A high red blood cell count has several possible causes, ranging from something as simple as dehydration to chronic lung disease, hormone therapies, or rarely, a bone marrow disorder. The most common cause is actually not having too many red blood cells at all. It’s having too little plasma (the liquid part of your blood), which makes the concentration of red cells appear higher than it really is. Sorting out which cause applies matters because the treatments are very different.
How High Is “High”?
Doctors typically flag a high red blood cell count based on two related measurements: hemoglobin (the oxygen-carrying protein inside red blood cells) and hematocrit (the percentage of your blood made up of red blood cells). The thresholds differ by sex. For men, a hematocrit above 49% or hemoglobin above 16.5 g/dL is considered elevated. For women, those cutoffs are a hematocrit above 48% or hemoglobin above 16.0 g/dL.
A single high reading doesn’t necessarily mean something is wrong. Your doctor will usually repeat the test, check your hydration status, and look at your other blood values before investigating further.
Dehydration: The Most Common Cause
When your body loses fluid from sweating, vomiting, diarrhea, or simply not drinking enough water, the liquid portion of your blood shrinks. Your red blood cells are the same number as before, but they’re packed into less fluid, so the concentration looks artificially high. MedlinePlus lists dehydration as the most common cause of an elevated hematocrit.
This is called “relative” erythrocytosis because your total red cell mass hasn’t actually increased. Rehydrating with fluids usually brings the numbers back to normal within hours to a day. If your hematocrit normalizes after rehydration, no further workup is typically needed.
Low Oxygen Levels and Lung Disease
Your body has a built-in feedback loop: when tissues don’t get enough oxygen, the kidneys release a hormone called erythropoietin, which tells the bone marrow to produce more red blood cells. Any condition that chronically lowers your blood oxygen can trigger this response.
Chronic obstructive pulmonary disease (COPD), including emphysema, is one of the most common culprits. Heart defects that mix oxygen-rich and oxygen-poor blood can do the same. Living at high altitude, where the air contains less oxygen, also stimulates extra red blood cell production. This is a normal adaptation, which is why people who move to places like Denver or Mexico City sometimes see their counts rise over weeks to months.
Sleep Apnea
Obstructive sleep apnea causes repeated drops in blood oxygen throughout the night. In theory, this intermittent oxygen deprivation should push the body to make more red blood cells, and the mechanism exists: the low oxygen activates a protein that ramps up erythropoietin production. In practice, though, it happens less often than you might expect. Research on over 500 sleep apnea patients found that only about 1.7% to 4.5% developed a genuinely elevated red blood cell count (the range depends on which cutoff values are used and whether patients on testosterone or with lung disease are excluded).
One reason sleep apnea rarely causes full-blown high counts is that oxygen levels bounce back to normal between apnea episodes. This rapid cycling between low and normal oxygen appears to trigger a self-correcting process where the body destroys some of the newly made red blood cells, preventing the count from climbing as high as it would with steady, sustained low oxygen.
Smoking
Cigarette smoke contains carbon monoxide, which binds to hemoglobin roughly 200 times more tightly than oxygen does. With a chunk of hemoglobin locked up carrying carbon monoxide instead of oxygen, your tissues sense an oxygen shortage, and the bone marrow responds by churning out more red blood cells to compensate. Heavy smokers can develop noticeably elevated counts that normalize over weeks to months after quitting.
Testosterone Therapy
Testosterone stimulates red blood cell production, and this effect is strong enough to be clinically significant. In a large clinical trial, 22% of men receiving testosterone therapy saw their hematocrit rise to 54% or higher. That level is considered high enough to require intervention, whether that means reducing the testosterone dose, pausing treatment, or removing blood through a procedure called phlebotomy. If you’re on testosterone replacement, expect your doctor to monitor your blood counts regularly, typically every few months during the first year and at least annually after that.
Anabolic steroids used for bodybuilding carry the same risk, often at higher doses and without medical monitoring.
Kidney Tumors and Other EPO-Producing Conditions
Certain tumors can produce erythropoietin on their own, independent of oxygen levels. Kidney cancer is the classic example. Research published by the American Society of Hematology confirmed that malignant kidney cells of a specific type can manufacture erythropoietin continuously, flooding the bloodstream with the hormone and driving red blood cell production far above normal. Patients with these tumors show measurably high erythropoietin levels on blood tests.
Other conditions that can raise erythropoietin include kidney cysts, certain liver tumors, and uterine fibroids. These are uncommon causes but worth investigating when the more common explanations have been ruled out.
Polycythemia Vera: A Bone Marrow Problem
Polycythemia vera (PV) is a different category entirely. Instead of the body responding to an outside signal like low oxygen or extra hormones, the bone marrow itself is overproducing red blood cells due to a genetic mutation. Over 90% of people with PV carry a specific mutation in the JAK2 gene, which essentially jams the “on” switch for blood cell production.
PV is a slow-growing blood cancer. The extra red blood cells thicken the blood, raising the risk of dangerous blood clots. These clots can cause heart attacks, strokes, or pulmonary embolisms. Over time, PV can also enlarge the liver and spleen, and in some cases it progresses to bone marrow scarring or leukemia. It’s a serious diagnosis, but it’s also manageable with treatment. Many people live for decades after diagnosis.
PV is relatively rare compared to the other causes on this list. When doctors find a persistently elevated red blood cell count that isn’t explained by dehydration, lung disease, smoking, altitude, or medications, they’ll typically test for the JAK2 mutation to check for PV.
What a High Count Feels Like
Mild elevations often cause no symptoms at all and are discovered incidentally on routine blood work. As the count climbs higher, symptoms tend to reflect thicker, slower-moving blood: headaches, dizziness, blurred vision, and a flushed or ruddy complexion. Some people notice itching after a warm shower (especially common in polycythemia vera), fatigue that seems paradoxical given the extra oxygen-carrying cells, or a feeling of fullness in the upper left abdomen from an enlarged spleen.
The most dangerous consequence of a significantly elevated count is blood clotting. Thickened blood is more likely to form clots in veins or arteries, which can lead to stroke, heart attack, or a clot in the lungs. This is why doctors take persistently high counts seriously even when symptoms are mild. The goal of treatment, regardless of the underlying cause, is to bring the count down to a safe range and address whatever is driving the overproduction.