Having too many red blood cells thickens your blood, slows circulation, and raises your risk of dangerous blood clots. The condition is called polycythemia, and it’s generally flagged when your hematocrit (the percentage of blood volume occupied by red blood cells) climbs above 49% in men or 48% in women. Some people discover it through routine bloodwork with no symptoms at all, while others experience headaches, dizziness, and unusual itching that can significantly affect daily life.
How Extra Red Blood Cells Affect Your Body
Red blood cells carry oxygen, so more of them might sound like a good thing. But blood has an ideal thickness for flowing smoothly through your vessels. When red blood cell levels climb too high, blood viscosity increases, and circulation suffers. In lab and animal studies, polycythemia increased blood viscosity by about 35% and reduced blood flow to resting muscles by the same amount. Active muscles fared better, losing less than 10% of their flow, because rhythmic contractions help push thicker blood through.
This sluggish circulation means your heart has to work harder to push blood through narrower capillaries, and organs that depend on steady blood flow, like your brain and lungs, become vulnerable. The thicker your blood, the more likely red blood cells are to clump and form clots in places they shouldn’t.
Clots, Stroke, and Cardiovascular Risk
The most serious consequence of too many red blood cells is an increased risk of blood clots. Clots can form in deep veins (deep vein thrombosis), travel to the lungs (pulmonary embolism), or block blood flow to the brain. A large Korean study of more than 207,000 men found that those with polycythemia had a 12% higher risk of ischemic stroke compared to men with normal red blood cell counts. That might sound modest, but it represents a meaningful and persistent increase in risk, especially for people who already have other cardiovascular risk factors like high blood pressure or diabetes.
Heart attacks, blood clots in the abdomen, and clots in unusual locations like the veins draining the liver are also more common. These clotting events are often the first sign that something is wrong, sometimes appearing before a person even knows their red blood cell count is elevated.
Symptoms You Might Notice
Many people with mildly elevated red blood cells feel nothing unusual. As levels climb higher, symptoms tend to emerge gradually. Common ones include headaches, blurred vision, dizziness, and a ruddy or flushed complexion, particularly in the face and hands. Some people feel an uncomfortable fullness or pressure in the upper left abdomen, caused by an enlarged spleen that’s working overtime to filter the excess cells.
One of the most distinctive symptoms is intense itching after a warm bath or shower. This affects roughly 68% of people with polycythemia vera, the most common bone marrow form of the condition. Most describe it as itching, though some experience stinging, tingling, or burning. It can last anywhere from minutes to over an hour and significantly reduces quality of life. People with this symptom also report higher levels of fatigue, pain, and shortness of breath overall.
Why Red Blood Cell Counts Rise
The causes fall into two broad categories: problems originating in the bone marrow, and the body’s response to something else going on.
Bone Marrow Overproduction
In polycythemia vera, the bone marrow produces red blood cells on its own, ignoring the normal signals that regulate production. This is driven by a genetic mutation called JAK2, present in about 95% of cases. The mutation isn’t inherited from your parents in most cases; it develops spontaneously during your lifetime. Polycythemia vera is a slow-growing blood cancer, but “cancer” here is a bit misleading. Most people live with it for decades with proper management. Still, over longer timeframes, roughly 5 to 14% of patients develop scarring in the bone marrow (myelofibrosis) within 15 years, and a smaller percentage develop acute leukemia.
Secondary Causes
Your body also ramps up red blood cell production when it senses it isn’t getting enough oxygen. This is a normal, healthy response that becomes a problem when the trigger is chronic. The most common secondary causes include:
- Smoking. Carbon monoxide from cigarettes binds to red blood cells in place of oxygen, creating a state of low oxygen throughout the body. The bone marrow responds by churning out more red blood cells to compensate. Studies consistently show that heavy smokers have significantly higher red blood cell counts, hemoglobin, and hematocrit compared to nonsmokers.
- Living at high altitude. Thinner air means less oxygen per breath, prompting the kidneys to signal for more red blood cell production. This is why people living above 8,000 feet often have naturally higher counts.
- Chronic lung or heart disease. Conditions like COPD, sleep apnea, or congenital heart defects reduce the oxygen reaching your tissues, triggering the same compensatory increase.
- Kidney tumors. Rarely, certain tumors produce the hormone that stimulates red blood cell production, driving counts up independently of oxygen levels.
How It’s Diagnosed
A complete blood count is usually the first clue. If your hematocrit is above 49% (men) or 48% (women), your doctor will want to confirm whether your total red blood cell mass is truly elevated or if you’re just dehydrated, which can temporarily concentrate your blood and mimic the condition. Normal red blood cell mass tops out around 36 mL per kilogram of body weight in men and 32 mL/kg in women.
From there, testing focuses on figuring out why. Blood oxygen levels, kidney function, and the hormone that drives red blood cell production (erythropoietin) help distinguish bone marrow problems from secondary causes. If polycythemia vera is suspected, a blood test for the JAK2 mutation and sometimes a bone marrow biopsy will confirm the diagnosis.
How Too Many Red Blood Cells Are Treated
The fastest and most straightforward treatment is therapeutic phlebotomy, which is essentially a controlled blood draw similar to donating blood. About 450 mL (roughly one pint) is removed per session, and sessions are repeated until hematocrit drops below the target, typically 45%. Clinical trials have shown that keeping hematocrit under 45% significantly lowers the rate of blood clots. After reaching the target, most people return every one to two months for maintenance draws to keep levels in check.
For polycythemia vera specifically, low-dose aspirin is commonly used alongside phlebotomy to reduce clotting risk. When phlebotomy alone isn’t enough to control counts, or when the disease is higher risk, medications that slow bone marrow production may be added.
For secondary polycythemia, the most effective treatment is addressing the underlying cause. Quitting smoking, treating sleep apnea with a breathing device, or managing chronic lung disease can gradually bring red blood cell levels back toward normal without any direct blood treatment. In people with chronic lung conditions whose hematocrit exceeds 56% and who have symptoms of thick blood, phlebotomy is used more cautiously, with a higher target of 50 to 52%.
What to Watch For Over Time
If you’ve been told your red blood cell count is high, the outlook depends heavily on the cause. Secondary polycythemia from a fixable trigger like smoking or untreated sleep apnea can often be reversed entirely. Polycythemia vera requires ongoing monitoring but is highly manageable for most people, with normal or near-normal life expectancy when hematocrit is kept under control.
The key risk to stay aware of is clotting. Sudden severe headache, vision changes, chest pain, leg swelling, or difficulty speaking can all signal a clot and warrant immediate medical attention. Regular blood counts let you and your doctor catch upward trends before they become dangerous, and staying well hydrated helps keep blood flowing smoothly between appointments.