An erythrocytosis workup is a structured sequence of tests designed to figure out why your red blood cell count or hemoglobin is persistently elevated, and whether it points to something that needs treatment. The process typically starts by confirming the elevation is real (not just dehydration), then branches based on your erythropoietin level and JAK2 mutation status to separate a bone marrow problem from an appropriate or inappropriate response elsewhere in the body. Getting this right matters because the causes range from benign lifestyle factors to a blood cancer called polycythemia vera, and treatment differs dramatically depending on the answer.
Confirming the Elevation Is Real
The first question in any erythrocytosis workup is whether the red blood cell count is genuinely increased or just appears elevated because plasma volume has dropped. Dehydration, heavy diuretic use, and chronic vomiting can all shrink the liquid portion of your blood, making the percentage of red cells (your hematocrit) look artificially high even though the total number of red cells is normal. This is called relative erythrocytosis, and it does not require any of the downstream testing described below. Red cell mass and plasma volume measurements can formally distinguish relative from absolute erythrocytosis, though in practice many clinicians begin by repeating the blood count after rehydration and looking for persistence of the elevated values before ordering more invasive studies.
Clinicians also look for obvious explanations before launching a full workup. A patient with known severe COPD whose hemoglobin runs slightly above the reference range is in a very different situation from a young adult with an unexplained hematocrit of 55%. Context shapes how aggressively the workup proceeds, but the core logic remains the same once the decision is made to pursue the cause.
The EPO Branch Point
Once absolute erythrocytosis is established, the single most informative next step is measuring serum erythropoietin, the hormone produced mainly by the kidneys that tells the bone marrow to make more red blood cells. An EPO level divides the diagnostic landscape in two. If EPO is low or suppressed, the marrow is overproducing red cells on its own, which points toward a primary bone marrow disorder. If EPO is normal or elevated, something outside the marrow is driving the production, which means secondary erythrocytosis.
This branching has been validated as a reliable and cost-effective first-intention test. A study in Haematologica assessing diagnostic value in patients with confirmed absolute erythrocytosis concluded that serum EPO measurement should be considered the first diagnostic test because it is simple, reliable, and inexpensive while effectively directing the rest of the workup.1Haematologica. Diagnostic value of serum erythropoietin level in patients with absolute erythrocytosis A low EPO level prompts testing for JAK2 mutations and possible bone marrow biopsy. A normal-to-high EPO level sends the workup in the direction of lung disease, heart defects, tumors, or genetic variants.
When EPO Is Low and JAK2 Testing
A suppressed EPO level strongly suggests the bone marrow is autonomously churning out red cells, and polycythemia vera is the leading concern. PV is a myeloproliferative neoplasm, a slow-growing blood cancer, and catching it matters because untreated PV carries significant risks of blood clots, stroke, and progression to more aggressive marrow diseases.
Almost all PV is driven by a mutation in the JAK2 gene. The most common is JAK2 V617F, present in roughly 95% of PV patients, with another small fraction carrying a mutation in exon 12 of the same gene. Because of this tight association, JAK2 mutation analysis has become the decisive diagnostic test. The 2008 WHO diagnostic criteria for PV elevated JAK2 mutation status to a major criterion, with supporting parameters including characteristic bone marrow appearance and a low EPO level.2PubMed. The 2008 WHO diagnostic criteria for polycythemia vera, essential thrombocythemia, and primary myelofibrosis In practice, a patient with confirmed erythrocytosis, low EPO, and a positive JAK2 mutation can be diagnosed without much ambiguity.3PubMed. Polycythemia vera
If JAK2 is negative despite a low EPO, the picture gets murkier. The clinician may order a bone marrow biopsy to look for other myeloproliferative disorders or consider rare primary erythrocytoses caused by mutations in the erythropoietin receptor itself. These receptor mutations make marrow cells hypersensitive to EPO, so even tiny amounts trigger excessive red cell production. One reported case involved a young girl with a novel EPOR mutation that truncated the receptor’s negative-regulation domain, leaving it perpetually “on.”4PubMed. A case of primary familial congenital polycythemia with a novel EPOR mutation
When EPO Is Normal or High
A normal-to-elevated EPO level redirects the workup toward secondary causes, and there are a lot of them. The body raises EPO when tissues do not get enough oxygen, so the clinician’s next job is figuring out whether the patient is genuinely hypoxic, whether something is producing EPO inappropriately, or whether a genetic quirk is fooling the oxygen-sensing system.
Hypoxia-Driven Causes
Chronic lung disease is the most common reason for secondary erythrocytosis in clinical practice. Conditions like COPD, pulmonary fibrosis, and chronic asthma reduce oxygen delivery, and the kidneys respond by ramping up EPO. Obstructive sleep apnea adds another layer: a study of patients with both COPD and overlapping sleep apnea found that polycythemia prevalence roughly doubled in those with both conditions compared to COPD alone, and that the percentage of sleep time spent with low oxygen saturation was independently associated with higher hemoglobin.5PubMed Central. Obstructive Sleep Apnea is Associated with an Increased Prevalence of Polycythemia in Patients with Chronic Obstructive Pulmonary Disease Sleep apnea on its own, however, tends to cause only minor bumps in hematocrit rather than frank erythrocytosis, so clinicians remain skeptical when a patient with mild sleep apnea shows a very high red cell count.6PubMed. Hematocrit levels in sleep apnea
Cyanotic congenital heart disease is another classic hypoxia-driven cause, particularly in the setting of Eisenmenger syndrome, where an untreated large heart defect leads to right-to-left shunting and chronic low oxygen levels. The erythrocytosis in these patients is compensatory, meaning the body is trying to boost oxygen-carrying capacity. Aggressive phlebotomy in these patients can actually be harmful, and clinicians are increasingly cautioned to reserve blood removal only for patients who have symptoms of hyperviscosity like headaches, blurred vision, or tingling rather than treating a number on a lab report.7PubMed Central. Erythrocytosis in congenital heart defects: hints for diagnosis and therapy from a clinical case Dehydration and iron deficiency can worsen hyperviscosity symptoms in these patients, and volume replacement or cautious iron therapy is often more appropriate than phlebotomy.8PubMed Central. Cyanotic congenital heart disease (CCHD) with symptomatic erythrocytosis
Inappropriate EPO Production
Some tumors secrete EPO on their own, independent of the body’s oxygen status. The classic culprits include renal cell carcinomas, hepatocellular carcinomas, cerebellar hemangioblastomas, pheochromocytomas, and uterine fibroids.9PubMed Central. Polycythemia with elevated erythropoietin production in a patient with a urinary stone and unilateral hydronephrosis When an otherwise unexplained erythrocytosis shows up with a markedly elevated EPO level and no obvious lung or heart explanation, imaging of the kidneys, liver, and brain becomes part of the workup. Even benign conditions like renal cysts or hydronephrosis can occasionally cause enough local kidney ischemia to drive EPO production.
Post-kidney-transplant erythrocytosis is another cause that fits here. After a successful transplant, the diseased native kidneys sometimes keep secreting EPO without responding to the normal feedback that should shut production down. The result is a kind of runaway EPO state, often with EPO levels that technically stay within the normal range but remain inappropriately high given the elevated red cell mass. Additional growth factors like angiotensin II and insulin-like growth factor 1 appear to contribute, which is why ACE inhibitors or angiotensin receptor blockers are the first-line treatment rather than phlebotomy.10PubMed. Posttransplant erythrocytosis
Smoking and Carbon Monoxide
Heavy smoking deserves its own mention because it causes erythrocytosis through a different mechanism than typical lung disease. Cigarette and cigar smoke delivers carbon monoxide, which binds to hemoglobin far more tightly than oxygen does. The resulting carboxyhemoglobin is useless for oxygen delivery, so the body compensates by making more red cells. In a study of patients with erythrocytosis, carboxyhemoglobinemia from smoking was identified as the cause of elevated hematocrit in the majority of cases, with either increased red cell volume or decreased plasma volume found in all 14 patients studied.11PubMed. Erythrocytosis associated with carboxyhemoglobinemia in smokers This means that asking about smoking history and, when relevant, checking a carboxyhemoglobin level is a basic part of the workup that can save a patient from unnecessary bone marrow biopsies.
Testosterone and Other Drug-Induced Causes
Exogenous testosterone is an increasingly common cause of erythrocytosis, given how widely testosterone replacement therapy is now prescribed for men with low levels. Testosterone stimulates red blood cell production directly through the marrow and indirectly by suppressing hepcidin, a hormone that regulates iron availability. A well-documented case described a 55-year-old man who developed secondary erythrocytosis following testosterone replacement, illustrating a scenario that clinicians encounter regularly in practice.12PubMed Central. Testosterone use causing erythrocytosis Monitoring hemoglobin is standard care for anyone on testosterone therapy, and dose adjustment or temporary cessation is the usual response when levels climb too high.
Recombinant EPO, originally developed for anemia in kidney disease, is another potential drug cause. Its misuse as a performance-enhancing agent in endurance sports has been widely documented. Sports authorities have developed blood-testing protocols to detect EPO doping, though limitations remain.13PubMed Central. Erythropoietin and blood doping In a clinical workup, a clinician who sees unexplained erythrocytosis with elevated EPO in an athlete or bodybuilder should at least consider exogenous EPO or anabolic steroid use, even though patients may not volunteer that history.
Hereditary Erythrocytosis and the Oxygen-Sensing Pathway
When the workup turns up no acquired cause, the possibility of a genetic basis comes into play. This is most relevant in younger patients or those with a family history of elevated hemoglobin. A distinct subset of secondary erythrocytoses are caused by mutations in the oxygen-sensing pathway, the molecular machinery that cells use to detect hypoxia and regulate EPO production.14PubMed Central. Genetic causes of erythrocytosis and the oxygen-sensing pathway
The key genes involved include VHL, EPAS1 (also called HIF-2α), and EGLN1 (also called PHD2). Mutations in any of these can trick the body into behaving as though oxygen levels are low when they are actually normal, driving persistent EPO elevation and red cell overproduction. Separately, hemoglobin variants with abnormally high oxygen affinity cause the same downstream effect through a different mechanism: because the altered hemoglobin holds onto oxygen too tightly and releases it to tissues poorly, the kidneys perceive tissue hypoxia and increase EPO. Roughly 100 such high-oxygen-affinity hemoglobin variants have been described, mostly involving mutations in the beta or alpha globin genes.15Haematologica. Identification of high oxygen affinity hemoglobin variants in the investigation of patients with erythrocytosis Mutations in the enzyme that produces 2,3-bisphosphoglycerate (BPGM) cause a similar problem, since this molecule normally helps hemoglobin release oxygen.16PubMed. Genetic basis of congenital erythrocytosis: mutation update and online databases
A p50 measurement, which tests how readily hemoglobin releases oxygen, is a useful screening tool when a high-oxygen-affinity variant is suspected. If the p50 is shifted leftward (meaning hemoglobin releases oxygen less readily), further hemoglobin electrophoresis and genetic sequencing can identify the specific variant.17PubMed Central. High-Oxygen-Affinity Hemoglobins-Case Series and Review of the Literature These hereditary forms are typically benign and require no treatment, but identifying them spares patients from years of unnecessary monitoring or treatment for a disease they do not have.
Why the Workup Matters for Risk
Elevated hematocrit is not just a number. Blood that carries too many red cells becomes thicker, and thicker blood is harder to pump and more prone to clotting. A large population-based study found that for every 5% increase in hematocrit, the risk of venous blood clots rose by about 25%, and the risk of unprovoked clots specifically rose by about 37%. Men with hematocrit values in the top fifth of the distribution had roughly 2.4 times the risk of unprovoked venous clots compared to those in the lowest two-fifths.18PubMed Central. Hematocrit and risk of venous thromboembolism in a general population. The Tromsø study This thrombotic risk is the primary reason clinicians push to identify and manage the underlying cause.
In polycythemia vera specifically, pruritus (itching) affects about 40% of patients, and it is characteristically triggered by contact with water at any temperature, a phenomenon called aquagenic pruritus.19PubMed Central. Polycythemia vera-associated pruritus and its management This symptom is sometimes the complaint that first brings a patient to medical attention and can be debilitating even when blood counts are only modestly elevated.
Treatment Depends Entirely on the Cause
The reason the workup matters so much is that management varies completely depending on the diagnosis. For polycythemia vera, the cornerstone is phlebotomy to keep hematocrit below 45%. A landmark trial randomized 365 PV patients to a hematocrit target of under 45% versus 45-50% and found that cardiovascular death or major thrombosis occurred in about 3% of the lower-target group versus nearly 10% of the higher-target group.20JAMA. Diagnosis and Treatment of Polycythemia Vera: A Review Patients at higher risk for clotting, generally those over 60 or with a prior clot, also receive cytoreductive therapy, most commonly hydroxyurea or interferon formulations. When those drugs fail or are not tolerated, ruxolitinib, a JAK inhibitor, is used as a second-line option.21PubMed Central. Treatment of hydroxyurea-resistant/intolerant polycythemia vera: a discussion of best practices
For secondary erythrocytosis driven by hypoxia, the treatment is improving oxygenation: supplemental oxygen for COPD, CPAP for sleep apnea, or surgical correction for congenital heart defects when feasible. For tumor-driven EPO secretion, removing the tumor usually resolves the erythrocytosis. For testosterone-induced cases, adjusting or stopping the testosterone is straightforward. For hereditary forms, often no treatment is needed at all beyond reassurance and monitoring.
High-Altitude Erythrocytosis
Living at high altitude is a physiological cause of erythrocytosis that clinicians in lowland practices may underappreciate. Everyone who moves to high altitude develops some increase in hemoglobin as a normal acclimatization response. A subset of long-term high-altitude residents, however, develops chronic mountain sickness (also called Monge disease), characterized by excessive erythrocytosis that goes beyond what is adaptive. The consensus threshold for excessive erythrocytosis at altitude is a hemoglobin of 19 g/dL or higher for women and 21 g/dL or higher for men, based on epidemiological data from communities in the Peruvian Andes living above 4,000 meters.22PubMed Central. Chronic Mountain Sickness: Clinical Aspects, Etiology, Management, and Treatment For patients who live at moderate altitudes and have mildly elevated hemoglobin, the workup should take residence elevation into account before pursuing more invasive testing.
Neonatal Erythrocytosis
Newborns represent a special case in erythrocytosis evaluation. Normal neonatal hematocrit values are much higher than adult values, and several delivery-room practices can push them even higher. Late cord clamping, which has become standard in many hospitals for its iron-supplementation benefits, results in a moderate rise in hematocrit and blood viscosity in the first hours of life.23American Journal of Obstetrics and Gynecology. The effect of Leboyer delivery on blood viscosity and other hemorheologic parameters in term neonates Most neonates tolerate this without issue, but in some cases, particularly premature infants or those with additional risk factors like maternal diabetes or twin-to-twin transfusion, neonatal polycythemia can cause sluggish circulation and symptoms requiring partial exchange transfusion. The point for the workup context is that neonatal erythrocytosis has an entirely different set of reference ranges and causes than adult erythrocytosis, and adult diagnostic algorithms do not apply.
Emerging Therapies for Polycythemia Vera
One frustrating aspect of PV management is that repeated phlebotomy, while effective at lowering hematocrit, progressively depletes iron stores. Patients often end up iron-deficient, which can cause fatigue, brain fog, and restless legs even as their hematocrit is controlled. This creates an uncomfortable paradox where treating the disease causes its own set of symptoms.
A newer approach aims to break this cycle. Rusfertide is a hepcidin mimetic, a drug that mimics the body’s natural iron-regulatory hormone to restrict how much iron reaches the bone marrow for red cell production. Phase 2 clinical trial results showed that rusfertide was highly effective at eliminating the need for therapeutic phlebotomies, normalizing blood counts, replenishing iron stores, and relieving symptoms like fatigue and itching in PV patients.24PubMed. Hepcidin mimetics in polycythemia vera: resolving the irony of iron deficiency and erythrocytosis The idea of controlling erythrocytosis by regulating iron supply rather than repeatedly draining blood represents a genuinely different therapeutic philosophy, and if later-phase trials confirm these results, it could substantially change how PV is managed day to day.