Understanding the Hematocrit Test
A hematocrit test is a standard component of a complete blood count (CBC) and measures the percentage of your blood volume made up of red blood cells. These red blood cells are responsible for carrying oxygen from your lungs to the rest of your body. A higher-than-normal hematocrit level is known as erythrocytosis, and while it might sometimes indicate a serious health problem, the reason is often much simpler and more benign.
Dehydration: The Most Common Factor
The primary and most common cause of high hematocrit is dehydration. When your body lacks sufficient fluids, the volume of blood plasma decreases. Since the number of red blood cells remains the same, they become more concentrated in the remaining blood volume, leading to an artificially high hematocrit percentage. This is known as relative erythrocytosis because it's an apparent increase, not a true overproduction of red blood cells.
Resolving dehydration, for instance by drinking more fluids, can quickly return your hematocrit to a normal range. This is why doctors often recommend rehydrating before retesting to rule out this common cause.
Secondary Causes of High Hematocrit
Beyond simple fluid loss, a high hematocrit level can be caused by conditions that lead to a chronic lack of oxygen, prompting the body to produce more red blood cells to compensate. This is known as secondary erythrocytosis. The most significant contributors to this category include:
Chronic Obstructive Pulmonary Disease (COPD)
COPD is a chronic inflammatory lung disease that causes obstructed airflow from the lungs. The body's response to reduced oxygenation is to increase the production of red blood cells to improve oxygen-carrying capacity.
Sleep Apnea
Obstructive sleep apnea, a disorder in which breathing repeatedly stops and starts during sleep, can cause intermittent hypoxia (low oxygen levels). This, in turn, stimulates the bone marrow to produce more red blood cells, leading to a higher hematocrit.
High Altitude Living
People who live at high altitudes, where the air has lower oxygen content, naturally have a higher hematocrit. Their bodies adapt to the reduced oxygen by producing more red blood cells to maintain an adequate oxygen supply throughout the body. When they move to lower altitudes, their hematocrit levels tend to decrease.
Smoking
Cigarette smoking causes an increase in carbon monoxide levels in the blood, which reduces the oxygen-carrying capacity of red blood cells. To counteract this, the body ramps up red blood cell production, resulting in an elevated hematocrit.
Primary Erythrocytosis: Polycythemia Vera
In rare cases, a high hematocrit is not a response to an external factor but a result of a primary bone marrow disorder called polycythemia vera (PV). In this condition, the bone marrow produces an excessive number of red blood cells, as well as white blood cells and platelets. Unlike secondary erythrocytosis, PV is a direct and abnormal overproduction of blood cells and requires specialized medical management.
Comparing Causes of High Hematocrit
| Feature | Dehydration (Relative Erythrocytosis) | Secondary Erythrocytosis | Polycythemia Vera (Primary Erythrocytosis) |
|---|---|---|---|
| Mechanism | Reduced plasma volume; red blood cells become more concentrated. | Body produces more red blood cells in response to chronic oxygen deficiency. | Abnormal bone marrow function leading to overproduction of blood cells. |
| Likelihood | Very Common | Common (e.g., in smokers, COPD patients) | Rare |
| Resolution | Resolves with rehydration. | Managed by treating the underlying condition (e.g., COPD, sleep apnea). | Requires specialized medical treatment to manage blood cell counts. |
What to Do About a High Hematocrit Reading
- Rule out dehydration first. If you have a high hematocrit reading, your doctor will likely ask about your fluid intake. Simple rehydration is often the first step in confirming if this is the cause.
- Report all symptoms. Tell your healthcare provider about any related symptoms, such as headaches, blurred vision, dizziness, or fatigue.
- Provide lifestyle context. Inform your doctor if you smoke, live at a high altitude, or have a history of heart or lung disease.
- Discuss medication use. Some medications, such as testosterone, can influence hematocrit levels.
- Consider further testing. If dehydration and other lifestyle factors are ruled out, your doctor may order additional tests to investigate potential underlying medical conditions, such as COPD or polycythemia vera.
The Importance of Professional Guidance
A high hematocrit result is a diagnostic clue, not a diagnosis in itself. It is crucial to have a healthcare professional interpret your results in the context of your overall health and lifestyle. For more information on blood tests, you can consult a trusted medical resource such as MedlinePlus. Never rely on online information alone for medical advice. A comprehensive evaluation by a doctor is the only way to determine the precise cause and appropriate course of action for your situation.
Conclusion: Your Next Steps
High hematocrit is a common finding, with dehydration being its most frequent cause. However, it can also point to underlying issues like lung disease or, rarely, a bone marrow disorder. Understanding the difference between these possibilities is key to an effective diagnosis. By working closely with your healthcare provider and providing a complete picture of your health, you can pinpoint the cause and ensure proper treatment, if necessary.
