A high red blood cell (RBC) count typically indicates your body is producing more oxygen-carrying cells than it needs, either because of an underlying health condition, a response to low oxygen levels, or sometimes just dehydration. Normal ranges fall between 4.7 and 6.1 million cells per microliter for men and 4.2 to 5.4 million cells per microliter for women. A result above these thresholds doesn’t automatically point to a serious problem, but it does warrant a closer look at what’s driving the increase.
Why Your Body Makes Too Many Red Blood Cells
Red blood cells carry oxygen from your lungs to every tissue in your body. When something disrupts the normal feedback loop that controls their production, numbers climb. The causes fall into two broad categories: problems originating in the bone marrow itself, and problems elsewhere in the body that signal the bone marrow to ramp up production.
In bone marrow disorders, a genetic defect causes the cells that mature into red blood cells to multiply on their own, without the usual signals telling them to do so. The most well-known version of this is polycythemia vera, a blood cancer driven by a specific gene mutation (called JAK2). It’s relatively rare, but it’s the first thing doctors want to rule out when your count is persistently high.
Far more commonly, the cause sits outside the bone marrow. Your body produces a hormone called erythropoietin, or EPO, that acts as a messenger telling bone marrow to make more red blood cells. Anything that triggers extra EPO production will push your count up. Low oxygen is the most frequent trigger: chronic lung disease, sleep apnea, smoking, and living at high altitude all reduce how much oxygen reaches your blood, so your body compensates by making more cells to carry what’s available.
Causes That Aren’t Really a “High Count”
Sometimes a blood test shows elevated red blood cells when the actual number of cells hasn’t increased at all. This happens with dehydration. Red blood cells are measured as a concentration in your blood plasma. When you lose fluid through sweating, vomiting, diarrhea, or simply not drinking enough water, the liquid portion of your blood shrinks while the cells stay the same. The result looks like a high count on paper but resolves once you rehydrate. If your doctor suspects this, they’ll often ask you to drink fluids and retest before investigating further.
Polycythemia Vera: The Bone Marrow Cause
Polycythemia vera is a slow-growing blood cancer where the bone marrow produces red blood cells, and often white blood cells and platelets, without restraint. It affects roughly 1 to 2 people per 100,000 each year and is most common in adults over 60. The hallmark is a hemoglobin level above 16.5 g/dL in men or 16 g/dL in women, combined with the JAK2 gene mutation found through a blood test.
What makes polycythemia vera distinctive is its symptom profile. Many people notice intense itching after a hot shower or bath, a sensation caused by overactive immune cells releasing histamine in the skin. Other common symptoms include headaches, dizziness, blurred vision, a feeling of fullness in the upper left abdomen (from an enlarged spleen), and a flushed or reddish complexion. Some people have no symptoms at all and only discover it through routine bloodwork.
The biggest concern with polycythemia vera is blood clots. Over time, it can also progress to scarring of the bone marrow or, rarely, to acute leukemia. Managing it centers on keeping the blood from becoming too thick, primarily through regular blood draws (therapeutic phlebotomy) that bring the hematocrit, the percentage of blood volume occupied by red cells, below 45%. Clinical trials have shown that maintaining hematocrit under 45% significantly lowers the rate of clot-related events like stroke and deep vein thrombosis.
Secondary Causes Worth Knowing
When the high count stems from something outside the bone marrow, doctors call it secondary erythrocytosis. The list of triggers is long, but a few deserve attention because they’re common and treatable.
- Chronic lung conditions and smoking. COPD, emphysema, pulmonary fibrosis, and heavy smoking all reduce blood oxygen levels. Your body responds by cranking up EPO and producing more red blood cells. Quitting smoking or improving lung function with treatment often brings the count down over time.
- Sleep apnea. Repeated drops in oxygen during the night trigger the same compensatory response. People with untreated sleep apnea frequently show elevated red blood cell counts that normalize after consistent use of a CPAP machine.
- Kidney and liver tumors. Certain tumors, particularly renal cell carcinoma, hepatocellular carcinoma, and cerebellar hemangioblastoma, can secrete EPO or EPO-like proteins on their own. This inappropriate hormone production drives red blood cell overproduction even when oxygen levels are perfectly normal.
- Testosterone use. Testosterone therapy, whether prescribed for low hormone levels or used for other reasons, stimulates EPO production. This is one of the most common causes doctors see in younger men with unexpectedly high counts.
How Thick Blood Causes Problems
The reason a high red blood cell count matters clinically is blood viscosity. More cells mean thicker blood, and thicker blood flows more slowly through small vessels. Data from a large Norwegian health survey found that every 5% rise in hematocrit increased the risk of venous blood clots by about 25%. People whose hematocrit fell in the top 20% of the population had a 1.5-fold higher risk of venous thrombosis compared to everyone else.
Thickened blood also affects the brain. Poor circulation through small cerebral vessels can cause persistent headaches, dizziness, confusion, and blurred vision. Some people describe a “foggy” feeling that clears after treatment brings their count down. In severe cases, particularly when hematocrit climbs above 56%, the risk extends to stroke, heart attack, and pulmonary embolism.
How Doctors Figure Out the Cause
A single high RBC reading on a routine blood panel is just a starting point. Your doctor will first want to confirm the result isn’t from dehydration by checking your hydration status and possibly retesting. If the count remains high, the next step is measuring your EPO level, and this one test narrows the possibilities considerably.
A low or suppressed EPO level suggests the bone marrow is overproducing cells on its own, pointing toward polycythemia vera or another bone marrow disorder. Your doctor will then order a test for the JAK2 gene mutation, which is positive in the vast majority of polycythemia vera cases. A high EPO level, on the other hand, means something outside the bone marrow is driving production. That prompts a search for the underlying cause: oxygen levels, lung function tests, a sleep study, kidney imaging, or a review of medications like testosterone.
What Treatment Looks Like
Treatment depends entirely on the cause. For secondary erythrocytosis, addressing the root problem is the priority. Treating sleep apnea, managing lung disease, adjusting testosterone doses, or removing a tumor that secretes EPO will typically bring the red blood cell count back toward normal on its own.
For polycythemia vera and other bone marrow disorders, therapeutic phlebotomy is the cornerstone of treatment. It works exactly like a blood donation: about 450 mL of blood is drawn per session. Sessions may happen frequently at first, sometimes daily, until hematocrit drops below the target of 45%. After that, most people need maintenance sessions every one to two months. The procedure takes about 15 to 30 minutes, and some people feel lightheaded or tired afterward, but it’s generally well tolerated.
For patients with lung disease whose hematocrit climbs above 56% and who develop symptoms of thick blood, phlebotomy targets are less aggressive, aiming to reduce hematocrit to around 50 to 52% rather than the lower targets used in polycythemia vera. The goal is to relieve symptoms without removing so many red blood cells that oxygen delivery suffers.
In polycythemia vera cases where phlebotomy alone isn’t enough to control the disease, doctors may add medications that slow bone marrow activity. Low-dose aspirin is also commonly used to reduce clot risk. With consistent management, most people with polycythemia vera live normal or near-normal lifespans, though they need ongoing monitoring for complications.