A high red blood cell (RBC) count means your blood contains more red blood cells than the typical range, which is 4.7 to 6.1 million cells per microliter for men and 4.2 to 5.4 million cells per microliter for women. In many cases, the cause is something straightforward like dehydration or living at high altitude. But a persistently elevated count can signal an underlying condition that needs attention, because extra red blood cells thicken your blood and raise the risk of clots.
Why Red Blood Cell Counts Rise
Your body makes red blood cells in the bone marrow, and the pace of production is controlled by a hormone called erythropoietin (EPO), which your kidneys release when they sense low oxygen levels. Anything that disrupts this system, whether it’s a lung condition, a bone marrow disorder, or simply not drinking enough water, can show up as a high RBC count on your lab results.
Doctors generally split the causes into three categories: a true overproduction of red blood cells by the bone marrow (primary), an appropriate but excessive response to something else going on in your body (secondary), or an apparent increase that’s really just concentrated blood (relative).
Relative Highs: When It’s Really About Fluid
Sometimes your RBC count looks high not because you have extra red blood cells, but because you don’t have enough plasma, the liquid portion of blood. Dehydration is the most common culprit. Vomiting, diarrhea, and diuretics (water pills often prescribed for high blood pressure) can all pull fluid out of your bloodstream, packing the same number of red blood cells into a smaller volume. Once you’re properly hydrated again, the count typically drops back to normal. This is called relative erythrocytosis, and it’s the least worrisome explanation for an elevated result.
Secondary Causes: Your Body Responding to Low Oxygen
When your tissues aren’t getting enough oxygen, your kidneys ramp up EPO production to stimulate more red blood cell growth. This is your body doing exactly what it’s designed to do, just more aggressively than usual. Common triggers include:
- Chronic lung disease like COPD or emphysema, which limits how much oxygen reaches your blood
- Living at high altitude, where the air contains less oxygen
- Smoking, which reduces the oxygen-carrying capacity of your blood
- Sleep apnea, which causes repeated drops in oxygen levels overnight
Certain medications also push RBC counts higher. Testosterone replacement therapy and anabolic steroids directly stimulate red blood cell production. So does synthetic erythropoietin, which is sometimes misused in sports as a performance enhancer.
In rare cases, tumors in the kidneys or liver secrete erythropoietin on their own, flooding the body with the hormone and driving up red blood cell production independent of oxygen levels. Kidney cancers and some conditions that reduce blood flow to the kidneys, like narrowed renal arteries, can have the same effect.
Primary Causes: A Bone Marrow Problem
When the bone marrow itself is the problem, the most well-known diagnosis is polycythemia vera (PV). This is a slow-growing blood cancer in which the bone marrow produces too many red blood cells regardless of what the body actually needs. Nearly all cases involve a specific gene mutation called JAK2, which essentially locks the bone marrow’s production switch in the “on” position.
PV is uncommon, but it’s the diagnosis doctors want to rule out when your RBC count stays elevated without an obvious explanation. A key clue is erythropoietin levels. In PV, EPO tends to run low (the bone marrow is overproducing on its own, so the kidneys dial back the signal). In secondary causes, EPO is usually normal or high. One study found average EPO levels of 3.4 mIU/mL in PV patients compared to 23.7 mIU/mL in people with secondary causes, with the normal range falling between 2.6 and 18.5.
Symptoms to Watch For
A mildly elevated RBC count often produces no symptoms at all, which is why it’s usually caught on routine bloodwork. As counts climb higher, thicker blood circulates less efficiently, and you might notice headaches, dizziness, fatigue, or blurred vision.
Polycythemia vera has a few distinctive symptoms. Itching after a warm bath or shower is a classic sign, caused by certain immune cells in the skin reacting to heat. You might also experience numbness or tingling in your hands and feet, feeling full after eating only a small amount (from an enlarged spleen pressing on your stomach), unusual bleeding from your gums or nose, or painful swelling in a joint, particularly the big toe.
Why High RBC Counts Matter for Your Health
The biggest concern with a persistently elevated RBC count is blood clots. Red blood cells are the primary driver of blood viscosity, and when there are too many of them, blood moves more slowly and clots form more easily. The effects are measurable: large epidemiologic studies have found that people with high hematocrit (the percentage of blood volume occupied by red blood cells) face more than double the risk of cardiovascular disease compared to those with low hematocrit.
The British Regional Heart Study found a 30% increase in serious heart disease among people with high hematocrit even after accounting for age, exercise habits, cholesterol, weight, and smoking. The risk of a first blood clot in the veins is about 1.5 times higher in men with elevated levels. Thicker blood also pushes red blood cells toward vessel walls, which concentrates platelets near the lining and makes them three to five times more likely to stick and start forming a clot.
These risks aren’t theoretical. The CYTO-PV trial, a landmark study in polycythemia vera, showed that keeping hematocrit below 45% through treatment reduced the risk of blood clots by nearly fourfold compared to allowing it to drift between 45% and 50%.
How Doctors Figure Out the Cause
If your RBC count comes back high on a routine blood test, the first step is usually straightforward: your doctor will check whether dehydration or a known medication could explain it. A repeat test after proper hydration can rule out relative erythrocytosis quickly.
If the count stays elevated, the next round of testing typically includes an EPO level. A low EPO points toward a bone marrow problem and usually triggers testing for the JAK2 gene mutation. A subnormal EPO level has about 96% specificity for polycythemia vera, meaning it very rarely shows up in people who don’t have PV. If EPO is normal or high, the focus shifts to secondary causes: lung function, oxygen saturation, sleep studies, kidney imaging, or a review of medications.
How a High RBC Count Is Treated
Treatment depends entirely on the cause. If dehydration is behind it, fluids solve the problem. If smoking or a medication is the trigger, addressing that root cause is the priority.
For polycythemia vera and other conditions where the body is making too many red blood cells, the most common treatment is therapeutic phlebotomy, which is essentially a controlled blood draw. About 500 mL of blood is removed at a time (similar to donating blood), initially every other day until hematocrit drops below target. For men, the goal is below 45%; for women, below 42%. Older patients or those with heart or cerebrovascular concerns have smaller volumes drawn, typically 200 to 300 mL twice a week. Once hematocrit reaches the target, you’ll have periodic draws, usually monthly, to keep it there.
For secondary erythrocytosis caused by chronic low oxygen, treating the underlying condition is the focus. That could mean better management of lung disease, using a CPAP machine for sleep apnea, or stopping testosterone therapy if that’s the driver. The goal is always to address whatever is telling your body to overproduce red blood cells in the first place.