A high red blood cell (RBC) count means your blood contains more oxygen-carrying 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 for women. Sometimes this is a temporary, harmless finding caused by dehydration or living at high altitude. Other times it signals an underlying condition that needs attention, because too many red blood cells thicken your blood and raise the risk of clots, stroke, and other complications.
Two Types of High RBC Count
Not every high reading means your body is producing extra red blood cells. The distinction matters because the cause changes what happens next.
Absolute erythrocytosis means your bone marrow is genuinely making more red blood cells than normal. This can be driven by a bone marrow disorder, chronic low oxygen levels, hormone changes, or certain medications.
Relative erythrocytosis means your red blood cell count looks elevated because you don’t have enough plasma (the liquid portion of blood) relative to your cells. Dehydration is the classic example. You haven’t gained extra cells; you’ve lost fluid. Once you rehydrate, the count typically returns to normal. This is one reason a single blood test showing a slightly high RBC count isn’t always cause for concern.
Common Causes
Low Oxygen Levels
Your body has a built-in feedback loop: when tissues don’t get enough oxygen, your kidneys release a hormone called erythropoietin (EPO), which tells your bone marrow to ramp up red blood cell production. Anything that chronically lowers your blood oxygen can trigger this response.
Living at high altitude is the most straightforward example. Thinner air means less oxygen per breath, and the body compensates by making more red blood cells to carry what’s available. The same mechanism kicks in with lung diseases like COPD and emphysema, or with obstructive sleep apnea, where breathing repeatedly stops during sleep. In sleep apnea specifically, EPO levels correlate with the amount of time a person’s oxygen saturation drops below 89%, meaning more severe cases drive stronger red blood cell production.
Heavy smoking also fits here. Carbon monoxide from cigarette smoke binds to the same spot on red blood cells that oxygen uses, effectively reducing your blood’s oxygen-carrying capacity. The body responds by making more cells to compensate.
Testosterone and Anabolic Steroids
Testosterone replacement therapy is one of the most common medication-related causes of a high RBC count. In men taking prescription testosterone, rates of elevated red blood cells run as high as 66.7%, with injectable formulations, higher doses, and older age all increasing the risk. Transgender men on testosterone also experience this, with rates varying by formulation but reaching up to 46.7% with testosterone pellets. The hormone appears to suppress signals that normally keep red blood cell production in check. If you’re on testosterone and your blood work comes back high, this is very likely the reason.
Polycythemia Vera
This is a rare but important cause. Polycythemia vera (PV) is a slow-growing blood cancer in which the bone marrow overproduces red blood cells on its own, without the normal EPO signal. About 95% of cases involve a specific gene mutation called JAK2 V617F that essentially jams the “on” switch for cell production. PV is less common than the other causes on this list, but it’s the one doctors most want to rule out when your count stays elevated without an obvious explanation.
Other Causes
Kidney tumors and certain other cancers can produce EPO independently, flooding the body with signals to make more red blood cells. Heart defects that mix oxygen-poor and oxygen-rich blood can trigger the same low-oxygen feedback loop described above. Performance-enhancing drugs, including EPO itself, are another well-known cause in athletes.
How Doctors Tell the Difference
When blood work shows a high RBC count, clinicians typically look at hemoglobin and hematocrit alongside it. Hemoglobin above 16.5 g/dL in men or 16.0 g/dL in women is the threshold that formally defines erythrocytosis. If your values cross that line, the next step is usually measuring your EPO level.
This single test does a lot of the sorting. A low EPO level suggests the bone marrow is overproducing cells on its own, pointing toward polycythemia vera. A high EPO level means something outside the bone marrow is driving production, whether that’s low oxygen, a tumor, or a medication. From there, genetic testing for the JAK2 mutation, oxygen saturation checks, or imaging may follow depending on which direction the results point.
Symptoms You Might Notice
A mildly elevated count often produces no symptoms at all, which is why it’s usually caught on routine blood work. As the count climbs higher, the blood becomes thicker and flows less easily. You might experience headaches, dizziness, blurred vision, fatigue, or a flushed or ruddy complexion, especially in the face.
One distinctive symptom linked to polycythemia vera is intense itching after contact with water, particularly warm or hot water. This affects a significant number of PV patients, and the itching typically hits the arms, legs, or torso within minutes of bathing. Most describe it as a deep itch or burning sensation, and it occurs without any visible rash. Worsening after hot water is reported far more often (57% of cases) than after cold. If you’ve noticed unexplained itching after showers alongside a high RBC count, it’s worth mentioning to your doctor.
Burning or redness in the hands and feet is another symptom associated with thickened blood. Some people also notice unusual fullness or pressure in the upper left abdomen, caused by an enlarged spleen working overtime to filter the excess cells.
Why a High Count Matters
The main risk of persistently elevated red blood cells is that thicker blood moves more slowly and clots more easily. This raises the chance of deep vein thrombosis, pulmonary embolism, heart attack, and stroke. One study in men hospitalized for acute stroke found that those with a hematocrit of 48% or higher had nearly three times the odds of dying in the hospital compared to those with mid-range levels. Hematocrit values above 45% have been linked to increased stroke risk more broadly.
These risks are most relevant when the count stays elevated over time. A one-time borderline reading on a day you were dehydrated is a very different situation from chronically high levels driven by an untreated condition.
What Happens Next
Treatment depends entirely on the cause. If dehydration is behind the reading, rehydrating solves it. If sleep apnea is the culprit, treating the apnea (usually with a CPAP machine) addresses the oxygen deficit and the RBC count tends to normalize. For testosterone-related elevations, your prescriber may adjust the dose or formulation, or monitor your blood work more frequently.
For polycythemia vera, the standard approach is periodic blood removal (similar to donating blood) to bring the count down and reduce clotting risk. Many patients also take a low-dose blood thinner. The goal is keeping hematocrit below 45%, which substantially lowers the risk of cardiovascular events. PV is a chronic condition, but with ongoing management most people live with it for decades.
If your blood work flagged a high RBC count for the first time, the most useful thing you can do is note whether any obvious triggers apply: recent dehydration, high altitude travel, testosterone use, or a known lung condition. Sharing that context helps your doctor decide whether a repeat test in a few weeks is enough or whether further workup is warranted.