people can manage their symptoms for decades with proper care.

Primary polycythemia (Polycythemia Vera) is a condition caused by a bone marrow problem, while secondary polycythemia is a reaction to an external factor, such as chronic oxygen deprivation. In primary polycythemia, erythropoietin levels are low, whereas in secondary polycythemia, they are typically high.