A high red blood cell (RBC) count means your blood contains more red blood cells than expected, either because your body is producing too many or because the liquid portion of your blood has decreased. 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 point to a serious problem, but it does signal that something is shifting the balance of your blood and needs a closer look.
Why Your RBC Count Might Be High
There are two fundamentally different ways your RBC count can end up elevated, and the distinction matters because it changes what happens next.
The first is called absolute erythrocytosis, which means your bone marrow is genuinely making more red blood cells than it should. This can happen because of a problem inside the bone marrow itself (a genetic defect that causes uncontrolled production) or because something outside the bone marrow is sending signals to ramp up production. That signal usually comes from a hormone called erythropoietin, or EPO, which tells your bone marrow to make more red blood cells whenever your body senses it isn’t getting enough oxygen.
The second is called relative erythrocytosis, and it’s more of an illusion. Your red blood cell count hasn’t actually increased. Instead, you’ve lost plasma, the liquid part of your blood. With less fluid in the mix, the same number of red blood cells becomes more concentrated. Dehydration is the most common cause. If you were dehydrated when your blood was drawn, your count may look high even though your body is producing a perfectly normal number of red blood cells.
Common Causes of Genuinely High RBC
When your body is truly overproducing red blood cells, the reasons fall into a few broad categories.
Low oxygen levels. This is the most common trigger. When your tissues aren’t getting enough oxygen, your body compensates by producing more red blood cells to carry what’s available. Chronic lung disease, sleep apnea, and smoking all cause this. Smokers are particularly affected because carbon monoxide from cigarettes binds to hemoglobin and reduces the blood’s oxygen-carrying capacity, prompting the body to make more cells. Living at high altitude has the same effect. When you move to a high-elevation area, your body detects the thinner air and begins producing extra red blood cells. Hematocrit (the percentage of your blood that’s red blood cells) typically reaches a new, higher steady state after a few weeks and stays there as long as you remain at altitude.
Medications and supplements. Testosterone therapy is a well-known cause of rising RBC counts. A class of diabetes medications called SGLT-2 inhibitors can also raise red blood cell levels, as can certain cancer treatment drugs. Anyone using supplemental EPO, whether prescribed or not, will see their count climb.
Tumors. Some tumors secrete EPO on their own, independent of oxygen levels. Kidney tumors, kidney cysts, liver tumors, certain brain tumors, and uterine fibroids can all trigger this kind of inappropriate red blood cell production.
Polycythemia Vera: A Bone Marrow Problem
Polycythemia vera (PV) is a blood cancer in which the bone marrow produces too many red blood cells because of a genetic mutation. In roughly 95% of people with PV, the problem traces to a specific mutation that locks a growth signal in the “on” position, bypassing the body’s normal controls. The bone marrow keeps making red blood cells even when there’s no signal from EPO telling it to do so.
PV is uncommon, but it’s the diagnosis doctors want to rule out when your count is persistently elevated without an obvious explanation like smoking, altitude, or dehydration. Left unmanaged, it can progress to more serious bone marrow diseases over time.
Symptoms to Watch For
Many people with a high RBC count feel nothing at all, especially in the early stages. The condition is often caught on routine bloodwork before any symptoms appear.
When symptoms do develop, they tend to reflect the blood becoming thicker and flowing less efficiently. Headaches, dizziness, blurred vision, and unusual fatigue are common. Some people notice numbness, tingling, or a burning sensation in their hands and feet. One distinctive symptom is intense itching after a warm bath or shower, which is particularly associated with polycythemia vera.
Other signs include feeling full quickly after eating, pain or bloating in the upper left abdomen from an enlarged spleen, unexpected nosebleeds or bleeding gums, and painful swelling in a single joint (often the big toe, similar to gout). None of these symptoms are specific to high RBC alone, which is why blood testing is essential for diagnosis.
Why High RBC Counts Are Taken Seriously
The main danger of persistently elevated red blood cells is blood clots. Thicker blood moves more slowly and is more prone to clotting. These clots can cause strokes, heart attacks, deep vein thrombosis, and pulmonary embolism. In polycythemia vera specifically, thrombosis is the most common complication. Arterial events (strokes and heart attacks) account for the majority, with strokes and transient ischemic attacks being the most frequent. Long-term data from clinical trials found that roughly 38% of PV patients experienced a thrombotic event over a 19-year follow-up period, and about 15% died from one.
That’s why even a modestly elevated count gets attention. The risk isn’t theoretical. It’s the primary way this condition causes harm.
What Happens After an Abnormal Result
A single high RBC reading usually prompts a repeat test, often with instructions to stay well-hydrated beforehand. This helps rule out dehydration as the cause. If the count stays elevated, your doctor will look at related numbers on the same blood panel, particularly hemoglobin and hematocrit. A hemoglobin above 16.5 g/dL in men or 16.0 g/dL in women is generally considered the threshold for polycythemia.
The next step is typically measuring your EPO level. This single test does a lot of the sorting. If EPO is high, the problem is almost certainly something outside the bone marrow: lung disease, sleep apnea, smoking, altitude, or a tumor producing EPO. Your doctor will investigate those possibilities with imaging or sleep studies. If EPO is low, that points toward the bone marrow itself overproducing cells on its own, and genetic testing for the mutation associated with polycythemia vera usually follows.
How High RBC Is Managed
Treatment depends entirely on the cause. If dehydration is responsible, rehydrating solves the problem. If smoking is the driver, quitting will gradually bring the count down as carbon monoxide levels in the blood normalize. If a medication like testosterone is behind the elevation, your doctor may adjust the dose or monitor you more frequently.
For conditions where the body is overproducing red blood cells and the count needs to come down, the most direct approach is therapeutic phlebotomy: having blood drawn at regular intervals, similar to donating blood. A typical session removes about 350 mL. The goal is to bring the hematocrit down to a target level, usually around 45% for polycythemia vera. Sessions may happen every few days initially, then taper to a maintenance schedule once the target is reached.
For polycythemia vera, phlebotomy is often combined with low-dose aspirin to reduce clotting risk. Some patients also take medication that slows the bone marrow’s production of blood cells. Clinical trial data shows this combination approach cuts the rate of serious cardiovascular events nearly in half compared to phlebotomy alone: about 8% versus 13% in one large trial. For secondary causes like sleep apnea or chronic lung disease, treating the underlying condition is the priority, since resolving the oxygen problem removes the signal telling the body to overproduce.