Polisitemia Vera

01Pathophysiology

Polycythemia vera (PV) is a clonal myeloproliferative neoplasm originating from a mutated hematopoietic stem cell. The central molecular defect is a gain-of-function point mutation in the JAK2 gene located on chromosome 9p. The JAK2 V617F mutation is present in roughly 95% of PV cases. In the remaining approximately 5%, a mutation in JAK2 exon 12 is found. This distinction matters because if V617F is negative, PV is not ruled out until exon 12 testing is performed.

Under normal conditions, the JAK2 tyrosine kinase is activated when erythropoietin binds its receptor on erythroid progenitor cells. In PV, the V617F mutation causes constitutive (ligand-independent) activation of the JAK-STAT signaling pathway, meaning the erythroid progenitors proliferate without requiring EPO stimulation. This is why serum EPO levels are characteristically suppressed in PV, which distinguishes it from secondary causes of erythrocytosis (where EPO is elevated as the driving stimulus).

The unregulated overproduction of red blood cells leads to increased blood viscosity. This hyperviscosity is directly responsible for many of the symptoms: headaches, dizziness, visual disturbances, and tinnitus all result from sluggish blood flow through cerebral and ocular microvasculature. Thrombotic events are the leading cause of morbidity and mortality in PV, driven both by hyperviscosity and by qualitative platelet abnormalities. Patients are at heightened risk for arterial thrombosis (stroke, myocardial infarction), venous thrombosis (deep vein thrombosis, pulmonary embolism), and notably splanchnic vein thrombosis such as Budd-Chiari syndrome (hepatic vein thrombosis) and portal vein thrombosis. A young patient presenting with Budd-Chiari syndrome should immediately prompt evaluation for PV and other myeloproliferative neoplasms.

The single most reliable discriminator on an exam is the EPO level. A low EPO with elevated Hb/Hct points to PV; an elevated EPO with the same lab picture points to a secondary cause. From there, JAK2 testing confirms PV, while history and imaging (chest CT for lung disease, renal imaging for tumors, sleep study for OSA) identify the secondary etiology.

Although PV is primarily an erythroid proliferative disorder, it is actually a panmyelosis: white blood cells and platelets are often elevated as well, reflecting the stem-cell-level nature of the clonal expansion. The splenomegaly seen in PV results from both extramedullary hematopoiesis and sequestration of the excess blood cells by the spleen.

Aquagenic pruritus, a hallmark symptom, occurs in up to 40% of patients and is characterized by intense itching within minutes of contact with warm water. The mechanism is thought to involve mast cell degranulation and histamine release triggered by temperature change, combined with elevated basophil counts seen in myeloproliferative disorders.

Erythromelalgia, another classic finding, presents as episodic burning pain with erythema and warmth in the extremities (particularly the hands and feet), caused by microvascular arteriolar inflammation and platelet-mediated thrombotic occlusion of small vessels. It responds dramatically to aspirin.

Over time, PV may undergo transformation. The two feared long-term complications are evolution into post-PV myelofibrosis (spent phase, occurring in approximately 10-20% over 15-20 years) and transformation into acute myeloid leukemia (occurring in roughly 5-10%, with risk increased by certain cytoreductive agents such as alkylating agents and, historically debated, prolonged hydroxyurea use).

02Classification and Clinical Manifestation

WHO 2022 Diagnostic Criteria

Diagnosis requires: All 3 major criteria, OR the first 2 major criteria plus the minor criterion.

Risk Stratification

Additional cardiovascular risk factors (diabetes, hypertension, dyslipidemia, smoking) contribute to the overall thrombotic risk profile but do not independently change the formal risk category in standard classification systems.

Key Clinical Manifestations by System

03Diagnostic Workup

Summary Table

Diagnostic Approach in Detail

Step 1: Identify the erythrocytosis. The workup typically begins when a routine CBC reveals an elevated hemoglobin or hematocrit. The first clinical question is whether this represents a true (absolute) increase in red cell mass or a relative polycythemia from hemoconcentration (e.g., dehydration). Repeating the CBC after adequate hydration can help clarify this. A persistently elevated Hb > 16.5 g/dL in men or > 16.0 g/dL in women warrants further investigation.

Step 2: Distinguish primary from secondary erythrocytosis. The serum erythropoietin level is the pivotal test here. A low EPO points toward a primary marrow disorder (PV), because the autonomous overproduction of red cells provides negative feedback that suppresses EPO release from the kidneys. A high or inappropriately normal EPO suggests a secondary cause such as chronic hypoxia (COPD, obstructive sleep apnea, high altitude, right-to-left cardiac shunts), EPO-secreting tumors (renal cell carcinoma, hepatocellular carcinoma, cerebellar hemangioblastoma), or exogenous EPO use.

Step 3: Obtain JAK2 mutation testing. If EPO is low and erythrocytosis is confirmed, JAK2 V617F testing should be ordered. A positive result, in the setting of elevated hemoglobin/hematocrit and low EPO, is often sufficient to confirm the diagnosis. If V617F is negative, proceed to JAK2 exon 12 mutation analysis, as roughly 3-5% of true PV cases carry this alternative mutation.

Step 4: Bone marrow biopsy. A bone marrow biopsy is recommended to fulfill WHO diagnostic criteria and is particularly valuable when there is a need to assess for early/prefibrotic disease, evaluate baseline reticulin fibrosis, and rule out other myeloproliferative neoplasms. The characteristic findings are hypercellularity for age, panmyelosis (trilineage proliferation), and clusters of pleomorphic mature megakaryocytes of varying sizes. Iron stores are frequently absent. Reticulin fibrosis grading at baseline helps track progression to post-PV myelofibrosis over time.

Key point for examinations: A normal SaO2 (> 92%) combined with a low EPO level effectively rules out secondary polycythemia and shifts the differential strongly toward PV.

04Management and Treatment

Summary Table

for all patients

Phlebotomy is the cornerstone of initial treatment. The goal is to achieve and maintain a hematocrit below 45%, which has been shown in the CYTO-PV trial to reduce cardiovascular events and death compared to a more lenient target. Sessions involve removing approximately 450-500 mL of blood, performed every 2-3 days initially if the hematocrit is very elevated, then spaced out to a maintenance schedule (often every 1-3 months) once the target is reached. Iron deficiency resulting from repeated phlebotomy is expected and should generally not be corrected with iron supplementation, as this would counteract the purpose of treatment. However, if the patient develops severely symptomatic iron deficiency (e.g., pica, restless leg syndrome, cognitive symptoms) that outweighs the benefit, cautious low-dose iron replacement with close hematocrit monitoring can be considered.

Low-dose aspirin (81 mg daily) is recommended for virtually all PV patients. The ECLAP trial demonstrated that aspirin reduces the combined risk of nonfatal myocardial infarction, nonfatal stroke, and venous thromboembolism without a meaningful increase in bleeding risk. Aspirin is also remarkably effective for erythromelalgia, often providing relief within hours. Aspirin should be avoided or used with caution in patients with extreme thrombocytosis (platelet count > 1,000,000 to 1,500,000/mcL) because of the risk of acquired von Willebrand disease: the very high platelet count leads to adsorption and proteolysis of von Willebrand factor multimers, creating a bleeding diathesis rather than a thrombotic state. In such cases, ristocetin cofactor activity should be checked before initiating aspirin.

For high-risk patients (age ≥ 60 or history of thrombosis)

Hydroxyurea is the first-line cytoreductive agent. It works by inhibiting ribonucleotide reductase, thereby reducing DNA synthesis in rapidly dividing hematopoietic cells. The typical starting dose is 15-20 mg/kg/day (roughly 500-1000 mg daily for most adults), titrated to achieve a hematocrit < 45%, white blood cell count > 3,000/mcL, and platelet count > 100,000/mcL. Patients on hydroxyurea require regular monitoring with CBC every 2-4 weeks during dose titration, then every 2-3 months once stable. Side effects include cytopenias (dose-dependent), oral ulcers, skin ulcers (particularly leg ulcers), nail changes, and rare mucocutaneous squamous cell changes. Hydroxyurea is absolutely contraindicated in pregnancy (teratogenic).

Pegylated interferon-alpha (notably ropeginterferon alfa-2b, which is FDA-approved for PV) is an alternative first-line cytoreductive agent and is preferred in younger patients, women of childbearing potential, and patients who are intolerant of or resistant to hydroxyurea. Interferon has the advantage of being able to induce molecular responses (reduction in JAK2 V617F allele burden) and is not considered leukemogenic. Ropeginterferon alfa-2b is administered subcutaneously every 2 weeks initially, then every 2-4 weeks during maintenance. Side effects include flu-like symptoms, fatigue, depression, autoimmune phenomena, and hepatotoxicity. Interferon is contraindicated in patients with severe psychiatric disorders (particularly untreated depression) and decompensated liver disease.

Ruxolitinib (a JAK1/JAK2 inhibitor) is indicated for patients who are resistant to or intolerant of hydroxyurea. It is effective at controlling hematocrit, reducing spleen size, alleviating pruritus, and improving constitutional symptoms. The starting dose is typically 10 mg twice daily, adjusted based on response and tolerability. Ruxolitinib carries risks of cytopenias, infections (particularly herpes zoster reactivation), and a withdrawal syndrome with rebound symptoms if discontinued abruptly. Patients should be screened for tuberculosis and hepatitis B before starting ruxolitinib.

Special situations

In pregnancy, hydroxyurea is contraindicated. The preferred management includes phlebotomy, low-dose aspirin, and pegylated interferon-alpha if cytoreduction is necessary. Pregnancy in PV carries elevated risks of thrombosis and pregnancy complications (preeclampsia, placental insufficiency, fetal loss), warranting close multidisciplinary management.

For extreme thrombocytosis (> 1,500,000/mcL), assess for acquired von Willebrand disease by measuring ristocetin cofactor activity. If acquired von Willebrand disease is confirmed, aspirin should be withheld until the platelet count is reduced with cytoreductive therapy.

05Differential Diagnosis and Distractors

polycythemia vera vs secondary erythrocytosis

other differential diagnoses

06Traps and High-Yield Pearls

The most common pitfall with PV questions is confusing primary and secondary erythrocytosis. Many students see an elevated hemoglobin or hematocrit and immediately reach for the right diagnosis, but then select the wrong EPO level. The rule is straightforward: in PV, the marrow is autonomously producing red cells independent of EPO, so EPO is suppressed. In secondary causes, EPO is the driver of the erythrocytosis, so it is elevated. If you remember only one lab value for PV, make it low EPO.

A second common trap involves the Budd-Chiari syndrome presentation. When a young patient (often a young woman on oral contraceptives) presents with hepatomegaly, ascites, and abdominal pain from hepatic vein thrombosis, students may focus on the thrombotic event itself and forget to evaluate the underlying cause. PV and other myeloproliferative neoplasms are among the most common causes of Budd-Chiari syndrome, and JAK2 testing should be ordered even when the CBC is not obviously abnormal (because the expanded plasma volume from liver disease can mask the elevated red cell mass).

Another frequently tested concept is the hematocrit target of < 45%. This number is firmly supported by trial data (CYTO-PV) and appears repeatedly. Students sometimes confuse this with a less strict target or forget that it applies to both men and women.

Do not forget the association between extreme thrombocytosis and paradoxical bleeding. When platelet counts exceed approximately 1,000,000-1,500,000/mcL, acquired von Willebrand disease can develop. In this setting, aspirin is potentially harmful because it worsens bleeding. The test to confirm this is ristocetin cofactor activity.

Finally, be aware that aquagenic pruritus can precede the diagnosis by years and may be the presenting complaint. A patient who describes intense itching after a hot shower, without a rash, should prompt a CBC and consideration of PV even if other symptoms are minimal.

The core competency being tested in PV questions is the ability to integrate a clinical presentation (plethora, hyperviscosity symptoms, unusual-site thrombosis, pruritus) with the correct laboratory profile (elevated Hb/Hct, low EPO, positive JAK2), differentiate primary from secondary erythrocytosis, and sequence the management correctly (phlebotomy and aspirin for all, add hydroxyurea for high-risk).