A hematocrit test, often part of a standard complete blood count (CBC), measures the percentage of your total blood volume made up of red blood cells. When this percentage is higher than the normal range, it indicates an underlying issue that needs to be addressed. A high hematocrit, also known as erythrocytosis, causes the blood to become thicker and more viscous, which can have a variety of health implications.
Understanding the Implications of High Hematocrit
Red blood cells are critical for transporting oxygen from the lungs to the body's tissues. While a high red blood cell count might seem like a good thing, a hematocrit level that is too high can impair blood flow. This increased thickness, or hyperviscosity, makes the blood more difficult for the heart to pump, forcing it to work harder and increasing the risk of serious complications like blood clots, heart attacks, and strokes. It is not a disease in itself but rather a symptom of another medical condition or an environmental factor.
The Primary Cause: Polycythemia Vera
For many, the most serious illness associated with a high hematocrit is polycythemia vera (PV). This is a rare, slow-growing form of blood cancer that originates in the bone marrow. In PV, the bone marrow produces too many red blood cells, and often an excess of white blood cells and platelets as well. This overproduction occurs independently of the body's normal regulatory signals, which is why it is classified as a primary erythrocytosis. A genetic mutation, most commonly in the JAK2 gene, is responsible for this condition.
Secondary and Other Causes of Elevated Hematocrit
While polycythemia vera is a critical cause, several other conditions and factors can lead to an elevated hematocrit, known as secondary erythrocytosis. In these cases, the body produces more red blood cells in response to an external stimulus or an underlying health problem.
Conditions Causing Low Blood Oxygen (Hypoxia)
- Chronic Obstructive Pulmonary Disease (COPD): Lung diseases like COPD and pulmonary fibrosis impair the lungs' ability to exchange oxygen, leading to persistently low blood oxygen levels. The body compensates by producing more red blood cells to carry what little oxygen is available.
- Congenital Heart Disease: Certain heart defects present from birth can prevent the heart from pumping blood efficiently, leading to poor oxygenation. The body increases red blood cell production to counteract this.
- Obstructive Sleep Apnea: This condition causes repeated pauses in breathing during sleep, leading to reduced oxygen levels. The body responds by making more red blood cells.
- High Altitude: Living at high altitudes, where the air has less oxygen, prompts the body to naturally increase its red blood cell count to enhance oxygen delivery.
- Smoking: Chronic smoking leads to carbon monoxide poisoning, which prevents red blood cells from carrying oxygen effectively. The body then produces more red blood cells to compensate.
Other Medical Conditions
- Kidney Tumors: Rarely, certain kidney tumors can produce excessive amounts of erythropoietin (EPO), a hormone that stimulates red blood cell production in the bone marrow.
- Dehydration: This is the most common cause of a high hematocrit on a standard blood test. When the body lacks sufficient fluids, the plasma volume in the blood decreases, making the red blood cell count appear artificially high. Rehydration typically resolves this issue.
Symptoms to Watch For
Symptoms of high hematocrit can vary depending on the underlying cause and the severity of the condition. Many symptoms are a direct result of the thickened blood slowing circulation. It's important to be aware of these signs and consult a doctor for a proper diagnosis.
- Headaches and dizziness
- Fatigue and weakness
- Shortness of breath, especially when lying down
- Flushed or reddened skin
- Vision problems, such as blurring or blind spots
- Itching, especially after a hot shower or bath (aquagenic pruritus)
- Abdominal discomfort or fullness due to an enlarged spleen
- Easy bruising or abnormal bleeding
Diagnostic Approach for High Hematocrit
When a high hematocrit reading is found, a doctor will perform further tests to determine what illness does a high hematocrit indicate. The diagnostic process is systematic and helps differentiate between primary and secondary causes.
- Repeat Blood Tests: The doctor will likely re-check the CBC to confirm the high hematocrit is persistent and not just due to a temporary factor like dehydration.
- Evaluate for Underlying Conditions: They will look for clinical signs of hypoxia (low oxygen levels) related to lung or heart problems.
- Measure Erythropoietin (EPO) Levels: EPO is the hormone that signals the bone marrow to produce red blood cells. A high EPO level suggests a secondary cause, while a low or normal level points toward primary conditions like polycythemia vera.
- Genetic Testing: If polycythemia vera is suspected, a blood test for the JAK2 gene mutation is typically performed. This is the key diagnostic tool for identifying PV.
- Bone Marrow Biopsy: In some complex cases, a bone marrow biopsy may be necessary to examine the bone marrow's cellular composition.
Treatment and Management
The treatment for a high hematocrit depends entirely on the root cause. It can range from simple lifestyle adjustments to ongoing medical therapy.
- For Dehydration: Increasing fluid intake is often enough to correct elevated hematocrit levels.
- For Secondary Causes: Addressing the underlying condition is the primary goal. This might involve treating sleep apnea, managing heart or lung disease, or quitting smoking.
- For Polycythemia Vera: The main treatment involves regular phlebotomy (blood removal) to reduce the red blood cell count and blood volume. This procedure, similar to blood donation, reduces blood thickness and lowers the risk of clotting. Low-dose aspirin is often prescribed to reduce clot risk further. For more advanced cases, myelosuppressive medications like hydroxyurea may be used to control blood cell production.
Comparison of Primary vs. Secondary Polycythemia
| Feature | Primary Polycythemia (e.g., Polycythemia Vera) | Secondary Polycythemia |
|---|---|---|
| Underlying Cause | Intrinsic bone marrow disorder, often due to a JAK2 gene mutation. | External factors or other diseases causing hypoxia or abnormal hormone production. |
| Erythropoietin (EPO) Levels | Low or normal. | Elevated in response to low oxygen. |
| Blood Cell Production | Uncontrolled overproduction of red blood cells (and often white blood cells and platelets). | Increased red blood cell production only, as a compensatory mechanism. |
| Symptoms | Itching (especially post-bath), splenomegaly, gout, and those of hyperviscosity. | Symptoms related to underlying condition (e.g., COPD, sleep apnea) and hyperviscosity. |
| Treatment Focus | Control bone marrow overproduction with phlebotomy and medication. | Address the root cause (e.g., improve oxygenation, quit smoking). |
Potential Complications
Regardless of the cause, an untreated high hematocrit poses significant health risks. The most serious concern is the increased likelihood of thrombosis, or blood clots, which can lead to life-threatening events like a stroke, heart attack, or pulmonary embolism. Chronic conditions like polycythemia vera can also lead to other issues:
- Enlarged Spleen (Splenomegaly): The spleen works overtime to filter excess blood cells and can become enlarged.
- Myelofibrosis: After years of overproduction, the bone marrow can develop scar tissue, leading to a condition called myelofibrosis, which can cause severe anemia.
- Gout: The rapid turnover of blood cells can increase uric acid levels, leading to painful gout flare-ups in the joints.
- Leukemia: In rare cases, polycythemia vera can progress to acute myelogenous leukemia (AML).
For more information on the diagnosis and treatment of various red blood cell disorders, visit the MedlinePlus Hematocrit Test page.
Conclusion
In summary, a high hematocrit is not an illness but a diagnostic sign that indicates an underlying issue, such as polycythemia vera, chronic lung or heart disease, sleep apnea, or simply dehydration. Since the risks, particularly blood clots, are significant, a prompt and accurate medical evaluation is crucial. Differentiating between primary and secondary causes through further testing, including EPO levels and genetic analysis, guides the appropriate treatment strategy to manage the condition and mitigate its dangers effectively.
