Erythrocytosis is not cancer in most cases. It’s a clinical finding meaning you have more red blood cells than normal, and the majority of causes are non-cancerous conditions like sleep apnea, chronic lung disease, or smoking. However, one specific type of erythrocytosis, called polycythemia vera, is a slow-growing blood cancer. The critical step after getting an elevated red blood cell count is figuring out which category you fall into.
What Erythrocytosis Actually Means
Erythrocytosis simply describes a higher-than-normal concentration of red blood cells in your blood. It’s a lab finding, not a diagnosis. Think of it like a fever: a fever tells you something is going on, but it doesn’t tell you what. Erythrocytosis works the same way. Dozens of different conditions can cause it, ranging from dehydration to rare bone marrow disorders.
Doctors break erythrocytosis into three broad categories. Relative erythrocytosis means your red blood cell count looks high because you’re dehydrated; your plasma volume has dropped, making the blood more concentrated, but you don’t actually have extra red cells. Absolute erythrocytosis means your body is genuinely producing too many red blood cells. That absolute category is then split further into primary (a problem in the bone marrow itself) and secondary (the bone marrow is responding to a signal from somewhere else in the body).
The One Type That Is Cancer
Polycythemia vera (PV) is the form of erythrocytosis that qualifies as cancer. It belongs to a group of diseases called myeloproliferative neoplasms, where the bone marrow overproduces blood cells due to a genetic mutation. In PV, a mutation in the JAK2 gene causes the bone marrow to churn out red blood cells uncontrollably. About 96% to 99% of people with PV carry this mutation.
PV is a slow-moving cancer, and many people live with it for decades. But it does carry real risks. Within 15 years of diagnosis, around 15% of PV cases progress to myelofibrosis, a more serious condition where scar tissue replaces healthy bone marrow. About 3% transform into acute myeloid leukemia within 10 years, which has a much worse prognosis. The more immediate concern for most PV patients is blood clots. In one large study, the rate of thrombotic events in PV patients was roughly 4.5 cases per 100 person-years, significantly higher than in people with other causes of erythrocytosis like sleep apnea.
The Many Non-Cancer Causes
Secondary erythrocytosis is far more common than PV, and none of these causes are cancers of the blood. They all share one basic mechanism: something in the body triggers extra production of erythropoietin (EPO), the hormone that tells your bone marrow to make more red blood cells. The bone marrow is working correctly; it’s just getting too strong a signal.
The most frequent triggers involve chronic low oxygen levels:
- Chronic lung disease (COPD, emphysema)
- Obstructive sleep apnea
- Smoking, which introduces carbon monoxide that reduces oxygen delivery
- Living at high altitude
- Cyanotic heart disease, where heart defects reduce blood oxygen
- Obesity hypoventilation syndrome
There’s also a less common category where EPO production is inappropriately high. Certain tumors, including kidney cancer, liver cancer, and some brain tumors, can secrete EPO on their own. So while erythrocytosis itself isn’t cancer, it can occasionally be a signal that a cancer elsewhere in the body is producing excess EPO. Kidney diseases, including cysts and polycystic kidney disease, can do this too without any malignancy involved.
Dehydration is probably the single most common reason for a mildly elevated red blood cell count on routine bloodwork. Once you rehydrate, the numbers normalize. No treatment needed.
How Doctors Tell the Difference
The diagnostic workup is straightforward. The two key tests are a serum EPO level and a JAK2 mutation test. These two results, combined with your symptoms and medical history, sort most people into the right category quickly.
If your EPO level is low (below about 2.9 mU/mL), that points toward PV. A low EPO is 92% specific for PV, meaning very few people with low EPO turn out to have something else. If your EPO is high (above about 15.1 mU/mL), that’s 98% specific for secondary erythrocytosis, meaning the bone marrow is responding to a legitimate signal rather than malfunctioning on its own.
A positive JAK2 mutation test essentially confirms PV. The current WHO diagnostic criteria also look at hemoglobin and hematocrit thresholds: hemoglobin above 16.5 g/dL in men or 16.0 g/dL in women, or hematocrit above 49% in men or 48% in women. Meeting these thresholds along with a JAK2 mutation and a low EPO level makes the diagnosis. A bone marrow biopsy may be done to confirm.
Symptoms Worth Paying Attention To
Many people with erythrocytosis feel nothing at all, and the finding shows up on routine bloodwork. When symptoms do appear, they tend to be vague: headaches, dizziness, blurred vision, and a reddish or ruddy complexion. These happen because thicker blood flows less efficiently.
One symptom is remarkably specific to polycythemia vera: intense itching after a warm bath or shower, called aquagenic pruritus. In a study of 441 PV patients, 68% experienced this symptom. Most described itching, though some felt stinging or burning. In nearly 65% of those affected, the itching started an average of 2.9 years before they were diagnosed with PV. If you’ve been dealing with unexplained post-shower itching alongside elevated red blood cell counts, that combination is a strong reason to pursue JAK2 testing.
How Erythrocytosis Is Managed
Treatment depends entirely on the cause. For secondary erythrocytosis, the priority is treating the underlying condition. If sleep apnea is driving your elevated counts, consistent use of a CPAP machine often brings red blood cell levels back toward normal. If smoking is the cause, quitting is the most effective intervention. For people living at high altitude, the erythrocytosis may be a normal physiological adaptation that doesn’t need treatment at all.
For polycythemia vera, the cornerstone of treatment is therapeutic phlebotomy, which is essentially a blood draw. Removing blood regularly lowers the red blood cell concentration and reduces the risk of clots. The target is to keep hematocrit below 45%, a threshold established by a clinical trial showing significantly fewer blood clots at that level. Most people need a session every one to two months for maintenance.
People with PV who are at high risk of blood clots, or who can’t tolerate frequent blood draws, may also take a medication that slows down blood cell production in the bone marrow. Low-dose aspirin is commonly used alongside phlebotomy to further reduce clotting risk.
For secondary erythrocytosis caused by chronic lung disease, phlebotomy is only recommended when hematocrit climbs above 56% and symptoms of blood thickening are present. The target in that situation is more lenient, typically keeping hematocrit between 50% and 52%, because these patients need the extra oxygen-carrying capacity that additional red blood cells provide.