A high red blood cell count, known medically as erythrocytosis, means your body is producing or retaining more red blood cells than expected. The causes range from something as simple as dehydration to chronic lung disease to a rare bone marrow condition called polycythemia vera. Because the reasons behind it vary so widely in seriousness, a single high reading on a blood test does not tell you much on its own. What matters is figuring out which category your situation falls into, and some of the clues come from symptoms you may already be noticing.
The Most Common and Least Alarming Cause
Before assuming anything is wrong with your bone marrow or lungs, it is worth knowing that the most frequent reason for a high red blood cell count on routine lab work is not that your body made too many red blood cells at all. When you are dehydrated, the liquid portion of your blood (plasma) shrinks, which concentrates the red blood cells you already have. Lab values like hemoglobin and hematocrit rise not because you have extra cells but because there is less fluid around them. This is sometimes called spurious or relative erythrocytosis, and it can show up after severe vomiting, prolonged diarrhea, or simply not drinking enough water in the days before a blood draw.1NCBI Bookshelf. Polycythemia
The practical takeaway is straightforward: if you had a stomach bug, were fasting, exercised heavily without rehydrating, or were otherwise fluid-depleted when your blood was drawn, your doctor will likely want to repeat the test after you have had a chance to rehydrate. If the numbers come back normal the second time, the mystery is solved. If they stay elevated, the investigation moves to the two main categories of true erythrocytosis.
When Your Body Makes More Red Blood Cells on Purpose
Your kidneys have oxygen sensors. When they detect that blood oxygen is consistently low, they release a hormone called erythropoietin (EPO), which tells your bone marrow to ramp up red blood cell production. This is a perfectly logical response. Your body is trying to compensate for the fact that each pass through the lungs is not loading up red blood cells with as much oxygen as it should. The result is called secondary erythrocytosis, and it is driven by that elevated EPO level.2NCBI Bookshelf. Secondary Polycythemia
Several conditions can trigger this chain of events:
- Chronic lung disease: COPD, emphysema, pulmonary fibrosis, and other conditions that reduce how efficiently your lungs transfer oxygen into your blood.
- Obstructive sleep apnea: Repeated pauses in breathing during sleep lower blood oxygen levels for hours every night, enough to push the kidneys into producing more EPO over time.
- Obesity hypoventilation syndrome: Excess weight around the chest and abdomen can mechanically restrict breathing, leading to chronic low oxygen that mirrors the effect of lung disease.
- Living at high altitude: At elevation, there is less oxygen in each breath. People who live above roughly 2,500 meters often develop mildly elevated red blood cell counts as a normal adaptation.
- Heavy smoking: Carbon monoxide from cigarette smoke binds to hemoglobin far more tightly than oxygen does, effectively reducing how much oxygen your red blood cells can carry. The body responds by making more of them.
In all of these situations, the elevated red blood cell count is a symptom of something else rather than a disease in itself. Treatment focuses on the underlying problem: managing COPD, treating sleep apnea with a CPAP machine, losing weight, or quitting smoking. Once oxygen levels improve, the signal to overproduce red blood cells fades.2NCBI Bookshelf. Secondary Polycythemia
When Your Bone Marrow Acts on Its Own
Primary erythrocytosis is a fundamentally different problem. Instead of the body responding logically to low oxygen, the bone marrow starts churning out red blood cells without being asked. The most well-known version of this is polycythemia vera (PV), a chronic blood cancer classified as a myeloproliferative neoplasm. In PV, a genetic change in bone marrow stem cells causes them to multiply in an uncontrolled way. The responsible mutation, an acquired variant in the JAK2 gene, is present in more than 95 percent of people with PV.3JAMA. Diagnosis and Treatment of Polycythemia Vera: A Review
It is worth emphasizing that “acquired” means you were not born with the mutation. It develops during your lifetime in the blood-forming cells of your bone marrow. PV is uncommon overall, but it is the condition doctors are most concerned about ruling in or out when red blood cell counts are persistently elevated without an obvious secondary cause like lung disease or smoking.4PubMed Central. Investigation and management of erythrocytosis
One key lab difference between primary and secondary erythrocytosis is the EPO level. In secondary cases, EPO is high because the kidneys are actively calling for more red blood cells. In PV, EPO is typically low or undetectable because the bone marrow is acting independently, and the body has no reason to signal for even more production. Your doctor can measure serum EPO with a simple blood test, and the result often points the investigation in the right direction.
Symptoms That Bring People to the Doctor
Many people discover they have a high red blood cell count incidentally, through routine blood work ordered for an unrelated reason. Mild elevations often produce no symptoms at all. But as the count climbs and blood becomes thicker and more viscous, a distinct set of problems can appear.
The symptoms most commonly reported include headaches, dizziness, blurred vision, and a flushed or ruddy complexion, especially on the face and hands. Some people notice itching after a warm shower or bath. You might also feel unusually fatigued, which seems counterintuitive since you have more oxygen-carrying cells, but thicker blood moves more sluggishly through small vessels, so tissues can actually receive less oxygen despite the surplus of red blood cells.
In polycythemia vera specifically, one of the more distinctive and bothersome symptoms is aquagenic pruritus, an intense itching triggered by contact with water. A study of PV patients found that about 41 percent experienced this symptom, and in more than half of those people, the itching started before they were ever diagnosed with PV. The itching was severe enough that roughly a third of affected patients avoided contact with water entirely.5PubMed Central / Medical Journals. Aquagenic Pruritus in Polycythemia Vera: Clinical Characteristics If you have been dealing with unexplained itching after showers for months or years, especially alongside any of the other symptoms listed above, it is worth mentioning to your doctor.
Why Thick Blood Is Dangerous
The biggest health risk of a persistently elevated red blood cell count, regardless of the cause, is blood clots. When hematocrit (the percentage of your blood volume made up of red blood cells) rises above the normal range, blood becomes thicker. Thicker blood flows more slowly, and slower-moving blood is more prone to clotting. This increased viscosity raises the risk of both arterial events like heart attacks and strokes, and venous events like deep vein thrombosis and pulmonary embolism.6Haematologica. Hematocrit and risk of venous thromboembolism in a general population. The Tromsø study
This is why doctors take erythrocytosis seriously even when the underlying cause turns out to be something relatively benign like sleep apnea. The apnea itself is treatable, but until it is treated, the extra red blood cells are thickening the blood and putting you at elevated cardiovascular risk. For people with PV, keeping hematocrit below a target threshold (usually around 45 percent) through periodic blood removal, called phlebotomy, is one of the main strategies for reducing clot risk.
How Doctors Figure Out the Cause
If your red blood cell count comes back high on a routine complete blood count (CBC), your doctor will first want to confirm that it is a real elevation and not a one-off from dehydration. That usually means repeating the blood test after adequate hydration. If the count is still elevated, the investigation branches based on a few straightforward questions and tests.
The first step is usually checking your EPO level. A low or suppressed EPO points toward primary erythrocytosis and triggers testing for the JAK2 mutation. A high EPO points toward secondary erythrocytosis and shifts the focus to finding what is driving the overproduction: oxygen saturation measurements, pulmonary function tests, a sleep study, or imaging of the kidneys, since certain kidney tumors can also secrete EPO.4PubMed Central. Investigation and management of erythrocytosis
Your medical history does a lot of the work here. A long-time smoker with mild COPD has an obvious candidate cause. Someone who recently moved to Denver from sea level has another. A person with no lung disease, no smoking history, normal oxygen levels, and a suppressed EPO is the one who gets the JAK2 test and, if positive, a referral to a hematologist. The diagnostic pathway is logical and stepwise, and most people do not need a bone marrow biopsy unless the blood tests leave the picture unclear.
Lifestyle Factors You Can Address
If your high red blood cell count is being driven by a correctable cause, addressing that cause is the single most effective thing you can do. For smokers, quitting removes the carbon monoxide burden and allows hemoglobin to carry oxygen normally again. Over weeks to months, the body dials back red blood cell production on its own. For people with untreated sleep apnea, consistent use of a CPAP device improves nighttime oxygen levels and can bring red blood cell counts down substantially.
Staying well hydrated matters at the margins. Chronic mild dehydration will not cause true erythrocytosis, but it can push borderline numbers into the abnormal range and make blood viscosity worse when red blood cell counts are already elevated. Drinking enough fluids is not a treatment for erythrocytosis, but it prevents your lab numbers from looking worse than the underlying situation warrants, and it helps reduce the viscosity risk that comes with thicker blood.
People who use testosterone replacement therapy should also be aware that exogenous testosterone stimulates red blood cell production. It is one of the more common medication-related causes of erythrocytosis in men, and your prescribing doctor should be monitoring your hematocrit periodically. If levels climb too high, the testosterone dose may need to be adjusted or the treatment paused.
Erythrocytosis in Athletes and Altitude Dwellers
Altitude training has been used by endurance athletes for decades precisely because it triggers the body to produce more red blood cells, boosting oxygen delivery to muscles. Spending weeks at elevation or sleeping in an altitude tent stimulates the same EPO-driven pathway that fires in people with chronic lung disease, just intentionally and temporarily. For these athletes, the elevated red blood cell count is a feature, not a problem, and counts return to baseline after returning to lower elevation.
Populations that live permanently at high altitude, such as communities in the Andes, the Tibetan Plateau, and parts of Ethiopia, present a more complicated picture. Some of these populations have evolved genetic adaptations that allow them to thrive at elevation without pushing red blood cell counts into dangerous territory. Others, particularly Andean highlanders, can develop a condition called chronic mountain sickness (Monge’s disease) in which red blood cell production overshoots to the point of causing symptoms like headaches, fatigue, and increased clotting risk. The same compensatory mechanism that works well in the short term can become harmful when it runs unchecked for years.
For anyone who lives at moderate altitude and has a mildly elevated red blood cell count, the key question is whether the elevation is proportional to the altitude. Your doctor can compare your numbers against reference ranges adjusted for your specific elevation rather than defaulting to sea-level norms, which would make almost everyone living above 1,500 meters look abnormal.
Rare and Unusual Causes
Most cases of erythrocytosis fall neatly into the categories above, but a few less common causes are worth knowing about. Certain tumors, particularly of the kidneys, liver, and brain, can secrete EPO or EPO-like substances and drive red blood cell production even when oxygen levels are normal. These are rare, but they are part of the reason doctors investigate further when someone has high EPO without an obvious respiratory explanation.
There are also inherited forms of erythrocytosis caused by mutations in genes that regulate oxygen sensing or hemoglobin structure. These are genuinely rare conditions, usually identified in childhood or adolescence, and they account for a small fraction of cases that do not fit the typical primary or secondary pattern. People with these conditions often have family members with similarly elevated counts, which is a helpful diagnostic clue.
Finally, some cases of erythrocytosis remain unexplained even after a thorough workup. The EPO is normal, the JAK2 test is negative, oxygen levels are fine, and there is no obvious drug or altitude cause. These cases are sometimes labeled idiopathic erythrocytosis, which is a clinical way of saying the cause is unknown. For these patients, management focuses on monitoring hematocrit levels and managing cardiovascular risk factors rather than treating a specific underlying condition.4PubMed Central. Investigation and management of erythrocytosis