Polycythemia Vera

Polycythemia vera (PV) is a chronic myeloproliferative neoplasm in which the bone marrow produces too many red blood cells, and often excess white blood cells and platelets as well. It is caused by an acquired genetic mutation — most commonly JAK2 V617F, present in approximately 95% of cases — that drives uncontrolled blood cell production. PV is not inherited and is not a cancer in the conventional sense, though it carries a risk of progression to more serious conditions if untreated.

The excess red blood cells thicken the blood, slowing circulation and increasing the risk of blood clots, which is the primary source of serious complications including stroke, deep vein thrombosis, and pulmonary embolism.

Why this matters for your blood test results

PV is most often first suspected from routine blood tests showing elevated hemoglobin, hematocrit, or red blood cell count
Elevated MXD%, eosinophils, or monocytes may also be present due to the generalised overproduction of bone marrow cells
JAK2 V617F mutation testing confirms the diagnosis in most cases; JAK2 Exon 12 testing is used when V617F is negative
Erythropoietin (EPO) levels are typically low or undetectable in PV — distinguishing it from secondary polycythaemia where EPO is elevated

Symptoms

Many people with PV have no symptoms at diagnosis and are identified incidentally on routine blood tests. When symptoms occur they include:

Headaches, dizziness, and visual disturbances from increased blood viscosity
Itching after bathing or showering (aquagenic pruritus) — a characteristic symptom driven by histamine release
Redness and burning of the hands and feet (erythromelalgia)
Fatigue and weakness
Enlarged spleen (splenomegaly) causing left-sided abdominal fullness
Spontaneous bruising or bleeding

Blood clot events — stroke, heart attack, DVT, or pulmonary embolism — may be the presenting event in some patients.

Causes and diagnosis

PV is caused by an acquired somatic mutation in the JAK2 gene, most commonly V617F, which constitutively activates the JAK-STAT signalling pathway driving unchecked blood cell production. The mutation is not inherited and arises spontaneously in a haematopoietic stem cell.

Diagnosis requires meeting WHO criteria, which include elevated haemoglobin or haematocrit, presence of a JAK2 mutation, and bone marrow biopsy findings showing hypercellularity with trilineage proliferation. Erythropoietin level is a key supporting test — a subnormal EPO strongly supports PV over secondary causes of polycythaemia.

Treatment

Treatment aims to reduce the risk of blood clot complications by controlling blood counts.

Therapeutic phlebotomy — regular removal of blood to keep haematocrit below target thresholds (typically <45% in males, <42% in females)
Low-dose aspirin — reduces clot risk in most patients
Cytoreductive therapy — hydroxyurea is the standard first-line agent for high-risk patients; interferon-alpha is increasingly used, particularly in younger patients
Ruxolitinib — a JAK1/JAK2 inhibitor used for hydroxyurea-resistant or intolerant disease

PV requires long-term specialist follow-up. A small proportion of patients progress over time to myelofibrosis or, rarely, acute leukemia.

FAQ

Is polycythemia vera serious? It is a chronic condition that requires ongoing management but is compatible with a good quality of life when well controlled. The main risks are blood clots and, in a minority of patients over time, progression to myelofibrosis or leukemia.

Is polycythemia vera hereditary? No. The JAK2 mutation that causes PV is acquired during a person's lifetime and is not passed to children.

Related biomarkers JAK2 V617F Mutation, JAK2 Exon 12 Mutation, Hemoglobin, Hematocrit, Red Blood Cell Count, Erythropoietin (EPO), White Blood Cells, Platelets, MXD%.

Polycythemia Vera

Why this matters for your blood test results

Symptoms

Causes and diagnosis

Treatment

FAQ

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