Erythrocytosis is an abnormal increase in the number of red blood cells circulating in your bloodstream, defined by a hemoglobin or hematocrit level above the normal threshold for your age and sex. It is not a single disease but a laboratory finding that can stem from dozens of different causes, ranging from a bone marrow disorder to living at high altitude to taking testosterone therapy. The distinction between harmless and dangerous forms hinges almost entirely on why the red cell count is elevated, which makes figuring out the underlying cause the central challenge for both patients and doctors.
What Erythrocytosis Actually Means
Your blood is a mix of cells and plasma, and the balance between those two fractions determines your hematocrit, the percentage of blood volume occupied by red blood cells. A high hematocrit or hemoglobin can happen in two fundamentally different ways. In relative erythrocytosis, you have a normal number of red cells but the liquid portion of blood has shrunk, concentrating everything. Dehydration, heavy sweating, and certain diuretic medications can do this. In absolute erythrocytosis, your body is genuinely producing more red blood cells than it should be.1PubMed Central. Investigation and management of erythrocytosis
The relative form is usually straightforward and resolves once plasma volume is restored. Absolute erythrocytosis is the form that requires a deeper investigation, because the causes range from genetic mutations present since birth to acquired cancers to chronic oxygen deprivation.
Primary Causes and the Bone Marrow
When the problem originates inside the bone marrow itself, doctors call it primary erythrocytosis. The marrow’s red blood cell factory is revving too high on its own, independent of the normal hormonal signals that regulate red cell production.
The most clinically significant primary cause is polycythemia vera, a slow-growing blood cancer classified among the myeloproliferative neoplasms. Nearly all cases are driven by a mutation in the JAK2 gene, with the V617F variant detected in roughly 98% of patients.2PubMed Central. Genetic Background of Polycythemia Vera This mutation makes red blood cell precursors hypersensitive to growth signals, so they proliferate even when the body is not asking for more oxygen-carrying capacity. People with polycythemia vera often have an enlarged spleen, itching after a warm shower, and a ruddy complexion, along with a meaningfully elevated risk of blood clots and, over years, potential progression to scarring of the marrow.
A rarer primary form is familial erythrocytosis, which typically shows up in childhood or adolescence. Several families have been found to carry mutations in the erythropoietin receptor gene. These mutations chop off a regulatory region on the receptor’s tail, leaving it stuck in an “on” position. People with these truncated receptors have elevated red cell counts and unusually low levels of erythropoietin (EPO), the kidney hormone that normally drives red cell production, because the receptor is so sensitive that even a whisper of EPO triggers a big response.3Blood. Two New EPO Receptor Mutations: Truncated EPO Receptors Are Most Frequently Associated With Primary Familial and Congenital Polycythemias Unlike polycythemia vera, familial erythrocytosis is not a cancer and does not progress to marrow scarring, though the thickened blood still poses some clotting risk.
Secondary Causes and the Oxygen Connection
Most erythrocytosis in the general population is secondary, meaning the bone marrow is responding appropriately to a signal, usually EPO, that is being overproduced for some reason. The most common reason is chronic hypoxia. When your tissues are not getting enough oxygen, a family of proteins called hypoxia-inducible factors, especially HIF-2, ramps up EPO production in the kidneys and liver.4PubMed Central. Regulation of erythropoiesis by hypoxia-inducible factors More EPO means more red blood cells, which means more oxygen-carrying capacity. The system is doing exactly what evolution designed it to do.
Common conditions that trigger this hypoxia-driven pathway include chronic obstructive pulmonary disease, severe sleep apnea, congenital heart defects that mix oxygenated and deoxygenated blood, and chronic heavy smoking (carbon monoxide in cigarette smoke binds hemoglobin and prevents it from carrying oxygen, effectively creating tissue-level hypoxia). Living at high altitude is another classic trigger, though as we will see, the body’s adaptation to altitude is more nuanced than simply making extra red cells.
Less commonly, EPO is produced not because of oxygen shortage but because a tumor is secreting it. Certain kidney cancers, liver tumors, and cerebellar hemangioblastomas can autonomously pump out EPO. Hemangioblastomas are especially interesting in this regard. A study examining 11 of these tumors found EPO messenger RNA in every single specimen, a consequence of losing function of the von Hippel-Lindau (VHL) tumor suppressor gene, which normally keeps the hypoxia-response pathway in check.5Blood. Coexpression of Erythropoietin and Vascular Endothelial Growth Factor in Nervous System Tumors Associated With von Hippel-Lindau Tumor Suppressor Gene Loss of Function Mutations in the VHL gene can also cause a congenital form of erythrocytosis even without tumor formation, because the oxygen-sensing machinery stays inappropriately activated.6PubMed Central. von Hippel-Lindau syndrome-related congenital polycythemia and response to belzutifan
Testosterone and Other Drug-Related Causes
One of the most rapidly growing causes of erythrocytosis is testosterone therapy. As more men receive testosterone for age-related hypogonadism or gender-affirming care, erythrocytosis has become one of the most frequent side effects clinicians encounter. Testosterone stimulates red blood cell production through multiple pathways: it triggers an initial rise in EPO, resets the EPO-hemoglobin balance point to a higher level, and suppresses hepcidin, the protein that controls how much iron is available for making new red cells.7PubMed Central. Testosterone therapy-induced erythrocytosis: can phlebotomy be justified? The result is that hemoglobin can climb well above the normal range within months of starting treatment, sometimes forcing a dose reduction or periodic blood removal.
The flip side of this biology is the use of recombinant EPO as a performance-enhancing drug. Injecting EPO directly raises hematocrit, which boosts the blood’s oxygen-carrying capacity. A systematic review of the available evidence found low-to-moderate quality data suggesting that recombinant EPO improves maximal exercise performance, including maximal power output and time to exhaustion. However, these benefits showed up almost exclusively during all-out maximal efforts, which may be less relevant to real competitive conditions where athletes rarely sustain absolute peak intensity for extended periods.8PubMed Central. Effect of erythropoietin on athletic performance: a systematic review and meta-analysis The practice carries real danger: pushing hematocrit too high thickens the blood to a point where clotting risk rises sharply.
Why Extra Red Cells Are Dangerous
The central hazard of erythrocytosis, regardless of cause, is hyperviscosity. Blood viscosity increases in a non-linear way as hematocrit climbs, meaning each additional percentage point of red cells thickens the blood more than the last. Thicker blood flows more slowly, which creates the conditions described in Virchow’s triad for clot formation: sluggish flow, damage to vessel walls, and a hypercoagulable state. Physically, higher hematocrit pushes platelets toward the vessel walls, increasing their contact with endothelium and raising the likelihood that a clot will form.9Journal of Thrombosis and Haemostasis. Red blood cells: the forgotten player in hemostasis and thrombosis
There is an interesting wrinkle in this story. At modest increases in hematocrit, the higher viscosity actually increases shear stress on the vessel lining, stimulating the release of nitric oxide, a vasodilator. This can paradoxically lower blood pressure slightly. But once hematocrit rises more than about 19% above baseline, the viscosity effect overwhelms the vasodilation, vascular resistance shoots up, and blood pressure rises.10PubMed. Paradoxical hypotension following increased hematocrit and blood viscosity This tipping point helps explain why mild erythrocytosis may go unnoticed for years while severe forms produce symptoms quickly.
Not all causes of erythrocytosis carry the same clotting risk. A study from a referral center in Mexico City compared thrombosis rates among patients with polycythemia vera, obstructive sleep apnea, and chronic lung disease. Polycythemia vera and chronic lung disease had significantly higher rates of blood clots and death than sleep apnea, with thrombosis rates of about 4.5 and 6.2 per 100 person-years, respectively, compared to roughly 1.5 per 100 person-years in sleep apnea patients.11PubMed Central. Causes of erythrocytosis and its impact as a risk factor for thrombosis according to etiology: experience in a referral center in Mexico City This suggests the underlying disease matters as much as the elevated red cell count in determining how dangerous the erythrocytosis actually is.
How Doctors Figure Out the Cause
The diagnostic workup for erythrocytosis follows a branching logic. The first step is confirming that the elevation is real and not just a snapshot of dehydration by repeating the blood count when you are well-hydrated. Once absolute erythrocytosis is confirmed, JAK2 mutation testing is the pivotal branch point. A positive JAK2 test essentially clinches the diagnosis of polycythemia vera. A negative result sends the investigation in a different direction.
When polycythemia vera has been ruled out, the serum EPO level becomes the next key fork. A low EPO suggests the bone marrow is overproducing red cells on its own, which in a JAK2-negative patient points toward a rare inherited EPO receptor mutation. A normal or elevated EPO suggests the marrow is responding to an external signal, and the search shifts to finding the source: lung function testing for hypoxia, sleep studies for apnea, hemoglobin oxygen affinity testing for high-affinity hemoglobin variants, and imaging for EPO-secreting tumors.12Leukemia. JAK2 unmutated erythrocytosis: current diagnostic approach and therapeutic views
Despite this structured algorithm, a meaningful fraction of patients end up labeled as “idiopathic erythrocytosis,” meaning the cause remains unknown after standard testing. Gene panel sequencing has improved this situation. When researchers applied a targeted panel of erythrocytosis-associated genes to patients who had gone through the traditional workup without a diagnosis, they identified known disease-causing variants in cases that had been missed by the step-by-step biochemical approach.13PubMed Central. Gene panel sequencing improves the diagnostic work-up of patients with idiopathic erythrocytosis and identifies new mutations The problem with the traditional method is that some mutations produce borderline or confusing biochemical results, so the branching logic sends you down the wrong path.
Treatment Depends Entirely on the Cause
There is no one-size-fits-all treatment for erythrocytosis, because managing it means managing whatever is driving it. For secondary forms caused by hypoxia, the priority is treating the underlying condition: supplemental oxygen for lung disease, CPAP therapy for sleep apnea, altitude descent or supplemental oxygen for altitude-related erythrocytosis. If the hypoxia can be corrected, the red cell count often comes down on its own.
For polycythemia vera, treatment is more involved. Phlebotomy, the periodic removal of blood to lower hematocrit, remains a cornerstone of management. The standard target is keeping hematocrit below 45%. Low-dose aspirin is used alongside phlebotomy to reduce clotting risk. For patients who need frequent phlebotomies, develop worsening symptoms, or have high-risk features, drug therapy is added. Hydroxyurea has been a standard cytoreductive agent for decades.
The introduction of ruxolitinib, a JAK1/JAK2 inhibitor, changed the landscape for patients inadequately controlled by standard therapy. In a pivotal trial, about 60% of patients on ruxolitinib achieved hematocrit control compared to 20% on standard therapy, and nearly half had at least a 50% reduction in their symptom burden compared to 5% in the control group.14PubMed Central. Ruxolitinib versus standard therapy for the treatment of polycythemia vera A follow-up trial confirmed these benefits even in patients without an enlarged spleen, broadening the drug’s applicability.15The Lancet Oncology. Ruxolitinib versus best available therapy in patients with polycythaemia vera without splenomegaly randomised in an open-label, phase 3b study (RESPONSE-2)
For testosterone-induced erythrocytosis, the options are reducing the testosterone dose, switching to a different formulation, or performing phlebotomy to keep hematocrit in a safe range. The question of whether phlebotomy actually prevents clotting events in this population is still debated, and the evidence base is thinner than many clinicians would like.
Erythrocytosis in Pregnancy and Newborns
Pregnancy creates an unusual physiological context for erythrocytosis. Blood volume normally expands dramatically during pregnancy, diluting red blood cells and pushing hematocrit down. In women with polycythemia vera, this natural dilution can mask the disease, but the underlying risks remain. Polycythemia vera in pregnancy is associated with higher thrombotic and placental risk, driven by erythrocytosis and hyperviscosity.16Seminars in Hematology. Management of myeloproliferative neoplasms in pregnancy: Essential thrombocythemia, polycythemia vera, and primary myelofibrosis Management typically involves close monitoring, aspirin, phlebotomy if needed, and careful attention to hydration.
Newborns present a different scenario. Neonatal polycythemia, defined as a venous hematocrit above 65% or hemoglobin above 22 g/dL, is relatively common and often reflects physiological adjustments to the transition from the low-oxygen womb environment. Most cases are benign, but when the high hematocrit causes hyperviscosity symptoms, such as sluggish feeding, breathing difficulty, or bluish discoloration, a partial exchange transfusion may be performed to bring the hematocrit down. Asymptomatic babies are generally managed with hydration and monitoring alone.17PubMed Central. Neonatal Polycythaemia
High-Altitude Populations and the Limits of Adaptation
Living at high altitude is the most natural trigger for erythrocytosis, and studying populations who have lived at elevation for thousands of years reveals fascinating variation in how human bodies cope. Tibetan and Andean highlanders have both adapted to chronic hypoxia, but they have done so in strikingly different ways. Andean populations tend to have markedly elevated hemoglobin levels, a classic erythrocytosis response. Tibetan populations, by contrast, maintain relatively normal hemoglobin levels despite living at comparable altitudes, relying instead on other physiological adjustments like increased blood flow and more efficient oxygen extraction.18PubMed Central. Two routes to functional adaptation: Tibetan and Andean high-altitude natives
The Andean strategy has a downside. Chronic mountain sickness, first described by Carlos Monge Medrano in 1925, is a condition affecting long-term high-altitude residents characterized by excessive erythrocytosis, low blood oxygen, and cognitive symptoms like headache, confusion, and poor concentration.19PubMed Central. Monge’s disease at 100 years: Revisiting the origins and endocrine mechanisms of chronic mountain sickness The condition becomes more common with age. In a male population living at 4,300 meters, researchers found that the prevalence of excessive erythrocytosis, low oxygen saturation, and chronic mountain sickness symptoms all increased progressively as men got older. The data suggest that as ventilatory drive naturally declines with age, the resulting deeper hypoxemia pushes red cell production into a pathological range. There may also be an element of the bone marrow becoming increasingly sensitive to a fixed level of hypoxemia over time.20PubMed. Pathophysiology and epidemiology of chronic mountain sickness
The Tibetan adaptation, which avoids excessive erythrocytosis, appears to sidestep this problem. This difference between the two populations is one of the clearest examples of evolutionary convergence producing genuinely different solutions: same environmental pressure, same functional outcome of successful oxygen delivery, but different physiological routes to get there.
Seals and the Spleen Trick
Humans are not the only animals that modulate their red blood cell count, and the most dramatic example comes from diving seals. Weddell seals effectively toggle their hematocrit on and off by storing enormous volumes of red blood cells in their spleens and then squeezing them out when they dive. One study measured hemoglobin jumping from about 17.5 g/dL at rest under anesthesia to roughly 22 g/dL in the first two minutes after surfacing from a dive, with hematocrit climbing from 44% to 55%. The researchers estimated that about 20 liters of red blood cells were sequestered in the spleen at rest, ready to be injected into the circulation on demand via catecholamine-driven splenic contraction.21PubMed. Splenic contraction, catecholamine release, and blood volume redistribution during diving in the Weddell seal
Work on hooded and harp seals revealed similar mechanics. When stimulated with epinephrine, the spleen of a 250-kg hooded seal expelled about 3.9 liters, roughly 13% of its total blood volume. The hematocrit of the blood draining from the spleen was an extraordinary 90%, essentially a concentrated slug of red cells. The contraction was driven through alpha-adrenergic receptors and happened within one to three minutes.22PubMed. Volume capacity and contraction control of the seal spleen This system represents a reversible, on-demand erythrocytosis perfectly suited to brief bouts of extreme oxygen need. Humans have a much smaller splenic reservoir and cannot replicate this trick in any meaningful way, but exercise and stress do cause a mild splenic contraction and a temporary bump in circulating red cells. The seal system is the extreme version of a principle that exists across many mammals.