Polycythemia vera (PV) is a slow-growing blood cancer in which your bone marrow produces too many red blood cells. The excess red cells thicken your blood, raising the risk of dangerous clots in arteries and veins. It’s a chronic condition, not a curable one, but with proper management most people live about 20 years after diagnosis.
What Happens Inside the Bone Marrow
Normally, your bone marrow takes cues from hormones like erythropoietin (EPO) to decide how many red blood cells to make. Think of it as a thermostat: when oxygen levels drop, EPO tells the marrow to ramp up production, and when levels are fine, it dials back down.
In roughly 95% of PV cases, a single genetic typo disrupts that feedback loop. A mutation in the JAK2 gene (called JAK2 V617F) makes the signaling pathway that controls blood cell production permanently overactive. Blood-forming stem cells become hypersensitive to EPO and other growth signals. They can even form new red blood cells with little to no EPO present at all. The result is a relentless overproduction of red cells, and often white cells and platelets too, regardless of what the body actually needs.
This mutation is acquired during a person’s lifetime rather than inherited at birth. Researchers still don’t fully understand what triggers it, but age is the biggest risk factor. PV is most commonly diagnosed in people over 60.
Common Symptoms
PV develops slowly, and some people learn about it only through a routine blood test that comes back with unusually high red blood cell counts. When symptoms do appear, they tend to reflect the consequences of thicker, more sluggish blood:
- Headaches and dizziness from reduced blood flow to the brain
- Fatigue that feels disproportionate to your activity level
- Blurred vision or ringing in the ears
- A ruddy or reddish complexion, especially in the face and hands
- Itching after a warm bath or shower (called aquagenic pruritus), one of the more distinctive signs of PV
- Burning pain, redness, and warmth in the hands or feet (erythromelalgia), caused by abnormal blood flow in small vessels
- An enlarged spleen, which can cause a feeling of fullness or discomfort in the upper left abdomen
The itching after water contact is particularly telling. It can range from mildly annoying to severe enough to make showers miserable, and it often appears years before a diagnosis is made.
How PV Is Diagnosed
Diagnosis relies on blood work, genetic testing, and sometimes a bone marrow biopsy. The current diagnostic thresholds focus on hemoglobin and hematocrit, which is the percentage of your blood volume occupied by red cells. For men, a hemoglobin above 16.5 g/dL or hematocrit above 49% raises suspicion. For women, the cutoffs are 16 g/dL and 48%.
If those numbers are elevated, your doctor will typically order a test for the JAK2 V617F mutation. A positive result, combined with the blood count findings, is often enough to confirm the diagnosis. In some cases a bone marrow biopsy is also performed to look at the pattern of cell production directly, but it isn’t always required.
Why Blood Clots Are the Main Danger
Thicker blood moves more slowly and is more prone to clotting. This is the most serious day-to-day risk of PV. In a large study tracking over 50,000 newly diagnosed patients, about 28% of those with PV experienced a thrombotic event during a median follow-up of roughly three years. The most common event was ischemic stroke, accounting for 46% of those clots.
Clots can also form in unusual locations, including the veins draining the liver or spleen (a condition called Budd-Chiari syndrome) and the deep veins of the legs. Heart attacks, pulmonary embolisms, and transient ischemic attacks (mini-strokes) round out the list. Age over 60 and a history of prior clots are the two factors that push someone into a higher risk category.
Treatment: Keeping Blood Thickness in Check
There is no treatment that eliminates the JAK2 mutation or cures PV outright. Instead, treatment focuses on keeping blood counts low enough to minimize clot risk and relieve symptoms.
Phlebotomy
The most immediate tool is therapeutic phlebotomy, which is essentially the same as donating blood. A unit of blood is drawn at regular intervals to bring your hematocrit below 45%. That 45% target is consistent across current guidelines because maintaining it has been shown to significantly reduce the rate of cardiovascular events and death. Early in treatment, phlebotomy may be needed every week or two. Once counts stabilize, the interval stretches to every few months for many people.
Low-Dose Aspirin
Nearly all PV patients take a daily low-dose aspirin. It reduces the stickiness of platelets and helps prevent clots. It’s also the first-line treatment for erythromelalgia, that burning pain in the hands and feet, often providing relief within hours.
Cytoreductive Medications
When phlebotomy alone isn’t enough to control blood counts, or when someone falls into the high-risk category, medications that slow the bone marrow’s overproduction enter the picture. Hydroxyurea has been the standard first-line drug for decades. It’s taken as a daily pill and works by broadly suppressing cell production in the marrow.
For patients who don’t tolerate hydroxyurea or whose disease doesn’t respond to it, two other options are available. Pegylated interferon is an injectable medication that can reduce the burden of JAK2-mutated cells over time and is increasingly used as a first-line alternative, especially in younger patients. Ruxolitinib, a targeted drug that directly blocks the overactive JAK signaling pathway, is approved as a second-line treatment for PV. It’s particularly effective at shrinking an enlarged spleen and controlling symptoms like itching and night sweats.
The Iron Deficiency Tradeoff
One counterintuitive aspect of PV management is iron. Repeated phlebotomy deliberately depletes your iron stores, because iron is a key ingredient in hemoglobin production. Less iron means the marrow can’t manufacture red cells as quickly, which helps keep hematocrit down. This is actually the intended effect.
The problem is that iron deficiency comes with its own set of symptoms: fatigue, brain fog, restless legs, brittle nails, and pica (cravings for non-food items like ice). Replacing iron through supplements or diet seems like the obvious fix, but in PV it often backfires. Giving iron fuels a surge in red cell production, sending hematocrit right back up and potentially requiring more frequent phlebotomy. Managing this tension is one of the trickier parts of living with PV. When iron deficiency symptoms become severe, doctors may add a cytoreductive medication so that phlebotomy can be spaced out further rather than simply giving iron back.
Long-Term Outlook and Disease Progression
PV is a chronic condition that most people manage for many years. Recent estimates put median survival at about 20 years from diagnosis, though individual outcomes vary widely depending on age, clot history, and how well blood counts are controlled.
A small percentage of patients see their disease evolve over time. About 12% develop post-PV myelofibrosis within 10 years, a condition where scar tissue gradually replaces healthy bone marrow and makes it harder to produce normal blood cells. The risk of transformation to acute myeloid leukemia is lower, around 3% at 10 years, but it carries a much more serious prognosis. These risks are part of why regular monitoring with blood tests and periodic check-ins remains important throughout life, even when symptoms are well controlled.