A high red blood cell (RBC) count usually means your body is producing extra red blood cells to compensate for low oxygen levels, or that your blood plasma volume has dropped and concentrated the cells you already have. Normal RBC 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 those ranges can stem from something as simple as dehydration or as significant as a bone marrow disorder.
Dehydration: The Most Common Cause
The most frequent reason for a high RBC reading isn’t that your body made too many red blood cells. It’s that you lost fluid. When plasma volume drops from dehydration, vomiting, diarrhea, or heavy sweating, the same number of red blood cells are packed into less liquid. Your count looks elevated on paper even though your bone marrow didn’t produce a single extra cell. This is called relative erythrocytosis, and it corrects itself once you rehydrate.
If your result was only slightly above range, think about your hydration status before the blood draw. Fasting overnight, exercising that morning, or simply not drinking enough water can push your numbers up temporarily. Your doctor may simply repeat the test after you’ve had adequate fluids.
Low Oxygen Triggers More Red Blood Cells
Your kidneys monitor oxygen levels in your blood. When oxygen drops, they release a hormone called erythropoietin (EPO) that tells your bone marrow to ramp up red blood cell production. This is the body’s attempt to carry more oxygen per unit of blood. Several common situations create this low-oxygen signal.
Smoking
Carbon monoxide from cigarette smoke binds to hemoglobin roughly 200 times more tightly than oxygen does. The resulting molecule can’t carry oxygen at all, which effectively suffocates your tissues even though you’re breathing normally. Your kidneys detect the oxygen shortfall and order more red blood cells to compensate. This effect is dose-dependent: the more you smoke, the higher the count tends to climb.
Living at High Altitude
Thinner air at elevation means less oxygen per breath. Your body responds by gradually increasing red blood cell production. Hematocrit, the percentage of your blood volume occupied by red blood cells, typically reaches a new steady state after a few weeks at altitude and stays there as long as you remain. If you recently moved to a higher elevation or returned from a mountain trip, that alone could explain your result.
Sleep Apnea
Obstructive sleep apnea causes repeated drops in blood oxygen throughout the night. Even though you’re at sea level, the intermittent choking and airway collapse creates a similar oxygen deficit to living at altitude. Your kidneys respond the same way, boosting EPO and driving red blood cell production upward. Many people with sleep apnea don’t realize they have it, so a high RBC count sometimes becomes the clue that leads to a sleep study.
Lung and Heart Disease
Chronic lung conditions like COPD or emphysema reduce how efficiently your lungs transfer oxygen. Congenital heart defects and heart failure can also lower oxygen delivery to tissues. In all these cases, the body’s compensatory response is the same: make more red blood cells to squeeze more oxygen-carrying capacity out of each heartbeat.
Medications That Raise RBC Counts
Testosterone therapy is one of the most common drug-related causes of high RBC. A systematic review in Blood Advances found erythrocytosis rates as high as 66.7% among men on prescription testosterone, with injectable formulations, higher doses, and older age increasing the risk. If you’re on testosterone replacement or taking anabolic steroids, this is very likely your answer.
A newer and perhaps surprising culprit is a class of diabetes medications called SGLT-2 inhibitors (empagliflozin, canagliflozin, dapagliflozin). Between 10% and 22% of people taking these drugs develop elevated red blood cell counts. Certain cancer treatments that block blood vessel growth can also raise counts, with rates up to 43.5% in some studies.
Polycythemia Vera: A Bone Marrow Disorder
In a small number of cases, a high RBC count points to polycythemia vera (PV), a condition where the bone marrow overproduces red blood cells on its own, without any oxygen-related trigger. PV is caused by a specific genetic mutation called JAK2, which is present in virtually all people with the condition. The mutation makes bone marrow cells behave as though they’re constantly receiving the signal to produce more red blood cells, even when oxygen levels are perfectly normal.
PV is diagnosed when hemoglobin exceeds 16.5 g/dL in men or 16.0 g/dL in women and the JAK2 mutation is confirmed through a blood test. It’s a chronic condition, not a cancer diagnosis in the traditional sense, but it does require ongoing management because it increases the risk of dangerous blood clots.
Why Thick Blood Is a Problem
Too many red blood cells make blood thicker and more sluggish. This increased viscosity raises the risk of clots forming in blood vessels, which can lead to stroke, heart attack, or pulmonary embolism (a clot in the lungs). The risk is most serious in polycythemia vera but applies to any cause of persistently elevated counts. Thicker blood also forces the heart to work harder to push it through the circulatory system, and over time can contribute to enlarged organs, particularly the spleen and liver.
How Doctors Figure Out the Cause
If your RBC count is elevated on a routine blood test, the first step is usually repeating the test while well-hydrated to rule out a false elevation from low plasma volume. If the count remains high, doctors look at your hemoglobin and hematocrit alongside the RBC number to confirm true erythrocytosis.
From there, the key fork in the road is an EPO level. If EPO is low or undetectable, the bone marrow is overproducing on its own, and testing for the JAK2 mutation comes next. If EPO is elevated, something outside the bone marrow is driving production, and the search shifts to oxygen-related causes: lung function tests, an overnight oxygen study for sleep apnea, or a review of medications.
Your medical history does a lot of the diagnostic work here. A smoker living in Denver who takes testosterone has three obvious explanations before anyone needs to consider rarer conditions. A nonsmoker at sea level on no medications with a persistently high count warrants a closer look. The pattern of your other blood values, your symptoms (headaches, itching after a hot shower, redness in the face, and dizziness are classic PV signs), and your lifestyle context all help narrow the list quickly.