Polycythemia vera (PV) is diagnosed using a set of criteria established by the World Health Organization. The current system requires meeting either all three major criteria or the first two major criteria plus a minor criterion. These criteria center on elevated blood counts, a specific genetic mutation, bone marrow findings, and a hormone level that helps rule out other causes of high red blood cells.
The Three Major Criteria
The first major criterion is an elevated hemoglobin or hematocrit level. The thresholds are sex-specific: hemoglobin above 16.5 g/dL in men or above 16.0 g/dL in women, or hematocrit above 49% in men or above 48% in women. An alternative measure, red blood cell mass more than 25% above the predicted normal value, also satisfies this criterion. These thresholds were lowered significantly in the 2016 revision. The hemoglobin cutoff for men dropped from 18.5 g/dL to 16.5 g/dL, a change designed to catch “masked” cases of PV that were being missed under the older, higher bar.
The second major criterion is a bone marrow biopsy showing a specific pattern called panmyelosis. This means the marrow is producing too many of all three major blood cell lines: red cells, white cells, and platelets. The marrow appears overly packed for the patient’s age, and the platelet-producing cells (megakaryocytes) vary noticeably in size rather than looking uniform. This biopsy pattern helps distinguish PV from other conditions that raise red blood cell counts.
The third major criterion is the presence of a JAK2 gene mutation. About 95% of people with PV carry the JAK2 V617F mutation. The remaining 2 to 5% carry a mutation in exon 12 of the same gene. Together, virtually all PV patients have one of these two mutations, making the JAK2 test one of the most decisive steps in diagnosis.
The Minor Criterion
The single minor criterion is a subnormal level of erythropoietin (EPO) in the blood. EPO is a hormone your kidneys produce to signal your bone marrow to make more red blood cells. In PV, the marrow is already overproducing red cells on its own because of the JAK2 mutation, so the body dials EPO production down. The WHO doesn’t specify a single numeric cutoff, but research suggests that an EPO level below about 5.3 IU/L provides high sensitivity for identifying PV, while a level below 3.1 IU/L is more specific. Normal EPO typically falls between roughly 5 and 26 IU/L.
This minor criterion matters most when a bone marrow biopsy isn’t available or isn’t definitive. If you meet the first two major criteria (elevated blood counts plus the JAK2 mutation) but don’t have a bone marrow biopsy confirming panmyelosis, a subnormal EPO level can complete the diagnosis instead.
How the Criteria Fit Together
There are two pathways to a formal diagnosis:
- All three major criteria met: elevated hemoglobin/hematocrit, characteristic bone marrow biopsy, and a JAK2 mutation.
- First two major criteria plus the minor criterion: elevated hemoglobin/hematocrit, a JAK2 mutation, and a subnormal EPO level (without needing the bone marrow biopsy).
In practice, blood work and a JAK2 test are usually the first steps. If the JAK2 mutation is positive and blood counts are above the WHO thresholds, a bone marrow biopsy or EPO level will typically confirm the diagnosis.
How PV Differs From Secondary Causes
Elevated red blood cell counts aren’t unique to PV. Chronic smoking, obesity, sleep apnea, and living at high altitude can all push hemoglobin and hematocrit above normal. This is called secondary erythrocytosis, and distinguishing it from PV is a major reason the WHO criteria exist.
The JAK2 mutation is the clearest dividing line. Secondary erythrocytosis patients test negative. Their EPO levels are typically normal or elevated, since their bodies are producing extra red cells in response to a real signal (like low oxygen from lung disease), not from a bone marrow defect. People with secondary erythrocytosis also tend to be younger, more likely to be male, more often obese (75% vs. 17% in one study), and more likely to smoke. Palpable spleen enlargement is far less common in secondary erythrocytosis (about 6%) compared to PV (about 23%).
That said, roughly one quarter of confirmed PV patients have normal EPO levels rather than low ones. A normal EPO result does not rule PV out, which is why the JAK2 mutation and bone marrow biopsy carry more diagnostic weight.
Symptoms That Prompt Testing
PV develops slowly, and many people have it for years before diagnosis. It’s often discovered incidentally through routine blood work showing high hemoglobin or hematocrit. When symptoms do appear, they can include headaches, dizziness, blurred vision, and fatigue. More distinctive signs include intense itching after a warm shower (called aquagenic pruritus), a feeling of fullness or pain in the upper left abdomen from an enlarged spleen, numbness or tingling in the hands and feet, painful swelling of the big toe from gout, and unusual bleeding from the nose or gums.
Risk Categories After Diagnosis
Once PV is confirmed, the next step is determining thrombosis risk, since blood clots are the most serious complication. The risk stratification is straightforward: you’re considered high risk if you’re over 60 or have a history of blood clots, and low risk if neither applies. This classification directly shapes treatment intensity, particularly whether medication to reduce red blood cell production is added alongside the standard approach of keeping hematocrit controlled through periodic blood draws.