A high hematocrit (HCT) level means the blood contains an elevated percentage of red blood cells (RBCs) relative to the total blood volume. Elevated HCT, or polycythemia, causes the blood to become thicker, which increases blood viscosity and forces the heart to work harder. This thicker blood moves slowly and carries an increased risk of forming dangerous blood clots, which can lead to stroke or heart attack. For most healthy adults, typical HCT ranges fall between 41% to 50% for men and 36% to 44% for women.
Identifying the Common Causes of Elevated Hematocrit
The fix for high hematocrit depends on correctly identifying the underlying cause, which generally falls into two main categories: transient or chronic. The most common and easily reversible cause is simple dehydration, which creates a relative polycythemia.
When the body lacks sufficient fluid, the plasma volume decreases, but the number of red blood cells remains the same, artificially inflating the HCT percentage. A more complex cause involves the body compensating for low oxygen delivery, known as secondary polycythemia.
Smoking is a frequent contributor because carbon monoxide (CO) in cigarette smoke binds to hemoglobin with an affinity 200 to 250 times greater than oxygen, effectively reducing the blood’s oxygen-carrying capacity and creating chronic tissue hypoxia. The kidneys respond to this perceived oxygen deficit by increasing the secretion of the hormone erythropoietin (EPO), which stimulates the bone marrow to produce more red blood cells.
Living at high altitudes, typically above 6,000 feet, produces a similar compensatory effect. The lower partial pressure of oxygen leads to chronic tissue hypoxia. The body adapts by releasing EPO, increasing red blood cell mass over time to maximize the oxygen-carrying capacity of the blood. Chronic lung conditions, such as severe Chronic Obstructive Pulmonary Disease (COPD), can also lead to secondary polycythemia by impairing oxygen exchange and causing persistent hypoxia.
In contrast to secondary causes, Polycythemia Vera (PV) represents primary polycythemia, where the cause originates in the bone marrow itself. PV is a chronic blood disorder where the bone marrow overproduces red blood cells, platelets, and white blood cells, independently of the body’s oxygen needs. This autonomous overproduction requires specialized medical treatment.
Immediate Lifestyle Adjustments for Reduction
For most cases of mild HCT elevation associated with lifestyle factors, immediate self-managed adjustments are the first line of action. Correcting fluid balance is often the fastest way to lower HCT if dehydration is the cause. Increasing water intake restores plasma volume, diluting the concentration of red blood cells and normalizing the HCT percentage.
It is also beneficial to reduce the consumption of substances that act as diuretics, such as excessive alcohol and caffeine. These substances promote fluid loss and increase urination, contributing to a reduced plasma volume and an artificially higher HCT. Moderating or eliminating them helps the body maintain better fluid retention.
For individuals who smoke, complete cessation is the mandatory and most effective lifestyle intervention for normalizing HCT levels. Quitting smoking removes the chronic source of carbon monoxide, allowing the body to recognize adequate oxygen delivery. This eliminates the need for compensatory EPO release, and red blood cell production eventually slows down, often leading to normalized HCT levels within weeks or months.
Dietary changes can play a supportive role, especially regarding iron intake, which is necessary for red blood cell production. If a doctor suspects elevated HCT is related to high iron stores, it may be advisable to temporarily limit high-heme iron sources like red meat and avoid iron-fortified foods and supplements. Consuming foods rich in compounds that inhibit non-heme iron absorption, such as tannins in tea or phytates in whole grains, can also help manage iron levels.
Medical Interventions and When They Are Necessary
When lifestyle modifications fail to bring HCT levels into a safe range, or if an underlying disease is suspected, medical intervention is required. Persistent high HCT requires a thorough medical workup to distinguish between secondary causes and a primary bone marrow disorder like Polycythemia Vera. This diagnostic process often involves testing for specific genetic mutations and measuring EPO levels to determine if the bone marrow is acting autonomously or responding to chronic hypoxia.
The standard treatment for dangerously high HCT, particularly in Polycythemia Vera, is therapeutic phlebotomy. This procedure is similar to a blood donation, where a measured amount of whole blood (typically 250 to 500 milliliters) is removed from a vein. Removing the blood effectively reduces the total red blood cell mass and decreases blood viscosity, immediately lowering the risk of thrombosis.
Phlebotomy sessions are repeated as needed to keep the HCT below a target level, often maintained below 45% for men and slightly lower for women. For patients with confirmed primary polycythemia or chronic conditions unresponsive to phlebotomy, a hematologist may prescribe cytoreductive therapy to suppress red blood cell production. These specialized medications, such as hydroxyurea or interferon alfa, work by slowing down the production of blood cells in the bone marrow.