Having high red blood cells means your blood contains a higher-than-normal concentration of red blood cells relative to plasma, the liquid portion of your blood. Normal ranges are 4.7 to 6.1 million cells per microliter for men and 4.2 to 5.4 million cells per microliter for women. A result above these ranges doesn’t automatically signal a serious problem, but it does warrant a closer look, because the cause can range from something as simple as dehydration to a chronic condition that needs ongoing treatment.
Two Types of High Red Blood Cells
Not all elevated readings mean the same thing. The distinction comes down to whether your body is actually producing extra red blood cells or whether the liquid part of your blood has dropped, making the cells appear more concentrated.
In absolute erythrocytosis, your bone marrow is genuinely churning out more red blood cells than it should. This is the type that typically needs medical investigation. In relative erythrocytosis, the number of red blood cells hasn’t changed at all. You simply have less plasma, so the cells are packed more tightly in the same space. Dehydration is the most common reason for this. Once you rehydrate, the count usually returns to normal.
Common Causes
Your body produces red blood cells in response to oxygen levels. When tissues don’t get enough oxygen, the kidneys release a hormone called erythropoietin that tells the bone marrow to make more. This is a normal, protective response, and it’s behind many of the most common causes of high red blood cells.
Chronic lung diseases like COPD are a frequent trigger, because damaged lungs can’t deliver enough oxygen to the bloodstream. Obstructive sleep apnea has a similar effect: repeated pauses in breathing overnight keep oxygen levels low for hours at a time. Living at high altitude, where the air contains less oxygen, naturally pushes red blood cell production higher. Smoking does the same thing. Carbon monoxide in cigarette smoke binds to red blood cells and displaces oxygen, so the body compensates by making more cells.
Kidney problems can also play a role, either because reduced blood flow to the kidneys mimics low oxygen or because certain kidney tumors produce erythropoietin on their own. Heart defects that allow oxygen-poor blood to mix with oxygen-rich blood are another, less common cause.
When the Bone Marrow Itself Is the Problem
In a small number of cases, the bone marrow overproduces red blood cells without any oxygen-related trigger. The most well-known version of this is polycythemia vera, a blood cancer in which a genetic mutation causes the marrow to make too many blood cells. Most people with polycythemia vera carry a specific mutation in the JAK2 gene, which was identified in 2005 and is now a standard part of diagnosis. About 20% of people with polycythemia vera have a blood clot as their first symptom, before the condition is even discovered.
Symptoms to Recognize
Mild elevations often produce no symptoms at all, and many people first learn about a high count from routine bloodwork. When symptoms do appear, they tend to reflect the thicker, slower-moving blood that comes with too many red blood cells.
Headaches, dizziness, and blurred vision are among the most common complaints. You might notice facial redness, particularly in your face, hands, and feet. Fatigue is surprisingly common despite having extra oxygen-carrying cells. Some people experience itchy skin, especially after a warm bath or shower. Other possible signs include nosebleeds, easy bruising, abdominal discomfort, confusion, joint pain from gout, and high blood pressure.
Why It Matters: Blood Clot Risk
The biggest health concern with persistently high red blood cells is the increased risk of blood clots. When blood becomes thicker, it moves more slowly and is more likely to clot in places it shouldn’t. Large epidemiologic studies have found the risk of cardiovascular disease is more than twofold higher in people with high hematocrit (the percentage of blood volume made up of red blood cells) compared to those with low hematocrit. The British Regional Heart Study found a 30% increase in major heart disease events in the high-hematocrit group, even after accounting for age, smoking, cholesterol, and other risk factors.
The risk extends to blood clots in the veins as well. One large study found a 1.5-fold increased risk of a first venous clot in men with high hematocrit. And data from the Framingham Study showed elevated stroke risk in women with high hematocrit levels. These aren’t small or theoretical risks, which is why doctors take persistently elevated counts seriously.
How Doctors Figure Out the Cause
A single high reading on a routine blood test usually leads to a repeat test, partly to rule out dehydration. If the count stays elevated, your doctor will look at related markers: hemoglobin (the oxygen-carrying protein inside red blood cells), hematocrit, and platelet and white blood cell counts.
From there, the workup depends on the suspected cause. Measuring erythropoietin levels helps sort out whether the bone marrow is being told to overproduce (high erythropoietin, pointing to a secondary cause like lung disease) or is acting on its own (low erythropoietin, suggesting polycythemia vera). Oxygen saturation testing can reveal whether chronic low oxygen is the trigger. If polycythemia vera is suspected, blood or bone marrow testing for the JAK2 gene mutation is the key diagnostic step. A bone marrow biopsy, where a small sample of the spongy tissue inside bone is removed, may also be done to examine the marrow directly.
How High Red Blood Cells Are Managed
Treatment depends entirely on the underlying cause. If dehydration is behind the elevated reading, drinking more fluids resolves it. If smoking is the driver, quitting allows red blood cell levels to gradually normalize. For sleep apnea, treating the breathing problem (typically with a CPAP machine) addresses the oxygen deficit that triggered the overproduction.
When the count is high enough to pose a clotting risk, the most direct intervention is therapeutic phlebotomy, which is essentially a controlled blood draw. It works the same way as donating blood: a unit is removed at regular intervals to bring the red blood cell concentration down. For people with polycythemia vera, the target is keeping hematocrit below 45%. A clinical trial called CYTO-PV showed that maintaining hematocrit below this threshold was associated with a nearly fourfold lower risk of blood clots compared to allowing it to drift between 45% and 50%.
For polycythemia vera and other bone marrow conditions, medications that slow cell production in the marrow may be added alongside phlebotomy, particularly for people at higher clotting risk. The goal is long-term management rather than a cure, and most people with polycythemia vera live with regular blood monitoring and periodic phlebotomy for years.
Lifestyle Factors That Affect Your Count
Beyond medical conditions, a few everyday factors can nudge red blood cell counts higher. Dehydration is the most common culprit for a falsely elevated result, so staying well-hydrated before bloodwork matters. Smoking reliably increases red blood cell production, and the effect is dose-dependent: heavier smokers tend to have higher counts. Even performance-enhancing drugs like testosterone and anabolic steroids can stimulate red blood cell production, which is one reason athletes using these substances face cardiovascular risks.
If your count is mildly elevated and your doctor isn’t concerned about a bone marrow disorder, addressing these modifiable factors is often all that’s needed. A repeat blood test after making changes can confirm whether the count has come back into range.