A high red blood cell (RBC) count usually means one of two things: your body is making more red blood cells than normal, or your blood has lost fluid and the existing cells are more concentrated. Normal ranges fall between 4.7 and 6.1 million cells per microliter for men and 4.2 to 5.4 million for women. Understanding which category your result falls into matters, because the causes range from something as simple as dehydration to conditions that need ongoing treatment.
Dehydration: The Most Common Explanation
The simplest reason for a high RBC count on a lab report is that your blood has less liquid in it than usual. Red blood cells float in plasma, the liquid portion of blood. When plasma volume drops, those same red blood cells become packed more tightly into a smaller volume of fluid. On a blood test, this looks like a high count, but your body hasn’t actually produced any extra cells.
This happens with dehydration from illness, exercise, heat, or not drinking enough water. It also occurs as a side effect of diuretics, which are commonly prescribed for high blood pressure. These medications push fluid out through the kidneys, reducing plasma volume. Once you rehydrate or adjust the medication, the count typically returns to normal. Your doctor may call this “relative” erythrocytosis, meaning the elevation is relative to the reduced fluid rather than a true increase in red blood cells.
Low Oxygen Levels Signal Your Body to Make More
When your tissues aren’t getting enough oxygen, your kidneys detect the shortage and release a hormone called erythropoietin (EPO). EPO travels to your bone marrow and tells it to ramp up red blood cell production. More red blood cells means more oxygen-carrying capacity, which is your body’s way of compensating. This process is behind several common causes of a genuinely elevated RBC count.
Living at High Altitude
At higher elevations, the air contains less oxygen per breath. Your kidneys respond by boosting EPO, and your bone marrow produces more red blood cells to compensate. This is a normal, healthy adaptation. People living in cities like Denver, Bogotá, or Addis Ababa naturally carry higher RBC counts than people at sea level, and it’s not a problem.
Smoking
Cigarette smoke contains carbon monoxide, which binds to the same spot on red blood cells that oxygen uses. This effectively disables a portion of your red blood cells from carrying oxygen, creating a state of chronic low oxygen even though your lungs may be working fine. Your kidneys sense the deficit and increase EPO production, pushing your RBC count upward. Quitting smoking allows the count to gradually normalize.
Lung Disease
Chronic obstructive pulmonary disease (COPD) and other lung conditions reduce how efficiently your lungs transfer oxygen into the blood. Research in patients with COPD found that 85% had elevated EPO levels, confirming the kidneys were actively responding to low oxygen by driving red blood cell production. The more severe the lung disease, the more pronounced this response can be.
Sleep Apnea
Obstructive sleep apnea causes repeated drops in blood oxygen throughout the night as the airway temporarily collapses during sleep. Even though oxygen levels recover between episodes, the cumulative effect of intermittent oxygen deprivation can trigger EPO release and lead to a higher RBC count. Treating sleep apnea with a CPAP machine or other therapies addresses the underlying oxygen problem and can bring the count down.
Testosterone and Other Medications
Testosterone is one of the most common medication-related causes of a high RBC count. It stimulates red blood cell production directly, and the effect is dose-dependent: higher doses lead to higher counts. In men taking prescription testosterone for low hormone levels, erythrocytosis rates reach as high as 66.7%. Transgender men on testosterone therapy see similar rates, with the elevation typically developing within the first one to three years of treatment.
Injectable forms carry a higher risk than gels or patches. Up to 2.7% of men on testosterone therapy develop blood clots as a complication, which is why regular blood monitoring is standard during treatment. Non-prescription anabolic steroids produce the same effect, though the elevation tends to resolve about three months after stopping a cycle.
A class of diabetes medications called SGLT-2 inhibitors can also raise RBC counts, with erythrocytosis rates ranging from 2.1% to 22% of users. The effect reverses after stopping the medication.
Polycythemia Vera: When Bone Marrow Overproduces
Polycythemia vera (PV) is a blood cancer in which the bone marrow produces too many red blood cells on its own, without being told to by EPO. In fact, EPO levels in PV are typically low, because the body recognizes it already has too many red blood cells and tries to shut off production. The bone marrow ignores that signal.
The root cause in nearly all cases is a mutation in a gene called JAK2, which acts like a stuck “on” switch for cell production. Bone marrow biopsies in PV show an overgrowth of not just red blood cells but often white blood cells and platelets too. PV is relatively rare and tends to develop gradually, often discovered incidentally on routine blood work. It requires ongoing management to keep blood counts in a safe range and reduce the risk of clotting.
Rare Causes Worth Knowing
Certain tumors can produce EPO independently, flooding the body with signals to make more red blood cells. Kidney cancer (renal cell carcinoma) is the most well-known example. In documented cases, EPO levels dropped to normal after the tumor was removed surgically, then climbed again if the cancer spread. Other EPO-producing tumors include some liver and adrenal cancers, though these are uncommon.
Heart defects that mix oxygen-rich and oxygen-poor blood can also cause chronically low oxygen levels and drive up RBC production. These are usually identified in childhood but occasionally go undetected into adulthood.
How Doctors Figure Out the Cause
When your RBC count comes back high, the first step is usually repeating the test after you’ve been well-hydrated. If the count remains elevated, a key branching point is measuring your EPO level. This single test helps separate the two main categories: if EPO is high, something is driving your body to make more red blood cells (low oxygen, a tumor, medications). If EPO is low, the bone marrow is overproducing on its own, which points toward polycythemia vera, and testing for the JAK2 mutation typically follows.
Your doctor will also look at your overall blood picture. A high RBC count alongside elevated white blood cells and platelets suggests a bone marrow problem. A high count with normal white cells and platelets, especially in a smoker or someone living at altitude, points toward a secondary cause.
Why a High RBC Count Matters
Red blood cells make blood thicker. The more you have, the harder your heart has to work to push blood through your vessels. When the hematocrit (the percentage of blood volume occupied by red blood cells) rises above 60%, blood viscosity increases exponentially, raising the risk of blood clots, stroke, and heart attack. Even at moderately elevated levels, thicker blood flows more slowly through small vessels, which can cause headaches, dizziness, blurred vision, and a flushed or reddish complexion.
The treatment depends entirely on the cause. Dehydration resolves with fluids. Smoking-related elevations improve with cessation. Sleep apnea responds to airway therapy. Medication-induced cases often improve with dose adjustments. Polycythemia vera is managed with periodic blood removal (phlebotomy) to keep the hematocrit in a safe range, sometimes combined with medications to slow marrow production. In all cases, identifying and addressing the underlying trigger is what brings the count back toward normal.