A high red blood cell count usually means one of three things: your body is responding to low oxygen levels, you’re dehydrated, or your bone marrow is overproducing cells on its own. The medical term for this is erythrocytosis, and it shows up when your hematocrit (the percentage of blood made up of red blood cells) rises above 50% in men or 44% in women. Most cases have a treatable or reversible cause.
How High Is “High”?
Normal hematocrit runs 41% to 50% for men and 36% to 44% for women. Results at the upper edge of normal can still be worth watching, but doctors typically investigate further when hematocrit climbs above 49% in men or 48% in women. At those levels, the World Health Organization considers polycythemia vera, a bone marrow disorder, as a possible diagnosis, though plenty of other explanations exist.
Your lab report may also flag a high hemoglobin level. The WHO threshold for concern is hemoglobin above 16.5 g/dL in men or above 16 g/dL in women. If both your hematocrit and hemoglobin are elevated, the finding is more significant than if only one is borderline.
Dehydration: The Most Common Culprit
Before assuming something serious is going on, consider whether you were dehydrated when your blood was drawn. Red blood cells float in plasma, the liquid portion of blood. When plasma volume drops from not drinking enough water, vomiting, diarrhea, or taking diuretics (water pills), the concentration of red blood cells goes up even though your body hasn’t made any extra. This is called relative erythrocytosis. Your red blood cell count looks high on paper, but the actual number of cells is normal.
A simple retest after rehydrating can confirm this. If your levels return to normal, no further workup is needed.
Low Oxygen Levels
Your kidneys monitor oxygen in your blood. When oxygen drops, they release a hormone called erythropoietin (EPO) that tells your bone marrow to make more red blood cells. This is a normal, healthy response, but it can push your count above the reference range. Several conditions trigger it:
- Smoking. Carbon monoxide from cigarettes binds to red blood cells and reduces their ability to carry oxygen. Your body compensates by producing more cells. Heavy smokers commonly have elevated hematocrit, and quitting often brings levels back down over weeks to months.
- Sleep apnea. Repeated breathing pauses during sleep cause intermittent drops in blood oxygen. Severe obstructive sleep apnea is a well-recognized driver of high red blood cell counts.
- Chronic lung disease. Conditions like COPD or pulmonary fibrosis reduce the lungs’ ability to transfer oxygen into the blood, triggering ongoing EPO release.
- Living at high altitude. Thinner air means less available oxygen. People who live above about 8,000 feet often have naturally higher red blood cell counts.
- Heart conditions. Some congenital heart defects allow oxygen-poor blood to mix with oxygen-rich blood, keeping overall oxygen levels low.
In all of these scenarios, the elevated count is your body’s attempt to solve an oxygen problem. Treating the underlying cause, whether that means CPAP therapy for sleep apnea or quitting smoking, typically brings red blood cells back toward normal.
Testosterone Therapy
If you’re on testosterone replacement therapy, this is one of the most likely explanations. Testosterone directly stimulates red blood cell production through multiple pathways, including an initial rise in EPO and changes to how your body regulates iron. Studies show testosterone therapy carries nearly four times the risk of erythrocytosis compared to placebo. The effect can be beneficial for men who were anemic before treatment, but it overshoots in others.
Lowering the testosterone dose is typically the first step. Addressing other risk factors that stack on top, like smoking or untreated sleep apnea, also helps.
Polycythemia Vera
Polycythemia vera (PV) is rarer but important to rule out when red blood cell counts are significantly elevated without an obvious cause. It’s a type of blood cancer where the bone marrow produces too many red blood cells on its own, independent of oxygen levels or EPO signals.
About 95% of PV cases involve a specific genetic mutation called JAK2 V617F. Normally, a protein called JAK2 helps regulate how your bone marrow responds to growth signals. The mutation disables JAK2’s built-in brake, giving it enhanced activity. The result is that blood cell precursors grow aggressively, sometimes even without the usual hormonal signals telling them to. This mutation is acquired during your lifetime, not inherited from your parents.
Diagnosing PV requires blood tests plus, in some cases, a bone marrow biopsy. A simple blood test for the JAK2 mutation can quickly confirm or rule out most cases.
Rare Causes Worth Knowing
Certain tumors can produce EPO on their own, tricking your bone marrow into overproducing red blood cells. About 1% to 5% of kidney cancers (renal cell carcinomas) are associated with this phenomenon. The malignant cells, which originate from kidney tubule tissue, produce EPO continuously regardless of oxygen levels. Removing the tumor resolves the erythrocytosis. Other EPO-secreting tumors are possible but uncommon.
Symptoms of Too Many Red Blood Cells
Mildly elevated counts often cause no symptoms at all, which is why many people discover the issue on routine bloodwork. As levels climb higher, blood becomes thicker and flows less easily. You might notice headaches, dizziness, blurred vision, or a reddish complexion, especially in the face and hands. One distinctive symptom is itchy skin after a warm bath or shower, which is particularly associated with polycythemia vera.
The real danger is blood clots. Thicker blood is more prone to clotting, which raises the risk of deep vein thrombosis, heart attack, and stroke. This is why doctors take persistently high red blood cell counts seriously even when you feel fine.
How High Red Blood Cells Are Managed
Treatment depends entirely on the cause. For dehydration, rehydrating is the fix. For smoking, sleep apnea, or lung disease, addressing the underlying oxygen problem is the priority. For testosterone therapy, a dose adjustment usually works.
For polycythemia vera, the main treatment is therapeutic phlebotomy: having blood drawn at regular intervals to bring hematocrit below 45%. This target has been shown to reduce the risk of blood clots. Low-dose aspirin is also standard to further lower clotting risk. Patients over 60 or those who have already had a blood clot may receive additional medication to slow red blood cell production.
Regardless of the cause, your doctor will likely recheck your blood counts after any intervention to confirm levels are moving in the right direction. A single high result on one lab test is a starting point for investigation, not a diagnosis on its own.